Molecular Metabolism

Futile lipid cycling is an ATP-wasting process proposed to participate in energy expenditure of mature fat-storing white adipocytes, given their inability to oxidize fat. The hallmark of activated brown adipocytes is to increase fat oxidation by uncoupling respiration from ATP synthesis. Whether ATP-consuming lipid cycling can contribute to BAT energy expenditure has been largely unexplored. Here we find that pharmacological inhibition of the mitochondrial pyruvate carrier (MPC) in brown adipocytes is sufficient to increase ATP-synthesis fueled by fatty acid oxidation, even in the absence of adrenergic stimulation. We find that elevated ATP-demand induced by MPC inhibition results from activation of futile lipid cycling. Furthermore, we identify that glutamine consumption and the Malate-Aspartate Shuttle are required for the increase in Energy Expenditure induced by MPC inhibition in Brown Adipocytes (MAShEEBA). These data demonstrate that futile energy expenditure through lipid cycling can be activated in BAT by altering fuel availability to mitochondria. Therefore, we identify a new mechanism to increase fat oxidation and energy expenditure in BAT that bypasses the need for adrenergic stimulation of mitochondrial uncoupling.

ObjectiveDelineating the nodal control points that maintain whole-body energy homeostasis is critical for understanding potential treatments of obesity and cardiometabolic diseases. The nutrient-sensing transcription factor MondoA is a regulator of skeletal muscle fuel storage, where muscle-specific inhibition improves glucose tolerance and insulin sensitivity. However, the role of MondoA in whole body energy metabolic homeostasis is not understood. MethodsGeneralized MondoA knockout (gKO) mice were generated and assessed for glucose tolerance and insulin sensitivity, body composition, energy expenditure, cold tolerance, and tissue specific transcriptional changes in response to high fat diet. Complementary studies in cultured human adipocytes assessed the impact of MondoA deficiency on substrate utilization and lipolysis. ResultsgKO mice are protected from diet-induced obesity and insulin resistance, through increased whole body energy expenditure. gKO mice exhibit reduced brown and inguinal white adipose tissue mass, without evidence of beiging. The gKO mice are hyperlactatemic and isolated MondoA-deficient adipocytes have increased 2-deoxyglucose uptake and glycolytic function. Lastly, gKO mice and KO adipocytes display increased circulating glycerol relative to free fatty acids in response to adrenergic stimulus consistent with elevated re-esterification. However, this phenotype is not recapitulated in adipocyte-specific KO mice. ConclusionsMondoA deficiency alters cellular sensing of nutrient availability and storage/utilization mechanisms. In the whole-body setting, this results in increased energy expenditure, potentially related to increased glucose uptake and glycolytic flux driving glycerol synthesis to supply high rates of lipolysis and lipid re-esterification. These results suggest that MondoA functions to maintain fuel storage and when lost, inter-organ futile cycling ensues. O_FIG O_LINKSMALLFIG WIDTH=140 HEIGHT=200 SRC="FIGDIR/small/680559v1_ufig1.gif" ALT="Figure 1"> View larger version (47K): org.highwire.dtl.DTLVardef@1ccf42dorg.highwire.dtl.DTLVardef@b2da28org.highwire.dtl.DTLVardef@1081aforg.highwire.dtl.DTLVardef@1b2168d_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical Abstract.C_FLOATNO 1) Global MondoA deficiency drives 2) tissue glucose uptake which in skeletal muscle is 3) converted and excreted as lactate, while in adipose tissue 4) triglyceride re-esterification requires 5) de novo glycerol synthesis to feed into the futile cycle. C_FIG

Background & AimsPNPLA3(I148M) is the strongest genetic risk factor for steatotic liver disease (SLD), but its functional role and tissue-specific regulation remain unclear. In mice, PNPLA3 is abundant in liver, yet undetectable in adipose depots. Here, we characterize the molecular mechanisms underlying these tissue-specific differences in PNPLA3 expression in mice to clarify its functional role and link to SLD risk. MethodsPnpla3 mRNA and PNPLA3 protein levels were quantified in liver and adipose depots of fasted and refed mice at 30{degrees}C and 6{degrees}C. Signaling pathways regulating PNPLA3 expression in adipocytes were examined using adrenergic agonists and pathway-specific modulators. Translation and proteasomal inhibitors were used during adrenergic stimulation to investigate the discordance between Pnpla3 mRNA and protein levels. Relationship between PNPLA3 levels and triglyceride (TG) fatty acid composition was also assessed. ResultsAt thermoneutrality, feeding strongly increased PNPLA3 levels in liver but it remained undetectable in adipose tissue of mice. Conversely, cold exposure or {beta}3-adrenergic stimulation had no effect on hepatic PNPLA3, but increased PNPLA3 >19-fold in brown adipose tissue (BAT), despite causing a >75% reduction in Pnpla3 mRNA, indicating robust post-translational regulation. In BAT, adrenergic signaling via cAMP/PKA and PI3K/AKT elevated PNPLA3 by reducing proteasomal degradation. PNPLA3 expression correlated with depletion of TG-long-chain polyunsaturated fatty acids (TG-LCPUFAs) in both liver and BAT, consistent with a role in lipid remodeling. ConclusionsThese findings reveal striking tissue- and context-specific regulation of PNPLA3, but a conserved association between its expression and TG-LCPUFAs levels, suggesting that PNPLA3 modulates lipid remodeling in response to metabolic stress and that disrupting this function may contribute to SLD susceptibility. Impact and implicationsDespite being the strongest genetic risk factor for SLD, PNPLA3s physiological role remains unclear. Using mouse models, this study reveals that PNPLA3 is regulated in a tissue-specific manner in response to feeding and cold exposure, thereby promoting remodeling of cellular lipids to adapt to dietary and environmental challenges. The localization of PNPLA3 action and its tissue-specific regulation are directly relevant to hepatologists and metabolic researchers aiming to understand its influence on intracellular lipid composition and its effects on disease susceptibility. Moreover, modulation of PNPLA3 turnover--and its impact on LCPUFAs remodeling--emerges as a potential therapeutic strategy for regulating lipid homeostasis in SLD. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=191 SRC="FIGDIR/small/684800v2_ufig1.gif" ALT="Figure 1"> View larger version (42K): org.highwire.dtl.DTLVardef@ee6ac7org.highwire.dtl.DTLVardef@a45d74org.highwire.dtl.DTLVardef@f39929org.highwire.dtl.DTLVardef@cc826b_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIPNPLA3 is regulated in a highly tissue-specific manner in mice. C_LIO_LIIn liver, feeding-but not cold exposure-induces PNPLA3 primarily through transcriptional mechanisms. C_LIO_LIIn adipose tissue, cold exposure-but not feeding-induces PNPLA3 through post-transcriptional mechanisms. C_LIO_LIIn adipose tissue, cold exposure increases PNPLA3 despite a reduction in Pnpla3 mRNA. C_LIO_LIPNPLA3 remodels lipids in liver and adipose tissue to maintain lipid homeostasis, a process disrupted in SLD. C_LI

Obesity affects one in three adults and is complicated by adipose inflammation, lipotoxicity and cell death. We previously identified RIPK1 as a genetic determinant of human obesity risk and adipose inflammation. Because RIPK1 is the apical kinase in the necroptosis pathway upstream of RIPK3 and the executioner protein MLKL, and emerging evidence links MLKL to lipid metabolism, MLKL has surfaced as a potential metabolic regulator. However, conflicting findings in Mlkl knockout mice fed a high fat diet have left its therapeutic relevance unresolved. MLKL has not been previously targeted through therapeutic knockdown in vivo in the context of diet-induced obesity. Here, we evaluated two independent MLKL antisense oligonucleotides (ASOs) in high fat diet (HFD)-fed C57BL/6J mice. In a 24-week progression model, MLKL ASO markedly reduced body weight, fat mass and hepatic steatosis compared with controls, while preserving lean mass. MLKL knockdown also lowered the respiratory exchange ratio, indicating a shift toward increased fat oxidation. In the intervention model, once obesity was established after 12 weeks of HFD feeding, both MLKL ASOs, and similarly, two independent RIPK1 ASOs, reversed weight gain and improved systemic glucose control. In vitro, MLKL-CRISPR/Cas9 knockout blocked 3T3-L1 adipogenesis, indicating a requirement for MLKL during adipocyte differentiation. However, in mature adipocytes, MLKL siRNA reduced palmitic acid-induced lipid accumulation, increased isoprenaline-stimulated lipolysis, and prevented TNF-mediated suppression of insulin-mediated AKT signalling and glucose uptake. Collectively, these findings demonstrate that partial MLKL suppression reprograms whole-body energy metabolism, enhances insulin sensitivity and limits diet-induced adiposity. MLKL, therefore, represents a promising and mechanistically novel therapeutic target for obesity and insulin resistance.

ObjectiveGlucokinase Regulatory Protein (GKRP) controls the activity of Glucokinase (GCK) to regulate liver glucose uptake and storage. Coding variants in GCKR, the gene encoding GKRP, strongly associate with fatty liver disease, hypertriglyceridemia, and hypercholesterolemia. Here, we sought to investigate the mechanisms by which a common GKRP variant affects hepatic lipid and cholesterol metabolism. MethodsWe developed mouse models to examine how the human GKRP P446L variant influences liver and systemic metabolism. Endogenous Gckr expression was ablated in adult mouse hepatocytes, together with re-expression of either human GKRP P446L or the reference GKRP protein. We assessed body weight, adiposity, systemic glucose homeostasis, and hepatic metabolites in mice expressing reference GKRP or GKRP P446L under multiple metabolic conditions. To determine whether the effects of GKRP P446L may result from reduced GCK activity, we analyzed mice with liver-specific deletion of Gck. ResultsHepatic expression of GKRP P446L resulted in reduced GKRP and GCK protein levels and elevated serum cholesterol. Hepatic deletion of Gck in mice recapitulated several effects of GKRP P446L, including increased hepatic cholesterol and triglyceride content. The elevated cholesterol was associated with increased cholesterogenic gene expression and cholesterol synthesis. Hepatic expression of an alternative hexokinase (HKII) normalized the effects of GCK-deficiency, suggesting that impaired glucose phosphorylation underlies the phenotype. ConclusionsThe GKRP P446L variant reduced GKRP protein abundance, and diminished GCK activity while increasing cholesterol levels. Loss of GCK elevated cholesterol and hepatic triglyceride levels. Collectively, these findings demonstrate that GCK suppresses hepatic cholesterol synthesis and lipid accumulation, suggesting that reduced GCK activity underlies the metabolic abnormalities associated with the GKRP P446L variant. HighlightsO_LIThe GKRP P446L variant reduces GKRP protein abundance and diminishes GCK activity. C_LIO_LIExpression of GKRP P446L in mouse hepatocytes increases serum cholesterol levels. C_LIO_LIHepatic GCK activity suppresses cholesterogenic gene expression and cholesterol synthesis. C_LI

BackgroundAs of 2023, approximately 100.1 million adults and 14.7 million children in the USA are obese. Many comorbidities develop with obesity, which impairs quality of life and burdens the health care system. Consequently, there is an urgent need for interventions and treatments to reverse obesity and its comorbidities and restore health. Sphingosine Kinase 1 (SphK1), a key enzyme in sphingolipid metabolism, produces sphingosine-1-phosphate (S1P), a bioactive lipid implicated in obesity and metabolic dysfunction. While global deletion of Sphk1 protects against diet-induced obesity, adipocyte-specific SPHK1 deficiency paradoxically promotes weight gain, glucose intolerance, and adipose inflammation. Given the known role of sphingolipids in adipose thermogenesis, we investigated whether Sphk1 regulates adipocyte beiging and mitochondrial function. MethodsWe assessed thermogenic responses in SphK1-deficient adipocytes and adipocyte-specific Sphk1 knockout (Ad-SphK1{Delta}) mice under basal and {beta}3-adrenergic stimulation using CL 316,243. Thermoneutral housing (30{degrees}C) and room temperature (23{degrees}C) conditions were used to minimize and assess ambient temperature effects on thermogenesis. Molecular, histological, and bioenergetic analyses were conducted across multiple adipose depots. Results{beta}3-adrenergic stimulation upregulated Sphk1 expression in mature white adipocytes, while SphK1-deficient adipocytes exhibited enhanced Ucp1 expression, indicating a suppressive role for SphK1 in beiging. In vivo, adipocyte-specific Sphk1 knockout (Ad-SphK1{Delta}) mice showed elevated Ucp1 expression in inguinal and gonadal white adipose tissue (iWAT, gWAT), both basally and after CL 316,243 treatment. These changes were accompanied by depot-specific alterations in adipocyte size and increased adiposity, independent of ambient temperature. Despite similar elevation of thermogenic markers, Sphk1 deletion had differential effect on mitochondrial function: iWAT showed increased mitochondrial content but reduced complex IV activity and ATP production, whereas gWAT showed reduced mitochondrial abundance without changes in respiration. ConclusionOur work suggests that Sphk1 may act as a negative regulator of thermogenic expression and affect mitochondrial function in a depot-specific manner. Loss of Sphk1 enhances beiging but compromises mitochondrial efficiency, revealing a complex role for the SphK1/S1P axis in adipose plasticity and metabolic regulation. These insights may inform future therapeutic strategies targeting sphingolipid pathways for obesity and metabolic disease.

ObjectiveThe metalloprotease ADAM17 (also called TACE) plays fundamental roles in homeostasis by shedding key signaling molecules from the cell surface. Although its importance for the immune system and epithelial tissues is well-documented, little is known about the role of ADAM17 in metabolic homeostasis. The purpose of this study was to determine the impact of ADAM17 expression, specifically in adipose tissues, on metabolic homeostasis. MethodsWe used histopathology, molecular, proteomic, transcriptomic, in vivo integrative physiological and ex vivo biochemical approaches to determine the impact of adipose tissue-specific deletion of ADAM17 upon adipocyte and whole organism metabolic physiology. ResultsADAM17adipoq-cre{Delta}/{Delta} mice exhibited a hypermetabolic phenotype characterized by elevated energy consumption and increased levels of adipocyte thermogenic gene expression. On a high fat diet, these mice were more thermogenic, while exhibiting elevated expression levels of genes associated with lipid oxidation and lipolysis. This hypermetabolic phenotype protected mutant mice from obesogenic challenge, limiting weight gain, hepatosteatosis and insulin resistance. Activation of beta-adrenoceptors by the neurotransmitter norepinephrine, a key regulator of adipocyte physiology, triggered the shedding of ADAM17 substrates, and regulated ADAM17 expression at the mRNA and protein levels, hence identifying a functional connection between thermogenic licensing and the regulation of ADAM17. Proteomic studies identified Semaphorin 4B (SEMA4B), as a novel ADAM17-shed adipokine, whose expression is regulated by physiological thermogenic cues that acts to dampen thermogenic responses in adipocytes. Transcriptomic data showed that cleaved SEMA4B acts in an autocrine manner in brown adipocytes to dampen the expression of genes involved in thermogenesis, adipogenesis, lipid uptake, storage and catabolism. ConclusionOur findings identify a novel ADAM17-dependent axis, regulated by beta-adrenoceptors and mediated by the ADAM17-cleaved form of SEMA4B, that may act to limit uncontrolled energy depletion during thermogenesis.

ObjectiveA catheter-free, non-radiolabeled method that permits in vivo measurement of tissue-specific glucose uptake does not exist. To address this gap, we sought to develop and validate a new, higher throughput mass spectrometry (MS)-based method that combines an injection of insulin with a non-radiolabeled glucose tracer, 2-fluoro-2-deoxyglucose (2FDG), to determine insulin-stimulated tissue-specific glucose clearance in conscious, unrestrained mice. MethodsInjections of saline or insulin with 2FDG were coupled with LC-Q Exactive Hybrid Quadrupole-Orbitrap (LC) MS-based measures of plasma 2FDG and tissue (2-fluoro-2-deoxyglucose-6-phosphate) 2FDGP to determine glucose clearance in mice under several different conditions. ResultsThe newly developed method was first applied to a dose response experiment in mice. Next, the ability of this method to quantify changes in glucose clearance in response to an insulin stimulus was assessed, and glucose clearance was compared between chow and high fat fed mice. Results from these studies showed that insulin-stimulated skeletal muscle and heart glucose clearance can be estimated following a bolus injection of tracer, and these fluxes are blunted in diet-induced obese mice. The broad applicability of this approach was then demonstrated by assessing glucose clearance in a mouse model with anticipated changes in insulin-stimulated skeletal muscle glucose metabolism. ConclusionsThe results validated a new LC-MS method to quantify insulin-stimulated tissue-specific glucose clearance in vivo without the use of catheters or radiolabeled tracers. The method offers great potential because it is designed for application to pre-clinical studies seeking high throughput tests and/or assays that can be coupled with discovery technologies such as genomics, proteomics and metabolomics. HIGHLIGHTSO_LIIn vivo glucose clearance can be estimated by a new non-radiolabeled method. C_LIO_LIThe plasma tracer to tracee ratio is required to determine tissue tracer phosphorylation. C_LIO_LIMeasures of plasma glucose and tracer kinetics are critical for data interpretation. C_LIO_LIThe new method can be combined with omics technologies such as metabolomics. C_LI

ObjectivePancreatic islets are nutrient sensors that regulate organismal blood glucose homeostasis. Glucagon release from the pancreatic -cell is important under fasted, fed, and hypoglycemic conditions, yet metabolic regulation of -cells remains poorly understood. Here, we identified a previously unexplored role for physiological levels of leucine, which is classically regarded as a {beta}-cell fuel, in the intrinsic regulation of -cell glucagon release. MethodsGcgCreERT:CAMPER and GcgCreERT:GCaMP6s mice were generated to perform dynamic, high-throughput functional measurements of -cell cAMP and Ca2+ within the intact islet. Islet perifusion assays were used for simultaneous, time-resolved measurements of glucagon and insulin release from mouse and human islets. The effects of leucine were compared with glucose and the mitochondrial fuels 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH, non-metabolized leucine analog that activates glutamate dehydrogenase), -ketoisocaproate (KIC, leucine metabolite), and methyl-succinate (complex II fuel). CYN154806 (Sstr2 antagonist), diazoxide (KATP activator, which prevents Ca2+-dependent exocytosis from , {beta}, and {delta}-cells), and dispersed -cells were used to inhibit islet paracrine signaling and identify -cell intrinsic effects. ResultsMimicking the effect of glucose, leucine strongly suppressed amino acid-stimulated glucagon secretion. Mechanistically, leucine dose-dependently reduced -cell cAMP at physiological concentrations, with an IC50 of 57, 440, and 1162 M at 2, 6, and 10 mM glucose, without affecting -cell Ca2+. Leucine also reduced -cell cAMP in islets treated with Sstr2 antagonist or diazoxide, as well as dispersed -cells, indicating an -cell intrinsic effect. The effect of leucine was matched by KIC and the glutamate dehydrogenase activator BCH, but not methyl-succinate, indicating a dependence on mitochondrial anaplerosis. Glucose, which stimulates anaplerosis via pyruvate carboxylase, had the same suppressive effect on -cell cAMP but with lower potency. Similarly to mouse islets, leucine suppressed glucagon secretion from human islets under hypoglycemic conditions. ConclusionsThese findings highlight an important role for physiological levels of leucine in the metabolic regulation of -cell cAMP and glucagon secretion. Leucine functions primarily through an -cell intrinsic effect that is dependent on glutamate dehydrogenase, in addition to the well-established -cell regulation by {beta}/{delta}-cell paracrine signaling. Our results suggest that mitochondrial anaplerosis-cataplerosis facilitates the glucagonostatic effect of both leucine and glucose, which cooperatively suppress -cell tone by reducing cAMP. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=173 SRC="FIGDIR/small/551113v2_ufig1.gif" ALT="Figure 1"> View larger version (44K): org.highwire.dtl.DTLVardef@1af2865org.highwire.dtl.DTLVardef@8508fborg.highwire.dtl.DTLVardef@10c009org.highwire.dtl.DTLVardef@1afb6e9_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LILeucine inhibits glucagon secretion from mouse and human islets C_LIO_LILeucine suppresses -cell cAMP via both direct and paracrine effects C_LIO_LIAnaplerosis via glutamate dehydrogenase is sufficient to suppress -cell cAMP C_LIO_LILeucine suppresses -cell cAMP and glucagon secretion more potently than glucose C_LI

ObjectiveWhite adipose tissue (WAT) expansion regulates energy balance and overall metabolic homeostasis. WAT absence or loss occurring through lipodystrophy and lipoatrophy contributes to the development of dyslipidemia, hepatic steatosis, and insulin resistance. We previously demonstrated the sole small ubiquitin-like modifier (SUMO) E2-conjuguating enzyme Ubc9 represses human adipocyte differentiation. Germline and other tissue-specific deletions of Ubc9 frequently cause lethality in mice. As a result, the role of Ubc9 during WAT development remains unknown. MethodsTo determine how Ubc9 impacts body composition and energy balance, we generated adipocyte-specific Ubc9 knockout mice (Ubc9a-KO). CRISPR/Cas9 gene editing inserted loxP sites flanking exons 3 and 4 at the Ubc9 locus. Subsequent genetic crosses to AdipoQ-Cre transgenic mice allowed deletion of Ubc9 in white and brown adipocytes. We measured multiple metabolic endpoints that describe energy balance and carbohydrate metabolism in Ubc9a-KO and littermate controls during postnatal growth. ResultsTo our surprise, Ubc9a-KO mice developed hyperinsulinemia and hepatic steatosis. Global energy balance defects emerged from dysfunctional WAT marked by pronounced local inflammation, loss of serum adipokines, hepatomegaly, and near absence of major adipose tissue depots. We observed progressive lipoatrophy that commences in the early adolescent period. ConclusionsOur results demonstrate that Ubc9 expression in mature adipocytes is essential for maintaining WAT expansion. Deletion of Ubc9 in fat cells compromised and diminished adipocyte function that provoked WAT inflammation and ectopic lipid accumulation in the liver. Our findings reveal an indispensable role for Ubc9 during white adipocyte expansion and endocrine control of energy balance.

BackgroundOlanzapine (OLA), a widely prescribed second-generation antipsychotic, is associated with adverse metabolic effects. We recently showed that oral OLA treatment in male mice induces weight gain and hepatic steatosis, whereas intraperitoneal (i.p.) administration leads to weight loss due to higher hypothalamic OLA levels and activation of brown adipose tissue. Since clinical studies report insulin resistance in individuals treated with OLA, here we investigated the impact of OLA i.p. treatment on insulin sensitivity, focusing on the liver- skeletal muscle axis. Material and MethodsWild-type male mice were treated with OLA (10 mg/kg, i.p.) for 8 weeks or received a single intrahypothalamic injection (15 nmol). Glucose homeostasis parameters were assessed. Mechanistic studies were performed in vagotomized mice, mice lacking JNK in either the hypothalamus or liver, mice overexpressing hepatic FGF21, and PTP1B-deficient mice (PTP1B-KO). ResultsOLA i.p. treatment induced systemic insulin resistance, pyruvate intolerance, and reduced insulin signaling in both liver and skeletal muscle. These effects were accompanied by increased hepatic JNK phosphorylation and IRS1 serine phosphorylation. A single intrahypothalamic OLA injection similarly impaired peripheral insulin action and activated hepatic JNK. Deletion of hypothalamic or hepatic JNK1, as well as vagotomy, prevented these defects. OLA reduced hepatic Fgf21 expression, an effect reversed by hypothalamic JNK1 deletion or vagotomy. Hepatic FGF21 overexpression prevented OLA-induced insulin resistance in skeletal muscle but not in liver. PTP1B-KO mice were protected from all metabolic impairments. ConclusionAlthough OLA i.p. treatment prevents weight gain, it decreases peripheral insulin sensitivity through a hypothalamus-liver axis driven by hypothalamic JNK1, which activates hepatic JNK via the vagus nerve, suppresses hepatic FGF21 and ultimately impairs insulin signaling in skeletal muscle. Importantly, the protection conferred by PTP1B deficiency against OLA-induced insulin resistance strongly suggests that targeting PTP1B might prevent metabolic comorbidities in patients under OLA treatment in a personalized manner.

The genetic determinants of the glucocorticoid receptor (GR) metabolic action remain largely unelucidated. This is a compelling gap in knowledge for the GR single nucleotide polymorphism (SNP) rs6190 (p.R23K), which has been associated in humans with enhanced metabolic health but whose mechanism of action remains completely unknown. We generated transgenic knock-in mice genocopying this polymorphism to elucidate how the mutant GR impacts metabolism. Compared to non-mutant littermates, mutant mice showed increased insulin sensitivity on regular chow and high-fat diet, blunting the diet-induced adverse effects on adiposity and exercise intolerance. Overlay of RNA-seq and ChIP-seq profiling in skeletal muscle revealed increased transactivation of Foxc1 and Arid5A genes by the mutant GR. Using myotropic adeno-associated viruses for in vivo overexpression or knockdown in muscle, we found that Foxc1 was required and sufficient for normal expression levels of insulin response pathway genes Insr and Irs1, promoting muscle insulin sensitivity. In parallel, Arid5a was required and sufficient to transcriptionally repress the lipid uptake genes C 36 and Fabp4, reducing muscle triacylglycerol accumulation. Moreover, the Foxc1 and Arid5a programs in muscle were divergently changed by glucocorticoid regimens with opposite metabolic outcomes in muscle. Finally, we found a direct human relevance for our mechanism of SNP action in the UK Biobank and All of Us datasets, where the rs6190 SNP correlated with pro-metabolic changes in BMI, lean mass, strength and glucose control according to zygosity. Collectively, our study leveraged a human nuclear receptor coding variant to unveil novel epigenetic regulators of muscle metabolism. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=150 HEIGHT=200 SRC="FIGDIR/small/586997v2_ufig1.gif" ALT="Figure 1"> View larger version (50K): org.highwire.dtl.DTLVardef@1af900borg.highwire.dtl.DTLVardef@119dcaforg.highwire.dtl.DTLVardef@e78baeorg.highwire.dtl.DTLVardef@1835427_HPS_FORMAT_FIGEXP M_FIG C_FIG

Animal behavior and metabolism are tightly coordinated with sleep-wake cycles governed by the brain in harmony with environmental light:dark cycles. Within the brain, the dorsomedial hypothalamic nucleus (DMH) has been implicated in the integrative control of feeding, energy homeostasis, and circadian rhythms [1], but the underlying cell types are unknown. Here, we identify a role for DMH leptin receptor-expressing neurons (DMHLepR) in these effects. Using a viral approach, we show that silencing DMHLepR neurons in adult mice not only increases body weight and adiposity, but also shifts circadian rhythms in feeding and metabolism into the light-cycle. Moreover, DMHLepR silencing abolishes the normal increase in dark-cycle locomotor activity characteristic of nocturnal rodents. Furthermore, DMHLepR-silenced mice fail to entrain to a restrictive change in food availability. Together, these findings identify DMHLepR neurons as critical determinants of the daily time of feeding and associated metabolic rhythms.

Aims/HypothesisPancreatic delta cells secrete somatostatin (SST), which can inhibit both alpha and beta cells of the pancreatic islet. By controlling insulin and glucagon release, delta cells play an important role in maintaining nutrient homeostasis. However, the mechanism by which a single inhibitory hormone inhibits both alpha and beta cells, which are often considered as functional antagonists in the counterregulatory control of blood glucose, has been a physiological riddle. Here, we solve this riddle through assessment of the contributions of alpha and beta cell specific somatostatin receptors to cell intrinsic behaviors and hormone release. MethodsIslets from mice constitutively expressing fluorescent sensors reporting on cyclic AMP and Ca2+ in both alpha and beta cells were imaged using stimuli to mimic the post-prandial state of a meal consisting of glucose and amino acids. This approach was coupled with cell specific somatostatin receptor antagonists to identify how somatostatin inhibits alpha and beta cell hormone output through modulation of cAMP and Ca2+ secondary messengers and paracrine interactions. ResultsOur results support and extend prior observation that somatostatin receptor 2 (SSTR2) is the only somatostatin receptor expressed by alpha cells, while somatostatin receptor 3 (SSTR3) is the only receptor expressed by mouse beta cells. Interestingly, SSTR2 and SSTR3 regulate downstream cAMP and Ca2+ signaling cascades differently within alpha and beta cells of intact islets. Stimulation of somatostatin receptors robustly inhibits cyclic AMP in alpha or beta cells. In contrast, stimulation of SSTR2 inhibits alpha cell Ca2+ with significantly greater potency compared to inhibition of beta cell Ca2+ via SSTR3. Despite the absence of SSTR2 on beta cells, blocking alpha cell SSTR2 during nutrient stimulation resulted in a significant increase in insulin release downstream of local release of glucagon. Conclusions/InterpretationOur observations address the physiological riddle of the delta cells role during the post-prandial phase where we demonstrate that somatostatin primarily inhibits alpha cell cAMP and Ca2+ via SSTR2, preventing glucagon release. Blocking SSTR2 resulted in an increase in locally released glucagon, which coupled with muted ability for SSTR3 to inhibit beta cell calcium under strong nutrient stimulation, results in potentiation of glucose stimulated insulin secretion from the beta cell. We conclude that the role of delta cells under nutrient stimulation is to modulate the volume of insulin release by tuning the strength of intra-islet paracrine potentiation of insulin secretion by glucagon, mediated via beta cell GLP1R. Research in ContextO_ST_ABSWhat is already known about this subject?C_ST_ABSO_LISomatostatin released by delta cells can attenuate glucagon release from the alpha cells and insulin release from the beta cells of the islet through inhibitory somatostatin receptors. C_LIO_LISomatostatin receptor 2 is a major receptor expressed on the mouse alpha cell surface, while mouse beta cells express SSTR3 on their primary cilia. C_LIO_LIParacrine signaling by alpha cell glucagon potentiates insulin release through the engagement of stimulatory glucagon-like peptide 1 receptors on beta cells. C_LI What are the key questions?O_LIHow do the cell specific somatostatin receptors of the alpha cell (SSTR2) and beta cell (SSTR3) differentially regulate cell-intrinsic Ca2+ and cAMP signaling and the indirect paracrine pathways that shape insulin secretion in the post-prandial state? C_LI What are the new findings?O_LIAlpha cells of the mouse islet express exclusively SSTR2 and not SSTR3. C_LIO_LISomatostatin is a more potent inhibitor of cytosolic calcium activity via alpha cell SSTR2 compared to beta cell SSTR3. C_LIO_LIPreventing somatostatin inhibition of the alpha cell in the presence of elevated glucose and amino acids potentiates insulin release through the engagement of glucagon-like peptide 1 receptors on the beta cell by glucagon. C_LI How might this impact on clinical practice in the foreseeable future?O_LIOur findings indicate that local feedback inhibition of beta cells is accomplished by tuning the strength of the paracrine potentiation of locally released glucagon. This is relevant in the context of developing treatments for hypoglycemia with alpha cell specific SSTR2-specific antagonists that could potentiate insulin release and potentially exacerbate hypoglycemic episodes. C_LI Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=177 SRC="FIGDIR/small/688371v1_ufig1.gif" ALT="Figure 1"> View larger version (71K): org.highwire.dtl.DTLVardef@1138ddaorg.highwire.dtl.DTLVardef@c613f9org.highwire.dtl.DTLVardef@49806eorg.highwire.dtl.DTLVardef@18ece86_HPS_FORMAT_FIGEXP M_FIG C_FIG

Intracellular fatty acids (FAs) activate and fuel non-shivering thermogenesis (NST) via uncoupling protein 1 (UCP1). Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) control FA availability. Since mice lacking ATGL in brown adipose tissue (BAT) exhibit intact recruitable adrenergic thermogenesis, we hypothesized that HSL-mediated FA release is sufficient to activate UCP1-dependent NST. We demonstrate that mice with inducible brown adipocyte-specific loss of ATGL and HSL (iBDKO) exhibit normal recruitable adrenergic thermogenesis upon prolonged cold exposure. Mechanistically, we show that BAT thermogenic capacity is impaired in cold-adapted iBDKO mice due to diminished mitochondrial numbers. Increased browning of white adipose tissue (WAT) in iBDKO mice indicates a shift in thermogenesis from BAT to WAT. Consistently, the loss of ATGL and HSL in BAT and WAT disrupts thermogenesis in both depots, resulting in blunted UCP1-dependent NST. Our study highlights the metabolic adaptability of adipose tissue and the critical role of intracellular lipolysis in regulating thermogenesis.

The R707W mutation in mitofusin 2, encoded by MFN2, causes a form of Multiple Symmetrical Lipomatosis (MFN2-MSL). This resembles sporadic, alcohol-associated MSL, combining loss of lower body adipose tissue with upper body adipose hyperplasia. Morbidity and sometimes mortality arise both from mechanical complications of head and neck adipose overgrowth, and metabolic complications. We reasoned that interventions that either mitigate the underlying cellular pathology, or that exacerbate it to induce selective death of hyperplastic adipose tissue may be beneficial. We thus assessed the effect of a metabolic or pharmacologic stressors or rapamycin in Mfn2R707W/R707W mice and or derived preadipocytes. 50mmol ethanol had little effect on WT or Mfn2R707W/R707W white preadipocytes, but increased mitochondrial content and blunted mitolysosome formation in Mfn2R707W/R707Wbrown preadipocytes. Daily consumption of 20% EtOH increased brown adipose tissue mass in female Mfn2R707W/R707W mice, and serum lactate in males. 200nM rapamycin - a candidate treatment - increased size and mitolysosome content of WT and Mfn2R707W/R707W white and brown preadipocytes, but these effects were blunted in Mfn2R707W/R707W cells. In male but not female Mfn2R707W/R707W mice, rapamycin reduced or reversed weight gain, reduced brown adipose mass, and increased serum Fgf21. Finally, a panel of other metabolic and pharmacological mitochondrial stressors solicited no selective death or ISR in Mfn2R707W/R707W preadipocytes. We conclude that ethanol mildly exacerbates MFN-MSL in mice, while rapamycin is tolerated. Lack of sensitisation to mitochondrial stressors implies that the MSL-inducing effect of MFN2 R707W may not be exerted through compromised oxidative phosphorylation.

Nuclei within the limbic system like the central amygdala (CeA) play a critical role in mediating fear, motivation, reward, and appetitive behavior. Although previous reports demonstrate the presence of the glucagon-like peptide-1 receptor (GLP-1R) in limbic nuclei, how limbic neurons mediate the actions of systemically administrated GLP-1R agonists is unclear. In this study, we investigated the CeAs response to peripherally administered GLP-1R agonist Exendin-4 (Ex-4) in vivo, and determined the functional requirement of select CeA neuron populations in acute Ex-4 induced hypophagia. Using fiber photometry, we observed that Ex-4 promoted a rapid and lasting activation of CeA neurons that was blocked by pretreatment with the GLP-1R antagonist Exendin-9. We then tested the functional requirement of CeA neuron activation in mediating Ex-4 induced hypophagia of standard grain chow using inhibitory chemogenetics. Chemogenetic inhibition of all CeA neurons significantly suppressed the hypophagic actions of Ex-4. Then using selective mouse Cre-drivers, we found that chemogenetic inhibition of protein kinase c delta (Prk-cd CeA) and GLP-1R (Glp1r CeA), but not somatostatin (SstCeA), neurons also attenuates the full hypophagic effect of Ex-4. Having observed that inhibition of Glp1rCeA modestly attenuated Ex-4 induced hypophagia of standard chow, we then tested whether these neurons might mediate Ex-4 suppression of energy-dense, palatable diet. We used intermittent high-fat diet (HFD) access and found that inhibition of Glp1rCeA neurons significantly rescued the reduction of HFD consumption by Ex-4. Collectively, these data demonstrate that the CeA responds to peripherally administered GLP-1R agonists and that multiple CeA neuron mediate GLP-1R agonist-mediated hypophagia.

Acyl-CoA binding protein (ACBP) plays a vital role in lipid metabolism by mediating the intracellular flux and utilization of long-chain acyl-CoAs. In this study we generated brown- and white adipose tissue specific knockout mice (Adipoq-Acbp-/-) and brown adipose tissue specific knockout mice (Ucp1-Acbp-/-) to investigate the role of ACBP in adipose tissue function. Here we demonstrate that loss of ACBP does not affect body weight, fat and lean mass, food intake and systemic energy expenditure, even under cold stress. Transcriptomic data show only minor changes in gene expression, whereas lipidomic profiling reveals a subtle increase in acyl-carnitines levels in brown adipose tissue. However, lipolytic activity in white adipose tissue as well as plasma glycerol, non-esterified fatty acid and triacylglycerol levels remained unaffected. In addition, no changes in mitochondrial respiration in BAT were observed. Taken together, our findings suggest that ACBP is dispensable for adipose tissue function and systemic energy metabolism, including thermoregulation.

ObjectiveBrown adipose tissue (BAT) dissipates energy via non-shivering thermogenesis, and it is a promising therapeutic target for metabolic disease. While most research focuses on thermogenic adipocytes, emerging data point to critical contributions from the surrounding stromal niche. Here, we investigated the role of adhesion G protein-coupled receptors (aGPCRs) in BAT function, focusing on Adgrf5 (Gpr116), a receptor enriched in endothelial cells. MethodsWe used single-nuclei RNA sequencing to map aGPCRs expression across mouse and human BAT. We then examined the consequences of Adgrf5(Gpr116) loss using global, brown adipocyte, and endothelial-specific knockout mouse models under acute and prolonged cold exposure. ResultsInducible endothelial deletion of Adgrf5(Gpr116) impaired the maintenance of thermogenic capacity during prolonged--but not acute--cold exposure. This was not associated with defective angiogenesis, but rather with endothelial fibro-inflammatory reprogramming. Single-nuclei RNA sequencing analysis revealed endothelial-to- mesenchymal transition (EndMT) features, including induction of mesenchymal markers, collagens, and metalloproteinases, and loss of barrier genes. Adgrf5(Gpr116)-deficient endothelial cells also exhibited cytoskeletal remodeling and activation of stress fiber pathways, implicating Adgrf5(Gpr116) as a mechanosensory safeguard of endothelial identity. ConclusionEndothelial Adgrf5(Gpr116) preserves thermogenic competence in BAT by suppressing EndMT and maladaptive matrix remodeling. Our findings establish vascular mechanosensing as a critical determinant of thermogenic tissue homeostasis. HighlightsO_LIAdhesion GPCRs are the second most abundant GPCR family in mouse and human brown fat C_LIO_LIAdhesion GPCRs are enriched in non-adipocyte cell types in brown fat and participate in cell-cell contact signaling C_LIO_LIEndothelial Adgrf5(Gpr116) is required for thermogenic adaptation during prolonged cold exposure C_LIO_LILoss of Adgrf5(Gpr116) induces fibro-inflammatory reprogramming and endothelial-to-mesenchymal transition (EndMT) C_LI

Adenosine is a widely distributed signaling molecule whose levels rise during conditions of metabolic stress, hypoxia, or inflammation. Adenosine is a homeostatic regulator of neuronal, cardiovascular, immune, and metabolic functions through activation of adenosine receptors. Here, we show that administration of adenosine rapidly elicits an immediate and pronounced excursion of glucose and non-esterified fatty acids (NEFA) in mice refed for four hours but is greatly attenuated in fasted mice. This adenosine-mediated postprandial response suggests that adenosine is a potent regulator of postprandial nutrient handling. Selective agonists and antagonists of A2A and A2B adenosine receptors demonstrate that activation of either is sufficient to evoke adenosines metabolic response, but both receptors must be inhibited simultaneously to abolish it. Adenosine strongly stimulates hepatic glucose production and adipose lipolysis, and this catabolic activity depends on sympathetic nerve activity as it requires autonomic signal transmission and is inhibited by blocking adrenergic receptors. Genetic ablation studies identify A2A and A2B receptors expressed on neurons, likely central neurons, as the primary site of action for mediating adenosines effects on whole-body metabolism. Collectively, these data demonstrate that acute adenosine administration promotes centrally mediated metabolic effects, particularly during the postprandial period. Rigorous dissection of signaling pathways shows that A2A and A2B receptors are individually sufficient and collectively necessary for adenosine-mediated glucose and fatty acid excursion.