Featured ArticlesVolume 21 | March 2019
|GDF15 acts synergistically with liraglutide but is not necessary for bariatric surgery weight loss Growth Differentiation Factor 15 (GDF15) analogues hold great promise as pharmacological treatments for obesity as GDF15 decreases food intake. However, little is known about the biology of this system and only recently was the receptor, GDNF Family Receptor Alpha Like (GFRAL), identified. In the present study, Frikke-Schmidt et al. wanted to determine whether established anorexic treatments, specifically bariatric surgery and glucagon-like peptide 1 agonism via liraglutide, might act via the GDF15/GFRAL system. The authors found that the GDF15/GFRAL system is not critical to the effects of vertical sleeve gastrectomy or liraglutide. This indicates that future therapies targeting the GDF15/GFRAL system might complement current therapeutic options.|
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Objective: Analogues of GDF15 (Growth Differentiation Factor 15) are promising new anti-obesity therapies as pharmacological treatment with GDF15 results in dramatic reductions of food intake and body weight. GDF15 exerts its central anorexic effects by binding to the GFRAL receptor exclusively expressed in the Area Postrema (AP) and the Nucleus of the Solitary Tract (NTS) of the hindbrain. We sought to determine if GDF15 is an indispensable factor for other interventions that cause weight loss and which are also known to act via these hindbrain regions.
Methods: To explore the role of GDF15 on food choice we performed macronutrient intake studies in mice treated pharmacologically with GDF15 and in mice having either GDF15 or GFRAL deleted. Next we performed vertical sleeve gastrectomy (VSG) surgeries in a cohort of diet-induced obese Gdf15-null and control mice. To explore the anatomical co-localization of neurons in the hindbrain responding to GLP-1 and/or GDF15 we used GLP-1R reporter mice treated with GDF15, as well as naïve mouse brain and human brain stained by ISH and IHC, respectively, for GLP-1R and GFRAL. Lastly we performed a series of food intake experiments where we treated mice with targeted genetic disruption of either Gdf15 or Gfral with liraglutide; Glp1r-null mice with GDF15; or combined liraglutide and GDF15 treatment in wild-type mice.
Results: We found that GDF15 treatment significantly lowered the preference for fat intake in mice, whereas no changes in fat intake were observed after genetic deletion of Gdf15 or Gfral. In addition, deletion of Gdf15 did not alter the food intake or bodyweight after sleeve gastrectomy. Lack of GDF15 or GFRAL signaling did not alter the ability of the GLP-1R agonist liraglutide to reduce food intake. Similarly lack of GLP-1R signaling did not reduce GDF15's anorexic effect. Interestingly, there was a significant synergistic effect on weight loss when treating wild-type mice with both GDF15 and liraglutide.
Conclusion: These data suggest that while GDF15 does not play a role in the potent effects of VSG in mice there seems to be a potential therapeutic benefit of activating GFRAL and GLP-1R systems simultaneously.[Hide abstract]
|FMRP controls lipid and glucose metabolism The Fragile X Mental Retardation Protein (FMRP) is an RNA-binding protein, which associates with polyribosomes to regulate mRNA translation. So far, FMRP functions and the mRNAs it targets have been explored mostly in the context of the central nervous system (CNS). Despite the wide expression of FMRP in peripheral tissues, the consequences of its absence outside the CNS are mostly unknown. Leboucher et al. demonstrate that loss of FMRP in mice markedly impacts glucose and lipid metabolism. They further show that loss of FMRP elevates hepatic protein synthesis and that FMRP likely controls the translation of key hepatic proteins involved in lipid metabolism. Finally, they provide clinical evidence that circulating metabolic markers are altered in Fragile X Syndrome patients.|
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Objectives: The Fragile X Mental Retardation Protein (FMRP) is a widely expressed RNA-binding protein involved in translation regulation. Since the absence of FMRP leads to Fragile X Syndrome (FXS) and autism, FMRP has been extensively studied in brain. The functions of FMRP in peripheral organs and on metabolic homeostasis remain elusive; therefore, we sought to investigate the systemic consequences of its absence.
Methods: Using metabolomics, in vivo metabolic phenotyping of the Fmr1-KO FXS mouse model and in vitro approaches, we show that the absence of FMRP induced a metabolic shift towards enhanced glucose tolerance and insulin sensitivity, reduced adiposity, and increased β-adrenergic-driven lipolysis and lipid utilization.
Results: Combining proteomics and cellular assays, we highlight that FMRP loss increased hepatic protein synthesis and impacted pathways notably linked to lipid metabolism. Mapping metabolomic and proteomic phenotypes onto a signaling and metabolic network, we predicted that the coordinated metabolic response to FMRP loss was mediated by dysregulation in the abundances of specific hepatic proteins. We experimentally validated these predictions, demonstrating that the translational regulator FMRP associates with a subset of mRNAs involved in lipid metabolism. Finally, we highlight that FXS patients mirror metabolic variations observed in Fmr1-KO mice with reduced circulating glucose and insulin and increased free fatty acids.
Conclusions: Loss of FMRP results in a widespread coordinated systemic response that notably involves upregulation of protein translation in the liver, increased utilization of lipids, and significant changes in metabolic homeostasis. Our study unravels metabolic phenotypes in FXS and further supports the importance of translational regulation in the homeostatic control of systemic metabolism.[Hide abstract]
|The role of C16:0 ceramide in the development of obesity and T2D: CerS6 inhibition as a novel therapeutic approachOversupply of saturated fats induces excess ceramide accumulation that leads to impaired insulin signaling and energy homeostasis, and eventually to insulin resistance and type 2 diabetes. The ceramide species C16:0 ceramide plays a key role in the development of insulin resistance. Raichur, Brunner, and colleagues investigated if the specific inhibition of ceramide synthase 6 (CerS6) might be suitable for a drug intervention approach using CerS6 antisense oligonucleotides to knock down CerS6. They report that CerS6 mediated C16:0 ceramide reduction could be an attractive target for the treatment of insulin resistance, obesity, and type 2 diabetes.|
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Objective: Ectopic fat deposition is associated with increased tissue production of ceramides. Recent genetic mouse studies suggest that specific sphingolipid C16:0 ceramide produced by ceramide synthase 6 (CerS6) plays an important role in the development of insulin resistance. However, the therapeutic potential of CerS6 inhibition not been demonstrated. Therefore, we pharmacologically investigated the selective ablation of CerS6 using antisense oligonucleotides (ASO) in obese insulin resistance animal models.
Methods: We utilized ASO as therapeutic modality, CerS6 ASO molecules designed and synthesized were initially screened for in-vitro knock-down (KD) potency and cytotoxicity. ASOs with >85% inhibition of CerS6 mRNA were selected for further investigations. Most promising ASOs verified for in-vivo KD efficacy in healthy mice. CerS6 ASO (AAGATGAGCCGCACC) was found most active with hepatic reduction of CerS6 mRNA expression. Prior to longitudinal metabolic studies, we performed a dose titration target engagement analysis with CerS6 ASO in healthy mice to select the optimal dose. Next, we utilized leptin deficiency ob/ob and high fat diet (HFD) induced obese mouse models for pharmacological efficacy study.
Results: CerS6 expression were significantly elevated in the liver and brown adipose, this was correlated with significantly elevated C16:0 ceramide concentrations in plasma and liver. Treatment with CerS6 ASO selectively reduced CerS6 expression by ∼90% predominantly in the liver and this CerS6 KD resulted in a significant reduction of C16:0 ceramide by about 50% in both liver and plasma. CerS6 KD resulted in lower body weight gain and accompanied by a significant reduction in whole body fat and fed/fasted blood glucose levels (1% reduction in HbA1c). Moreover, ASO-mediated CerS6 KD significantly improved oral glucose tolerance (during oGTT) and mice displayed improved insulin sensitivity. Thus, CerS6 appear to play an important role in the development of obesity and insulin resistance.
Conclusions: Our investigations identified specific and selective therapeutic valid ASO for CerS6 ablation in in-vivo. CerS6 should specifically be targeted for the reduction of C16:0 ceramides, that results in amelioration of insulin resistance, hyperglycemia and obesity. CerS6 mediated C16:0 ceramide reduction could be a potentially attractive target for the treatment of insulin resistance, obesity and type 2 diabetes.[Hide abstract]
|The impact of exercise on mitochondrial dynamics and the role of Drp1 in skeletal muscleMitochondrial networks exhibit a life cycle including biogenesis, rearrangement of the network via fission-fusion, and removal of damaged or unneeded mitochondria by autophagic turnover, or mitophagy. No one study has systematically interrogated the impact of acute exercise and long-term training on all three phases of the mitochondrial life cycle. Moore et al. examined how the mitochondrial life cycle responds to three different endurance exercise interventions: acute exercise, chronic exercise training, and acute exercise after chronic exercise training. This research led to the identification of a novel role for the mitochondrial fission regulator Dynamin related protein 1 (Drp1) during acute exercise.|
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Objective: Mitochondria are organelles primarily responsible for energy production, and recent evidence indicates that alterations in size, shape, location, and quantity occur in response to fluctuations in energy supply and demand. We tested the impact of acute and chronic exercise on mitochondrial dynamics signaling and determined the impact of the mitochondrial fission regulator Dynamin related protein (Drp)1 on exercise performance and muscle adaptations to training.
Methods: Wildtype and muscle-specific Drp1 heterozygote (mDrp1+/−) mice, as well as dysglycemic (DG) and healthy normoglycemic men (control) performed acute and chronic exercise. The Hybrid Mouse Diversity Panel, including 100 murine strains of recombinant inbred mice, was used to identify muscle Dnm1L (encodes Drp1)-gene relationships.
Results: Endurance exercise impacted all aspects of the mitochondrial life cycle, i.e. fission-fusion, biogenesis, and mitophagy. Dnm1L gene expression and Drp1Ser616 phosphorylation were markedly increased by acute exercise and declined to baseline during post-exercise recovery. Dnm1L expression was strongly associated with transcripts known to regulate mitochondrial metabolism and adaptations to exercise. Exercise increased the expression of DNM1L in skeletal muscle of healthy control and DG subjects, despite a 15% ↓(P = 0.01) in muscle DNM1L expression in DG at baseline. To interrogate the role of Dnm1L further, we exercise trained male mDrp1+/− mice and found that Drp1 deficiency reduced muscle endurance and running performance, and altered muscle adaptations in response to exercise training.
Conclusion: Our findings highlight the importance of mitochondrial dynamics, specifically Drp1 signaling, in the regulation of exercise performance and adaptations to endurance exercise training.[Hide abstract]
|Insulin action in the brain regulates mitochondrial stress responses and reduces weight gainProper control of mitochondrial function in the brain is of utter importance for normal brain function and physiology. Mitochondrial dysfunction has been observed in neurodegenerative and metabolic disorders, such as type 2 diabetes. Normally, mitochondria adapt to altered nutrient supply via the mitochondrial stress response. Wardelmann, Blümel, Rath, and colleagues found that brain insulin signaling impacts mitochondrial stress responsiveness in the hypothalamus and thus affects mitochondrial function and metabolism. Their data offer new insight into how brain insulin regulates neuronal health, acts as a neuroprotective hormone, and regulates metabolism.|
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Objective: Insulin action in the brain controls metabolism and brain function, which is linked to proper mitochondrial function. Conversely, brain insulin resistance associates with mitochondrial stress and metabolic and neurodegenerative diseases. In the present study, we aimed to decipher the impact of hypothalamic insulin action on mitochondrial stress responses, function and metabolism.
Methods: To investigate the crosstalk of insulin action and mitochondrial stress responses (MSR), namely the mitochondrial unfolded protein response (UPRmt) and integrated stress response (ISR), qPCR, western blotting, and mitochondrial activity assays were performed. These methods were used to analyze the hypothalamic cell line CLU183 treated with insulin in the presence or absence of the insulin receptor as well as in mice fed a high fat diet (HFD) for three days and STZ-treated mice without or with insulin therapy. Intranasal insulin treatment was used to investigate the effect of acute brain insulin action on metabolism and mitochondrial stress responses.
Results: Acute HFD feeding reduces hypothalamic mitochondrial stress responsive gene expression of Atf4, Chop, Hsp60, Hsp10, ClpP, and Lonp1 in C57BL/6N mice. We show that insulin via ERK activation increases the expression of MSR genes in vitro as well as in the hypothalamus of streptozotocin-treated mice. This regulation propagates mitochondrial function by controlling mitochondrial proteostasis and prevents excessive autophagy under serum deprivation. Finally, short-term intranasal insulin treatment activates MSR gene expression in the hypothalamus of HFD-fed C57BL/6N mice and reduces food intake and body weight development.
Conclusions: We define hypothalamic insulin action as a novel master regulator of MSR, ensuring proper mitochondrial function by controlling mitochondrial proteostasis and regulating metabolism.[Hide abstract]
|An extra copy of Down syndrome critical region 1-4 results in impaired glucose homeostasisThe prevalence of diabetes in children with Down syndrome is threefold higher than in unaffected children. Additionally, metabolic syndrome and type 2 diabetes occur at relatively early ages in those with Down syndrome. However, the molecular basis of dysregulated glucose homeostasis in patients with Down syndrome is not well understood. Seo et al. investigated the role of Down syndrome critical region 1-4 (DSCR1-4) in the liver by introducing a single extra copy of DSCR1-4 into mice. Their analysis reveals that a single extra copy of DSCR1-4 increases hepatic glucose production and expression of gluconeogenic genes, resulting in pathological states such as insulin resistance and pyruvate intolerance.|
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Objectives: During fasting, hepatic gluconeogenesis is induced to maintain energy homeostasis. Moreover, abnormal dysregulation of hepatic glucose production is commonly observed in type 2 diabetes. However, the signaling components controlling hepatic glucose production to maintain normal glucose levels are not fully understood. Here, we examined the physiological role of Down syndrome critical region 1–4 (DSCR1-4), an endogenous calcineurin signaling inhibitor in the liver that mediates metabolic adaptation to fasting.
Methods: We assessed the effect of cyclosporine A, an inhibitor of calcineurin signaling on gluconeogenic gene expression in primary hepatocytes. DSCR1-4 expression was examined in diet- and genetically-induced mouse models of obesity. We also investigated the metabolic phenotype of a single extra copy of DSCR1-4 in transgenic mice and how DSCR1-4 regulates glucose homeostasis in the liver.
Results: Treatment with cyclosporin A increased hepatic glucose production and gluconeogenic gene expression. The expression of DSCR1-4 was induced by refeeding and overexpressed in obese mouse livers. Moreover, transgenic mice with a single extra copy of DSCR1-4 exhibited pyruvate intolerance and impaired glucose homeostasis. Mechanistically, DSCR1-4 overexpression increased phosphorylation of the cAMP response element-binding protein, which led to elevated expression levels of gluconeogenic genes and, thus, enhanced hepatic glucose production during fasting.
Conclusions: A single extra copy of DSCR1-4 results in dysregulated hepatic glucose homeostasis and pyruvate intolerance. Our findings suggest that nutrient-sensitive DSCR1-4 is a novel target for controlling hepatic gluconeogenesis in diabetes.[Hide abstract]