Featured ArticlesVolume 28 | October 2019
|Skeletal muscle Akt in the regulation of muscle mass and glucose homeostasisThe serine/threonine kinase Akt (protein kinase B) has been implicated as a central regulator of insulin action in skeletal muscle. A prevailing hypothesis is that a reduction in insulin-stimulated Akt activation decreases glucose uptake leading to hyperglycemia and systemic insulin resistance. To directly assess the role of skeletal muscle Akt on muscle growth and glucose homeostasis, Jaiswal et al. deleted Akt2 or both Akt1 and Akt2 specifically in the skeletal muscles of mice. While mice bearing an Akt2 deletion alone showed no effect on muscle growth and glucose metabolism, congenital deletion of both the Akt isoforms resulted in a drastic reduction in muscle mass and a mild defect in glucose tolerance.|
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Objective: Skeletal muscle insulin signaling is a major determinant of muscle growth and glucose homeostasis. Protein kinase B/Akt plays a prominent role in mediating many of the metabolic effects of insulin. Mice and humans harboring systemic loss-of-function mutations in Akt2, the most abundant Akt isoform in metabolic tissues, are glucose intolerant and insulin resistant. Since the skeletal muscle accounts for a significant amount of postprandial glucose disposal, a popular hypothesis in the diabetes field suggests that a reduction in Akt, specifically in skeletal muscle, leads to systemic glucose intolerance and insulin resistance. Despite this common belief, the specific role of skeletal muscle Akt in muscle growth and insulin sensitivity remains undefined.
Methods: We generated multiple mouse models of skeletal muscle Akt deficiency to evaluate the role of muscle Akt signaling in vivo. The effects of these genetic perturbations on muscle mass, glucose homeostasis and insulin sensitivity were assessed using both in vivo and ex vivo assays.
Results: Surprisingly, mice lacking Akt2 alone in skeletal muscle displayed normal skeletal muscle insulin signaling, glucose tolerance, and insulin sensitivity despite a dramatic reduction in phosphorylated Akt. In contrast, deletion of both Akt isoforms (M-AktDKO) prevented downstream signaling and resulted in muscle atrophy. Despite the absence of Akt signaling, in vivo and ex vivo insulin-stimulated glucose uptake were normal in M-AktDKO mice. Similar effects on insulin sensitivity were observed in mice with prolonged deletion (4 weeks) of both skeletal muscle Akt isoforms selectively in adulthood. Conversely, short term deletion (2 weeks) of skeletal muscle specific Akt in adult muscles impaired insulin tolerance paralleling the effect observed by acute pharmacological inhibition of Akt in vitro. Mechanistically, chronic ablation of Akt induced mitochondrial dysfunction and activation of AMPK, which was required for insulin-stimulated glucose uptake in the absence of Akt.
Conclusions: Together, these data indicate that chronic reduction in Akt activity alone in skeletal muscle is not sufficient to induce insulin resistance or prevent glucose uptake in all conditions. Therefore, since insulin-stimulated glucose disposal in skeletal muscle is markedly impaired in insulin-resistant states, we hypothesize that alterations in signaling molecules in addition to skeletal muscle Akt are necessary to perturb glucose tolerance and insulin sensitivity in vivo.[Hide abstract]
|The GIP receptor in brown adipose tissueInsulinotropic polypeptide (GIP) is a gut-derived hormone that potentiates glucose-dependent insulin secretion via cognate receptors on islet β-cells. It also exerts a wide range of extra-pancreatic actions. Beaudry et al. studied the physiological roles of GIP in brown adipose tissue (BAT) using tissue-specific deletions of the GIP receptor. They found the BAT GIP receptor associated with changes in gene expression, IL-6 expression and secretion, oxygen consumption ex vivo, lipid utilization, the defence of body temperature, and metabolism.|
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Objective: Glucose-dependent insulinotropic polypeptide (GIP) is secreted from the gut in response to nutrient ingestion and promotes meal-dependent insulin secretion and lipid metabolism. Loss or attenuation of GIP receptor (GIPR) action leads to resistance to diet-induced obesity through incompletely understood mechanisms. The GIPR is expressed in white adipose tissue; however, its putative role in brown adipose tissue (BAT) has not been explored.
Methods: We investigated the role of the GIPR in BAT cells in vitro and in BAT-specific (GiprBAT−/−) knockout mice with selective elimination of the Gipr within the Myf5+ expression domain. We analyzed body weight, adiposity, glucose homeostasis, insulin and lipid tolerance, energy expenditure, food intake, body temperature, and iBAT oxygen consumption ex vivo. High-fat diet (HFD)-fed GiprBAT−/− mice were studied at room temperature (21 °C), 4 °C, and 30 °C ambient temperatures.
Results: The mouse Gipr gene is expressed in BAT, and GIP directly increased Il6 mRNA and IL-6 secretion in BAT cells. Additionally, levels of thermogenic, lipid and inflammation mRNA transcripts were altered in BAT cells transfected with Gipr siRNA. Body weight gain, energy expenditure, and glucose and insulin tolerance were normal in HFD-fed GiprBAT−/− mice housed at room temperature. However, GiprBAT−/− mice exhibited higher body temperatures during an acute cold challenge and a lower respiratory exchange ratio and impaired lipid tolerance at 21 °C. In contrast, body weight was lower and iBAT oxygen consumption was higher in HFD-fed mice housed at 4 °C but not at 30 °C.
Conclusions: The BAT GIPR is linked to the control of metabolic gene expression, fuel utilization, and oxygen consumption. However, the selective loss of the GIPR within BAT is insufficient to recapitulate the findings of decreased weight gain and resistance to obesity arising in experimental models with systemic disruption of GIP action.[Hide abstract]
|Dysregulation of PP2A-Akt interaction contributes to SNRK deficiency induced insulin resistance Sucrose non-fermenting related kinase (SNRK) is expressed abundantly in adipose tissues. It is a regulator of adipose inflammation and energy homeostasis in both mice and humans. Li and colleagues hypothesized that SNRK play important roles in adipose glucose metabolism in mice. They show that SNRK ablation is sufficient to inhibit insulin-regulated AKT phosphorylation and glucose uptake in adipose tissues and identify a novel SNRK pathway regulating insulin signaling in adipose tissue.|
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Objective: We previously identified Sucrose non-fermenting related kinase (SNRK) as a regulator of adipose inflammation and energy homeostasis. In this study, we aimed to investigate the role of SNRK in insulin signaling in white (WAT) and brown adipose tissue (BAT).
Methods: Adipose tissue specific (SNRK deficiency in both WAT and BAT) and BAT specific knockout mouse models were employed. Phosphoproteomic studies were conducted to identify the novel SNRK pathway regulating insulin signaling in adipose tissue.
Results: SNRK ablation is sufficient to inhibit insulin-stimulated AKT phosphorylation and glucose uptake in both WAT and BAT. Phosphoproteomic study using SNRK deficient versus wild type BAT samples revealed 99% reduction of phosphorylation on Serine 80 of PPP2R5D, the regulatory subunit of Protein phosphatase 2A (PP2A). Drastic (142.5-fold) induction of phosphorylation on Serine 80 of PPP2R5D was observed in SNRK-deficient primary brown adipocytes overexpressing SNRK compared to control protein. In vitro phosphorylation reaction followed by targeted phosphoproteomic detection further confirms that human recombinant SNRK is able to phosphorylate human recombinant PPP2R5D. Dephosphorylated PPP2R5D promotes constitutive assembly of PP2A-AKT complex, therefore inhibits insulin-induced AKT phosphorylation and subsequent glucose uptake in both BAT and WAT. Knockdown of PPP2R5D in adipocytes can improve insulin sensitivity in adipocytes without SNRK expression.
Conclusions: Our findings demonstrate that SNRK regulates insulin signaling through controlling PPP2R5D phosphorylation, which subsequently impacts PP2A activity and then AKT phosphorylation in both WAT and BAT. SNRK may represent a promising potential target for treating insulin resistance-related metabolic disorders.[Hide abstract]
|cAMP-miR-203-IFN-γ network regulates fat browning and glucose toleranceSeveral miRNAs have recently emerged as important regulators of brown and beige adipocyte differentiation and function. The microRNA miR-203 is enriched in brown fat. Guo et al. studied the role of miR-203 in vivo and found that upregulation of miR-203 could be used for the suppression of the interferon-γ (IFN-γ) signaling pathway. They also provide mechanistic insights that the cAMP-miR-203-IFN-γ network regulates inflammation response and white adipose tissue browning.|
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Objective: Brown and beige adipocytes in humans and rodents are specialized to burn lipids for heat generation as a natural defense against cold and obesity, which is advantageous to metabolic homeostasis. MicroRNAs as another regulatory layer to regulate metabolic homeostasis attracted a lot of attentions. Our previous work revealed microRNA (miR)-203 as a brown adipocyte-enriched microRNA involved in brown adipocytes development. However, the potential role of miR-203 in adipose tissue metabolic homeostasis has not been determined in vivo. In this study, we investigate the potential role of miR-203 in subcutaneous white adipose tissue (sub-WAT) browning and metabolic homeostasis.
Methods: We investigated the relationship between miR-203 and energy homeostasis in adipose tissue from cold exposed, high fat diet (HFD) fed, ob/ob and db/db mice. The functions of miR-203 on sub-WAT browning were validated through miR-203 knockdown or overexpression. The miR-203 targeted signal pathway was screened by RNAseq analysis. Luciferase report assay, western blot, and qPCR were performed to establish the miR-203 related upstream and downstream signal pathway in vivo and in vitro. The functions of miR-203 on obesity and metabolic homeostasis were validated through GTT/ITT and western blot on high fat diet-induced obesity in C57 mice. ELISA was used to determine the concentration of IFN-γ. Flow cytometry analysis was performed to determine the infiltration of macrophages in adipose tissue.
Results: MiR-203 expression positively correlates with energy expenditure, and overexpression of miR-203 could enhance sub-WAT browning in normal diet (ND) condition. Mechanistically, the expression of miR-203 is activated by cAMP-dependent C/EBPβ up-regulation. Subsequently, miR-203 inhibits IFN-γ signal pathway activation by directly targeting Lyn, which is an activator of Jak1-Stat1. Moreover, the forced expression of miR-203 could improve insulin sensitivity and resist high fat diet-induced obesity by inhibiting IFN-γ.
Conclusions: MicroRNA-203 (miR-203) promotes white adipose tissue browning in cold exposed mice and improves glucose tolerance in HFD fed mice by repressing IFN-γ. Since miR-203 is activated by cAMP-dependent C/EBPβ up-regulation and directly represses IFN-γ signal pathway, we declare that miR-203 acts as a messenger between cAMP signal pathway and IFN-γ signal pathway.[Hide abstract]
|Tbx15 is required for adipocyte browning induced by adrenergic signalingTBX15 (T-box 15) is a transcription factor that plays an important role in the adipogenesis of primary brown and beige adipocyte precursors. To further elucidate its role in adipose tissues, Sun, Zhao, and colleagues generated adipose-specific Tbx15 knockout mice. They demonstrate that Tbx15 is important for adipocyte thermogenesis and browning in adipose tissue in vivo. Interventions that increase TBX15 expression may provide new therapeutic strategies to promote thermogenesis and combat obesity and related metabolic disorders.|
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Objective: The T-box gene Tbx15 is abundantly expressed in adipose tissues, especially subcutaneous and brown fat. Although its expression is correlated with obesity, its precise biological role in adipose tissue is poorly understood in vivo. Here we investigated the function of Tbx15 in brown adipose thermogenesis and white adipose browning in vivo.
Methods: In the present study, we generated adipose-specific Tbx15 knockout (AKO) mice by crossing Tbx15 floxed mice with adiponectin-Cre mice to delineate Tbx15 function in adipose tissues. We systematically investigated the influence of Tbx15 on brown adipose thermogenesis and white adipose browning in mice, as well as the possible underlying molecular mechanism.
Results: Upon cold exposure, adipocyte browning in inguinal adipose tissue was significantly impaired in Tbx15 AKO mice. Furthermore, ablation of Tbx15 blocked adipocyte browning induced by β3 adrenergic agonist CL 316243, which did not appear to alter the expression of Tbx15. Analysis of DNA binding sites using chromatin-immunoprecipitation (ChIP) revealed that TBX15 bound directly to a key region in the Prdm16 promoter, indicating it regulates transcription of Prdm16, the master gene for adipocyte thermogenesis and browning. Compared to control mice, Tbx15 AKO mice displayed increased body weight gain and decreased whole body energy expenditure in response to high fat diets.
Conclusions: Taken together, these findings suggest that Tbx15 regulates adipocyte browning and might be a potential target for the treatment of obesity.[Hide abstract]
|Double bond configuration of palmitoleate is critical for atheroprotectionA significant reduction in cardiovascular disease risk can be achieved by replacing saturated fatty acids with mono- or poly-unsaturated fatty acids. Palmitoleate is a mono-unsaturated fatty acid that has beneficial effects on metabolism and atherosclerosis. However, it is unknown whether the cis and trans isoforms of palmitoleate have the same effect. Cimen and colleagues directly compared the effects of cis- and trans-palmitoleate on inflammation and atherosclerosis in a hypercholesterolemic mouse model and found that positive effects were restricted to the cis form only.|
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Objective: Saturated and trans fat consumption is associated with increased cardiovascular disease (CVD) risk. Current dietary guidelines recommend low fat and significantly reduced trans fat intake. Full fat dairy can worsen dyslipidemia, but recent epidemiological studies show full-fat dairy consumption may reduce diabetes and CVD risk. This dairy paradox prompted a reassessment of the dietary guidelines. The beneficial metabolic effects in dairy have been claimed for a ruminant-derived, trans fatty acid, trans-C16:1n-7 or trans-palmitoleate (trans-PAO). A close relative, cis-PAO, is produced by de novo lipogenesis and mediates inter-organ crosstalk, improving insulin-sensitivity and alleviating atherosclerosis in mice. These findings suggest trans-PAO may be a useful substitute for full fat dairy, but a metabolic function for trans-PAO has not been shown to date.
Methods: Using lipidomics, we directly investigated trans-PAO's impact on plasma and tissue lipid profiles in a hypercholesterolemic atherosclerosis mouse model. Furthermore, we investigated trans-PAO's impact on hyperlipidemia-induced inflammation and atherosclerosis progression in these mice.
Results: Oral trans-PAO supplementation led to significant incorporation of trans-PAO into major lipid species in plasma and tissues. Unlike cis-PAO, however, trans-PAO did not prevent organelle stress and inflammation in macrophages or atherosclerosis progression in mice.
Conclusions: A significant, inverse correlation between circulating trans-PAO levels and diabetes incidence and cardiovascular mortality has been reported. Our findings show that trans-PAO can incorporate efficiently into the same pools that its cis counterpart is known to incorporate into. However, we found trans-PAO's anti-inflammatory and anti-atherosclerotic effects are muted due to its different structure from cis-PAO.[Hide abstract]
|Short-term cold exposure supports human Treg inductionRegulatory T cells (Tregs) are key in controlling tissue homeostasis and function including local fat depots, where they can suppress inflammatory processes. It has been previously shown that short-term cold exposure can induce Tregs in mice. Becker et al. studied the effect of short-term cold exposure on Tregs in humans and show that cold stimulation supports Treg induction also in humans.|
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Objective: Obesity and type-2 diabetes (T2D) are metabolic diseases that represent a critical health problem worldwide. Metabolic disease is differentially associated with fat distribution, while visceral white adipose tissue (VAT) is particularly prone to obesity-associated inflammation. Next to their canonical function of immune suppression, regulatory T cells (Tregs) are key in controlling adipose tissue homeostasis. Towards understanding the molecular underpinnings of metabolic disease, we focus on how environmental-metabolic stimuli impinge on the functional interplay between Tregs and adipose tissue. Here, cold exposure or beta3-adrenergic signaling are a promising tool to increase energy expenditure by activating brown adipose tissue, as well as by reducing local inflammation within fat depots by supporting immunosuppressive Tregs. However, in humans, the underlying mechanisms that enable the environmental-immune crosstalk in the periphery and in the respective tissue remain currently unknown.
Methods: We used combinatorial approaches of next generation humanized mouse models and in vitro and in vivo experiments together with beta3-adrenergic stimulation to dissect the underlying mechanisms of human Treg induction exposed to environmental stimuli such as cold. To test the translational relevance of our findings, we analyzed samples from the FREECE study in which human subjects were exposed to individualized cooling protocols. Samples were analyzed ex vivo and after in vitro Treg induction using qRT-PCR, immunofluorescence, as well as with multicolor flow cytometry and cell sorting.
Results: In vivo application of the beta3-adrenergic receptor agonist mirabegron in humanized mice induced thermogenesis and improved the Treg induction capacity of naïve T cells isolated from these animals. Using samples from the human FREECE study, we demonstrate that a short-term cold stimulus supports human Treg induction in vitro and in vivo. Mechanistically, we identify BORCS6 encoding the Ragulator-interacting protein C17orf59 to be significantly induced in human CD4+ T cells upon short-term cold exposure. Strong mTOR signaling is known to limit successful Treg induction and thus likely by interfering with mTOR activation at lysosomal surfaces, C17orf59 improves the Treg induction capacity of human naïve T cells upon cold exposure.
Conclusions: These novel insights into the molecular underpinnings of human Treg induction suggest an important role of Tregs in linking environmental stimuli with adipose tissue function and metabolic diseases. Moreover, these discoveries shed new light on potential approaches towards tailored anti-inflammatory concepts that support human adipose tissue homeostasis by enabling Tregs.[Hide abstract]
|Temporal expression patterns of the melatoninergic system in the thymus of childrenMelatonin from the pineal gland is best known for its regulation of the circadian rhythm. However, melatonin synthesis also occurs in other organs, one of which is the thymus. Melatonin synthesis changes with age. To assess whether the thymic melatoninergic system shows changes related to development, Cruz-Chamorro et al. studied the enzyme expression required for melatonin synthesis as well as the nuclear and membrane melatonin receptors in normal human thymuses from children of different ages. They found that the thymic melatoninergic system is present and especially active from the first days of human life and its expression is inversely correlated with the age of the human thymus.|
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Objective: To obtain greater knowledge of the extra-pineal sources of melatonin during development, the amount of indolamine and the expression levels of the last two enzymes involved in its biosynthesis, Arylalkylamine N-acetyltransferase (AANAT) and acetylserotonin O-methyltransferase (ASMT), were analyzed in the human thymus from children from three different age groups (from days to years). The melatonin membrane and nuclear receptor expression levels also were studied.
Methods: Quantitative reverse transcriptase PCR and western blot were performed to investigate the receptor and enzyme expression levels. The results were examined and correlated with the ages of the thymuses.
Results: We found high levels of indolamine in the thymuses of newborns (younger than 1 month), which decreased during development; thymuses from the months (from 2 to 11 months) and years (from 1 to 12 years) groups showed lower levels. A similar decline was also observed in the mRNA of the AANAT enzyme and the expression levels of melatonin receptors. However, ASMT expression was exactly the opposite, with low levels in the newborn group and higher levels in the years group. Our results show that the thymic synthesis of melatonin occurs very early in childhood. Additionally, this is the first report that is focused on melatonin receptors expression in the human thymus.
Conclusions: Considering the limited melatonin synthesis performed by the newborn pineal gland, we suggest that the high levels of melatonin found in human thymus in this experimental group arise from synthesis in the tissue itself, which could be contributing to the immune efficiency at the thymic level.[Hide abstract]
|How CGRP maintains the stability of neuromuscular junctionα-Calcitonin gene-related peptide (CGRP) is a 37-amino-acid neuropeptide that is released at neuro-muscular junctions (NMJ) and stabilizes nicotinic acetylcholine receptors as well as muscle structure. However, the mechanism of this action has remained unclear. Machado and colleagues found that the increased NMJ stability is mediated by mechanistic target of rapamycin complex 1 and the suppression of autophagy. In addition, the present data show that the calcium-dependent proteolytic system is also inhibited by CGRP, a mechanism that could also be important for the maintenance of the synapse.
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Objective: Although it is well established that a-calcitonin gene-related peptide (CGRP) stabilizes muscle-type cholinergic receptors nicotinic subunits (AChR), the underlying mechanism by which this neuropeptide regulates muscle protein metabolism and neuromuscular junction (NMJ) morphology is unclear.
Methods: To elucidate the mechanisms how CGRP controls NMJ stability in denervated mice skeletal muscles, we carried out physiological, pharmacological, and molecular analyses of atrophic muscles induced by sciatic nerve transection.
Results: Here, we report that CGRP treatment in vivo abrogated the deleterious effects on NMJ upon denervation (DEN), an effect that was associated with suppression of skeletal muscle proteolysis, but not stimulation of protein synthesis. CGRP also blocked the DEN-induced increase in endocytic AChR vesicles and the elevation of autophagosomes per NMJ area. The treatment of denervated animals with rapamycin blocked the stimulatory effects of CGRP on mTORC1 and its inhibitory actions on autophagic flux and NMJ degeneration. Furthermore, CGRP inhibited the DEN-induced hyperactivation of Ca2+-dependent proteolysis, a degradative system that has been shown to destabilize NMJ. Consistently, calpain was found to be activated by cholinergic stimulation in myotubes leading to the dispersal of AChR clusters, an effect that was abolished by CGRP.
Conclusions: Taken together, these data suggest that the inhibitory effect of CGRP on autophagy and calpain may represent an important mechanism for the preservation of synapse morphology when degradative machinery is exacerbated upon denervation conditions.[Hide abstract]
|Cognitive dysfunction in diabetic rats is prevented by pyridoxamineIt is increasingly evident that people with diabetes have a higher risk of developing Alzheimer’s disease or other central nervous system disorders related to cognitive decline. Pyridoxamine is one of the three interconvertible members of vitamin B6, an essential cofactor in several processes including synthesis of neurotransmitters. Protective effects of pyridoxamine from several secondary complications of diabetes have previously been described. Kassab et al. demonstrate that robust recognition memory deficits in diabetic rats are correlated with dysfunction in brain metabolism. Pyridoxamine treatment prevented these recognition memory deficits.|
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Objective: The impact of diabetes mellitus on the central nervous system is less widely studied than in the peripheral nervous system, but there is increasing evidence that it elevates the risk of developing cognitive deficits. The aim of this study was to characterize the impact of experimental diabetes on the proteome and metabolome of the hippocampus. We tested the hypothesis that the vitamin B6 isoform pyridoxamine is protective against functional and molecular changes in diabetes.
Methods: We tested recognition memory using the novel object recognition (NOR) test in streptozotocin (STZ)-induced diabetic, age-matched control, and pyridoxamine- or insulin-treated diabetic male Wistar rats. Comprehensive untargeted metabolomic and proteomic analyses, using gas chromatography-mass spectrometry and iTRAQ-enabled protein quantitation respectively, were utilized to characterize the molecular changes in the hippocampus in diabetes.
Results: We demonstrated diabetes-specific, long-term (but not short-term) recognition memory impairment and that this deficit was prevented by insulin or pyridoxamine treatment. Metabolomic analysis showed diabetes-associated changes in 13/82 identified metabolites including polyol pathway intermediates glucose (9.2-fold), fructose (4.9-fold) and sorbitol (5.2-fold). We identified and quantified 4807 hippocampal proteins; 806 were significantly altered in diabetes. Pathway analysis revealed significant alterations in cytoskeletal components associated with synaptic plasticity, glutamatergic signaling, oxidative stress, DNA damage and FXR/RXR activation pathways in the diabetic rat hippocampus.
Conclusions: Our data indicate a protective effect of pyridoxamine against diabetes-induced cognitive deficits, and our comprehensive ‘omics datasets provide insight into the pathogenesis of cognitive dysfunction enabling development of further mechanistic and therapeutic studies.[Hide abstract]
|Direct and indirect effects of liraglutide on hypothalamic neurons Glucagon-like peptide 1 (GLP-1) is a gut derived hormone that plays a key role in regulation of glucose metabolism. In addition, GLP-1 suppresses feeding. The acute effects of GLP-1 receptor activation in melanocortin neurons mirrors that of the description of leptin. Liraglutide is a GLP-1 receptor agonist. He et al. find that leptin and GLP-1 directly modulate neuronal excitability of melanocortin neurons in an additive manner. They also describe a melanocortin pre-synaptic network altered in response to GLP-1.|
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Objective: The long-acting glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, stimulates insulin secretion and efficiently suppresses food intake to reduce body weight. As such, liraglutide is growing in popularity in the treatment of diabetes and chronic weight management. Within the brain, liraglutide has been shown to alter the activity of hypothalamic proopiomelanocortin (POMC) and Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons. Moreover, the acute activities of POMC and NPY neurons have been directly linked to feeding behavior, body weight, and glucose metabolism. Despite the increased usage of liraglutide and other GLP-1 analogues as diabetic and obesity interventions, the cellular mechanisms by which liraglutide alters the activity of metabolically relevant neuronal populations are poorly understood.
Methods: In order to resolve this issue, we utilized neuron-specific transgenic mouse models to identify POMC and NPY neurons for patch-clamp electrophysiology experiments.
Results: We found that liraglutide directly activated arcuate POMC neurons via TrpC5 channels, sharing a similar mechanistic pathway to the adipose-derived peptide leptin. Liraglutide also indirectly increases excitatory tone to POMC neurons. In contrast, liraglutide inhibited NPY/AgRP neurons through post-synaptic GABAA receptors and enhanced activity of pre-synaptic GABAergic neurons, which required both TrpC5 subunits and K-ATP channels. In support of an additive role of leptin and liraglutide in suppressing food intake, leptin potentiated the acute effects of liraglutide to activate POMC neurons. TrpC5 subunits in POMC neurons were also required for the intact pharmacological effects of liraglutide on food intake and body weight. Thus, the current study adds to recent work from our group and others, which highlight potential mechanisms to amplify the effects of GLP-1 agonists in vivo. Moreover, these data highlight multiple sites of action (both pre- and post-synaptic) for GLP-1 agonists on this circuit.
Conclusions: Taken together, our results identify critical molecular mechanisms linking GLP-1 analogues in arcuate POMC and NPY/AgRP neurons with metabolism.[Hide abstract]
|HSP70 induces liver X receptor pathway and cholesterol reduction Heat shock protein 70 (HSP70) is a chaperone that facilitates protein folding and transport, with essential roles in maintaining cell homeostasis and survival. In addition to its chaperone function in proteostasis, HSP70 can act as a signaling molecule. Gungor, Vanharanta, et al. studied the effects of rHSP70 on human primary monocyte-derived macrophage foam cells that accumulate cholesterol, which leads to artherosclerosis. They found that HSP70 exerts atheroprotective effects and that this can be mechanistically explained by HSP70 mediated stimulation of the liver X receptor, the master regulator of cholesterol removal.|
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Objective: Heat Shock Proteins (HSPs) maintain cellular homeostasis under stress. HSP70 represents a major stress-inducible family member and has been identified as a druggable target in inherited cholesterol-sphingolipid storage diseases. We investigated if HSP70 modulates cholesterol accumulation in more common conditions related to atherogenesis.
Methods: We studied the effects of recombinant HSP70 in cholesterol-laden primary macrophages from human blood donors and pharmacological HSP70 upregulation in high-cholesterol diet fed zebrafish.
Results: Recombinant HSP70 facilitated cholesterol removal from primary human macrophage foam cells. RNA sequencing revealed that HSP70 induced a robust transcriptional re-programming, including upregulation of key targets of liver X receptors (LXR), master regulators of whole-body cholesterol removal. Mechanistically, HSP70 interacted with the macrophage LXRalpha promoter, increased LXRalpha and its target mRNAs, and led to elevated levels of key proteins facilitating cholesterol efflux, including ATP-binding cassette transporters A1 and G1. Pharmacological augmentation of endogenous HSP70 in high-cholesterol diet fed zebrafish activated LXR and its target mRNAs and reduced cholesterol storage at the whole organism level.
Conclusions: These data demonstrate that HSP70 exerts a cholesterol lowering effect in primary human cells and animals and uncover a nuclear action of HSP70 in mediating cross-talk between HSP and LXR transcriptional regulation.[Hide abstract]
|Temporal plasticity of insulin and incretin secretion and insulin sensitivity following sleeve gastrectomy Bariatric procedures like vertical sleeve gastrectomy (VSG) alleviate diabetic hyperglycemia in 40-50% of patients. To further study the changes of key glucoregulatory factors after VSG over time, Douros et al. assessed glycemic regulation, incretin secretion, insulin sensitivity, and islet function over 90 days in mice with VSG and sham-operated controls pair-fed to match caloric intake. They found that an acute postoperative enhancement of intrinsic islet function persists over time. This surgical effect is superior to that seen with caloric restriction or weight loss alone. Critically, the early enhancements to incretin secretion and islet function diminish over time as insulin sensitivity increases, indicating modulation of these factors after surgery.|
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Objective: Bariatric surgery acutely improves glucose control, an effect that is generally sustained for years in most patients. The acute postoperative glycemic reduction is at least partially mediated by enhanced incretin secretion and islet function, and occurs independent of caloric restriction, whereas the sustained improvement in glucose control is associated with increased insulin sensitivity. However, studies in humans with bariatric surgery suggest that these elevations are not static but undergo coordinated regulation throughout the postoperative time course. The studies described here test the hypothesis that incretin secretion, islet function, and peripheral insulin sensitivity undergo temporal regulation following bariatric surgery as a means to regulate glucose homeostasis.
Methods: Incretin secretion, islet function, and insulin sensitivity in mice with vertical sleeve gastrectomy (VSG) were compared to sham-operated controls that were pair-fed for 90d, matching food consumption and body-weight between groups.
Results: Glucose clearance and insulin secretion were enhanced in VSG mice compared to controls during mixed-meal tolerance tests (MMTT) at 12 and 80 days postoperatively, as were prandial GLP-1, GIP, and glucagon levels. Insulin sensitivity was comparable between groups 14d after surgery, but significantly greater in the VSG group at day 75, despite similar body-weight gain between groups. Glucose stimulated insulin secretion was greater in VSG mice compared to controls in vivo (I.P. glucose injection) and ex vivo (islet perifusion) indicating a rapid and sustained enhancement of β-cell function after surgery. Notably, glycemia following a MMTT was progressively higher over time in the control animals but improved in the VSG mice at 80d despite weight regain. However, meal-stimulated incretin secretion decreased in VSG mice from 10 to 80 days postoperative, as did meal-stimulated and I.P. glucose-stimulated insulin secretion. This occurred over the same time period that insulin sensitivity was enhanced in VSG mice, suggesting postoperative islet output is tightly regulated by insulin demand.
Conclusions: These data demonstrate a dynamic, multifactorial physiology for improved glucose control after VSG, whereby rapidly elevated insulin secretion is complimented by later enhancements in insulin sensitivity. Critically, the glucose lowering effect of VSG is demonstrably larger than that of caloric-restriction, suggesting these adaptations are mediated by surgical modification of gastrointestinal anatomy and not weight-loss per se.[Hide abstract]
|mTORC1 and CB1 signaling regulate inputs onto the hypothalamic PVN in response to energy availabilityThe paraventricular nucleus (PVN) of the hypothalamus plays key roles in the regulation of energy balance. However, whether excitatory glutamatergic transmission from pro-opiomelanocortin (POMC) neurons affects the activity of PVN parvocellular neurons is not known. Mechanistic target of rapamycin complex 1 (mTORC1) controls intracellular energy availability. Cannabinoid receptor type 1 (CB1R) can affect the mTOR pathway. Mazier et al. show that mTORC1 activity in POMC neurons regulates glutamatergic inputs to PVN parvocellular neurons and that this function is exerted via control of pre-synaptic CB1R.|
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Objective: The hypothalamic paraventricular nucleus (PVN) is a key target of the melanocortin system, which orchestrates behavioral and metabolic responses depending on energy availability. The mechanistic target of rapamycin complex 1 (mTORC1) and the endocannabinoid type 1 receptor (CB1R) pathways are two key signaling systems involved in the regulation of energy balance whose activity closely depends upon energy availability. Here we tested the hypothesis that modulation of mTORC1 and CB1R signaling regulates excitatory glutamatergic inputs onto the PVN.
Methods: Patch-clamp recordings in C57BL/6J mice, in mice lacking the mTORC1 component Rptor or CB1R in pro-opio-melanocortin (POMC) neurons, combined with pharmacology targeting mTORC1, the melanocortin receptor type 4 (MC4R), or the endocannabinoid system under chow or a hypercaloric diet.
Results: Acute pharmacological inhibition of mTORC1 in C57BL/6J mice decreased glutamatergic inputs onto the PVN via a mechanism requiring modulation of MC4R, endocannabinoid 2-AG mobilization by PVN parvocellular neurons, and retrograde activation of presynaptic CB1R. Further electrophysiology studies using mice lacking mTORC1 activity or CB1R in POMC neurons indicated that the observed effects involved mTORC1 and CB1R-dependent regulation of glutamate release from POMC neurons. Finally, energy surfeit caused by hypercaloric high-fat diet feeding, rapidly and time-dependently altered the glutamatergic inputs onto parvocellular neurons and the ability of mTORC1 and CB1R signaling to modulate such excitatory activity.
Conclusions: These findings pinpoint the relationship between mTORC1 and endocannabinoid-CB1R signaling in the regulation of the POMC-mediated glutamatergic inputs onto PVN parvocellular neurons and its rapid alteration in conditions favoring the development of obesity.[Hide abstract]