Featured ArticlesVolume 7 | No. 1 | January 2018
|Overexpression of nicotinamide phosphoribosyl transferase augments exercise enduranceNicotinamide adenine dinucleotide (NAD+) is an essential co-substrate for several enzyme classes. The NAD+ salvage pathway is the dominant pathway for NAD+ biosynthesis in mammals. In this pathway, nicotinamide phosphoribosyl transferase (NAMPT) plays a central role. NAMPT protein content in skeletal muscle is positively correlated with mitochondrial function, insulin sensitivity, and oxidative capacity in humans. Costford, Brouwers, et al. generated a mouse transgenic line that overexpressed NAMPT in skeletal muscle. They reveal a fascinating interaction between elevated NAMPT activity in skeletal muscle and voluntary exercise that manifests as a striking improvement of exercise endurance. |
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Objective: Nicotinamide phosphoribosyl transferase (NAMPT) is the rate-limiting enzyme in the salvage pathway that produces nicotinamide adenine dinucleotide (NAD+), an essential co-substrate regulating a myriad of signaling pathways. We produced a mouse that overexpressed NAMPT in skeletal muscle (NamptTg) and hypothesized that NamptTg mice would have increased oxidative capacity, endurance performance, and mitochondrial gene expression, and would be rescued from metabolic abnormalities that developed with high fat diet (HFD) feeding.
Methods: Insulin sensitivity (hyperinsulinemic-euglycemic clamp) was assessed in NamptTg and WT mice fed very high fat diet (VHFD, 60% by kcal) or chow diet (CD). The aerobic capacity (VO2max) and endurance performance of NamptTg and WT mice before and after 7 weeks of voluntary exercise training (running wheel in home cage) or sedentary conditions (no running wheel) were measured. Skeletal muscle mitochondrial gene expression was also measured in exercised and sedentary mice and in mice fed HFD (45% by kcal) or low fat diet (LFD, 10% by kcal).
Results: NAMPT enzyme activity in skeletal muscle was 7-fold higher in NamptTg mice versus WT mice. There was a concomitant 1.6-fold elevation of skeletal muscle NAD+. NamptTg mice fed VHFD were partially protected against body weight gain, but not against insulin resistance. Notably, voluntary exercise training elicited a 3-fold higher exercise endurance in NamptTg versus WT mice. Mitochondrial gene expression was higher in NamptTg mice compared to WT mice, especially when fed HFD. Mitochondrial gene expression was higher in exercised NamptTg mice than in sedentary WT mice.
Conclusions: Our studies have unveiled a fascinating interaction between elevated NAMPT activity in skeletal muscle and voluntary exercise that was manifest as a striking improvement in exercise endurance.[Hide abstract]
|Neurturin promotes motor neuron recruitment and neuromuscular junction formationPeroxisome-proliferator-activated receptor γ coactivator-1α (PGC-1α) proteins are key regulators of mitochondrial biogenesis and energy metabolism with important roles in the biology of skeletal muscle. PGC-1α1 has been identified as a regulator of neuromuscular junction (NMJ) structure and activity. To investigate how PGC-1α isoforms affect NMJs and the role of retrograde signaling during NMJ formation, Mills, Taylor-Weiner, and colleagues used an in vitro microfluidic NMJ model. PGC-1α1 expression resulted in pre-synaptic and post-synaptic NMJ changes. Using a bioinformatics approach, the authors identified the myokine neurturin as a key component in PGC-1α1 mediated neurite recruitment to muscle.
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Objective: We examined whether skeletal muscle overexpression of PGC-1α1 or PGC-1α4 affected myokine secretion and neuromuscular junction (NMJ) formation.
Methods: A microfluidic device was used to model endocrine signaling and NMJ formation between primary mouse myoblast-derived myotubes and embryonic stem cell-derived motor neurons. Differences in hydrostatic pressure allowed for fluidic isolation of either cell type or unidirectional signaling in the fluid phase. Myotubes were transduced to overexpress PGC-1α1 or PGC-1α4, and myokine secretion was quantified using a proximity extension assay. Morphological and functional changes in NMJs were measured by fluorescent microscopy and by monitoring muscle contraction upon motor neuron stimulation.
Results: Skeletal muscle transduction with PGC-1α1, but not PGC-1α4, increased NMJ formation and size. PGC-1α1 increased muscle secretion of neurturin, which was sufficient and necessary for the effects of muscle PGC-1α1 on NMJ formation.
Conclusions: Our findings indicate that neurturin is a mediator of PGC-1α1-dependent retrograde signaling from muscle to motor neurons.[Hide abstract]
|Macrophages sensing oxidized DAMPs reprogram their metabolism Macrophages have been shown to play essential roles in maintenance of tissue homeostasis as well as in regulation of induction and resolution of inflammation in response to tissue injury or infection. The functional plasticity of macrophages has been tied to changes in their cellular metabolism. Serbulea and colleagues show that macrophages sense danger associated molecular patterns (DAMPs) to reprogram their metabolism towards a redox-regulatory phenotype accompanied by antioxidant and inflammatory gene expression.|
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Objective: Macrophages control tissue homeostasis and inflammation by sensing and responding to environmental cues. However, the metabolic adaptation of macrophages to oxidative tissue damage and its translation into inflammatory mechanisms remains enigmatic.
Methods: Here we identify the critical regulatory pathways that are induced by endogenous oxidation-derived DAMPs (oxidized phospholipids, OxPL) in vitro, leading to formation of a unique redox-regulatory metabolic phenotype (Mox), which is strikingly different from conventional classical or alternative macrophage activation.
Results: Unexpectedly, metabolomic analyses demonstrated that Mox heavily rely on glucose metabolism and the pentose phosphate pathway (PPP) to support GSH production and Nrf2-dependent antioxidant gene expression. While the metabolic adaptation of macrophages to OxPL involved transient suppression of aerobic glycolysis, it also led to upregulation of inflammatory gene expression. In contrast to classically activated (M1) macrophages, Hif1α mediated expression of OxPL-induced Glut1 and VEGF but was dispensable for Il1β expression. Mechanistically, we show that OxPL suppress mitochondrial respiration via TLR2-dependent ceramide production, redirecting TCA metabolites to GSH synthesis. Finally, we identify spleen tyrosine kinase (Syk) as a critical downstream signaling mediator that translates OxPL-induced effects into ceramide production and inflammatory gene regulation.
Conclusions: Together, these data demonstrate the metabolic and bioenergetic requirements that enable macrophages to translate tissue oxidation status into either antioxidant or inflammatory responses via sensing OxPL. Targeting dysregulated redox homeostasis in macrophages could therefore lead to novel therapies to treat chronic inflammation.[Hide abstract]
|Mitochondrial fission is associated with UCP1 activity Brite adipocytes, brown adipocytes that are interspersed in white adipose tissue, are promising targets for the treatment of human obesity. Compared to white adipocytes, brown and brite adipocytes possess a higher mitochondrial content and express the uncoupling protein 1 (UCP1), which facilitates a proton leak and the uncoupling of the respiratory chain. Pisani, Barquissau, et al. characterized the properties of mitochondria during the conversion of human white to brite adipocytes using the human Multipotent Adipose Derived Stem Cell (hMADS) model. They found that human brite adipocyte mitochondria had an enhanced oxidative capacity and sustained fission.|
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Objective: Thermogenic adipocytes (i.e. brown or brite/beige adipocytes) are able to burn large amounts of lipids and carbohydrates as a result of highly active mitochondria and enhanced uncoupled respiration, due to UCP1 activity. Although mitochondria are the key organelles for this thermogenic function, limited human data are available.
We characterized changes in the mitochondrial function of human brite adipocytes, using hMADS cells as a model of white- to brite-adipocyte conversion. We found that profound molecular modifications were associated with morphological changes in mitochondria. The fission process was partly driven by the DRP1 protein, which also promoted mitochondrial uncoupling.
Conclusions: Our data demonstrate that white-to-brite conversion of human adipocytes relies on molecular, morphological and functional changes in mitochondria, which enable brite/beige cells to carry out thermogenesis.[Hide abstract]
|Fatty acid oxidation is required for brown adipose tissue maintenance and thermogenic programingBrown adipose tissue (BAT) is tasked with maintaining body temperature by consuming fatty acids via nonshivering thermogenesis under cold environmental temperatures. To understand the role of fatty acid oxidation to adipose tissue structure, function, and physiology, Gonzalez-Hurtado et al. subjected mice with an adipose-specific defect in fatty acid oxidation to disparate thermogenic stimuli. They show that pharmacologic thermogenic agonists induce a loss of uncoupling protein 1 (UCP1) and BAT morphology and failed to induce UCP1 and thermogenic programing in white adipose tissue (WAT). These data show that fatty acid oxidation is critical for the maintenance of the brown adipocyte phenotype, particularly under conditions of metabolic stress.|
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Objective: To determine the role of fatty acid oxidation on the cellular, molecular, and physiologic response of brown adipose tissue to disparate paradigms of chronic thermogenic stimulation.
Methods: Mice with an adipose-specific loss of Carnitine Palmitoyltransferase 2 (Cpt2A−/−), that lack mitochondrial long chain fatty acid β-oxidation, were subjected to environmental and pharmacologic interventions known to promote thermogenic programming in adipose tissue.
Results: Chronic administration of β3-adrenergic (CL-316243) or thyroid hormone (GC-1) agonists induced a loss of BAT morphology and UCP1 expression in Cpt2A−/− mice. Fatty acid oxidation was also required for the browning of white adipose tissue (WAT) and the induction of UCP1 in WAT. In contrast, chronic cold (15 °C) stimulation induced UCP1 and thermogenic programming in both control and Cpt2A−/− adipose tissue albeit to a lesser extent in Cpt2A−/− mice. However, thermoneutral housing also induced the loss of UCP1 and BAT morphology in Cpt2A−/− mice. Therefore, adipose fatty acid oxidation is required for both the acute agonist-induced activation of BAT and the maintenance of quiescent BAT. Consistent with this data, Cpt2A−/− BAT exhibited increased macrophage infiltration, inflammation and fibrosis irrespective of BAT activation. Finally, obese Cpt2A−/− mice housed at thermoneutrality exhibited a loss of interscapular BAT and were refractory to β3-adrenergic-induced energy expenditure and weight loss.
Conclusions: Mitochondrial long chain fatty acid β-oxidation is critical for the maintenance of the brown adipocyte phenotype both during times of activation and quiescence.[Hide abstract]
|Vacuolar protein sorting 13C inhibits lipolysis in brown adipocytesA defining feature of brown adipocytes (BAs) is the presence of multiple small lipid droplets (LDs) that integrate triglyceride storage and mobilization. Vacuolar protein sorting 13C (VPS13C) is a protein that occurs in LDs. Ramseyer et al. find that VPS13C occupies a unique subdomain on BA LDs. Deletion of VPS13C augments basal and β-adrenergic induced lipolysis and this is likely due to increased adipose tissue triglyceride lipase trafficking to LDs. The targeting of VPS13C to a distinctive LD subdomain suggests a specialized role in the subcellular control of lipolysis and the trafficking of lipolytic products between LDs and other organelles.
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Objective: Brown adipose tissue (BAT) thermogenesis depends on the mobilization and oxidation of fatty acids from intracellular lipid droplets (LD) within brown adipocytes (BAs); however, the identity and function of LD proteins that control BAT lipolysis remain incomplete. Proteomic analysis of mouse BAT subcellular fractions identified vacuolar protein sorting 13C (VPS13C) as a novel LD protein. The aim of this work was to investigate the role of VPS13C on BA LDs.
Methods: Biochemical fractionation and high resolution confocal and immuno-transmission electron microscopy (TEM) were used to determine the subcellular distribution of VPS13C in mouse BAT, white adipose tissue, and BA cell culture. Lentivirus-delivered shRNA was used to determine the role of VPS13C in regulating lipolysis and gene expression in cultured BA cells.
Results: We found that VPS13C is highly expressed in mouse BAT where it is targeted to multilocular LDs in a subspherical subdomain. In inguinal white adipocytes, VPS13C was mainly observed on small LDs and β3-adrenergic stimulation increased VPS13C in this depot. Silencing of VPS13C in cultured BAs decreased LD size and triglyceride content, increased basal free fatty acid release, augmented the expression of thermogenic genes, and enhanced the lipolytic potency and efficacy of isoproterenol. Mechanistically, we found that BA lipolysis required activation of adipose tissue triglyceride lipase (ATGL) and that loss of VPS13C greatly increased the association of ATGL to LDs.
Conclusions: VPS13C is present on BA LDs where is targeted to a distinct subdomain. VPS13C limits the access of ATGL to LD and loss of VPS13C elevates lipolysis and promotes oxidative gene expression.[Hide abstract]
|Constant hepatic ATP concentrations during prolonged fasting and absence of effects of Cerbomed Nemos® on parasympathetic tone and hepatic energy metabolismThe vagus nerve may be key to controlling glucose homeostasis by mediating the peripheral effects of insulin signaling in the central nervous system. Transcutaneous auricular vagus nerve stimulation (taVNS) can be applied to non-invasively activate the central projections of the auricular branch of the vagus nerve. Gancheva and colleagues designed a randomized, controlled, crossover clinical study in which they found that the procedure neither affects hepatic glucose metabolism nor hepatocellular lipid and ATP content in healthy humans. No differences in circulating glucoregulatory hormones and pancreatic polypeptide levels between active and sham stimulation indicate a lack of parasympathetic tone modulation with taVNS, which is confirmed by the absence of alterations in cardiac autonomic function.
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Objective: Brain insulin-induced improvement in glucose homeostasis has been proposed to be mediated by the parasympathetic nervous system. Non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) activating afferent branches of the vagus nerve may prevent hyperglycemia in diabetes models. We examined the effects of 14-min taVNS vs sham stimulation by Cerbomed Nemos® on glucose metabolism, lipids, and hepatic energy homeostasis in fasted healthy humans (n = 10, age 51 ± 6 yrs, BMI 25.5 ± 2.7 kg/m2).
Methods: Heart rate variability (HRV), reflecting sympathetic and parasympathetic nerve activity, was measured before, during and after taVNS or sham stimulation. Endogenous glucose production was determined using [6,6-2H2]glucose, and hepatic concentrations of triglycerides (HCL), adenosine triphosphate (ATP), and inorganic phosphate (Pi) were quantified from 1H/31P magnetic resonance spectroscopy at baseline and for 180 min following stimulation.
Results: taVNS did not affect circulating glucose, free fatty acids, insulin, glucagon, or pancreatic polypeptide. Rates of endogenous glucose production (P = 0.79), hepatic HCL, ATP, and Pi were also not different (P = 0.91, P = 0.48 and P = 0.24) between taVNS or sham stimulation. Hepatic HCL, ATP, and Pi remained constant during prolonged fasting for 3 h. No changes in heart rate or shift in cardiac autonomic function from HRV towards sympathetic or parasympathetic predominance were detected.
Conclusions: Non-invasive vagus stimulation by Cerbomed Nemos® does not acutely modulate the autonomic tone to the visceral organs and thereby does not affect hepatic glucose and energy metabolism. This technique is therefore unable to mimic brain insulin-mediated effects on peripheral homeostasis in humans.[Hide abstract]
|Metabolic adaptation to intermittent fasting is independent of PPARANumerous studies have shown that intermittent fasting can prevent or delay the onset of metabolic diseases. Given that fasting is characterized by the depletion of hepatic glycogen, increased lipid utilization, and elevated serum ketone bodies and that peroxisome proliferator-activated receptor alpha (PPARA) is a major regulator of fatty acid oxidation (FAO) and ketogenesis, PPARA is considered a key mediator of the fasting response. Li, Brocker, et al. placed wild-type and Ppara-null mice on an every-other-day fasting (EODF) regimen and/or treated with the potent PPARA agonist Wy-14643 to explore the effect of PPARA on the adaptive response to fasting. The results suggest that although PPARA deficiency aggravates acute fasting-induced steatosis, EODF elicits a pronounced metabolic adaptation to prevent fasting-induced hepatic steatosis independently of PPARA.|
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Objective: Peroxisome proliferator-activated receptor alpha (PPARA) is a major regulator of fatty acid oxidation and severe hepatic steatosis occurs during acute fasting in Ppara-null mice. Thus, PPARA is considered an important mediator of the fasting response; however, its role in other fasting regiments such as every-other-day fasting (EODF) has not been investigated.
Methods: Mice were pre-conditioned using either a diet containing the potent PPARA agonist Wy-14643 or an EODF regimen prior to acute fasting. Ppara-null mice were used to assess the contribution of PPARA activation during the metabolic response to EODF. Livers were collected for histological, biochemical, qRT-PCR, and Western blot analysis.
Results: Acute fasting activated PPARA and led to steatosis, whereas EODF protected against fasting-induced hepatic steatosis without affecting PPARA signaling. In contrast, pretreatment with Wy-14,643 did activate PPARA signaling but did not ameliorate acute fasting-induced steatosis and unexpectedly promoted liver injury. Ppara ablation exacerbated acute fasting-induced hypoglycemia, hepatic steatosis, and liver injury in mice, whereas these detrimental effects were absent in response to EODF, which promoted PPARA-independent fatty acid metabolism and normalized serum lipids.
Conclusions: These findings indicate that PPARA activation prior to acute fasting cannot ameliorate fasting-induced hepatic steatosis, whereas EODF induced metabolic adaptations to protect against fasting-induced steatosis without altering PPARA signaling. Therefore, PPARA activation does not mediate the metabolic adaptation to fasting, at least in preventing acute fasting-induced steatosis.[Hide abstract]
|Detection of free fatty and bile acids by ileal GLP-1 secreting cellsEnteroendocrine cells (EECs) are found scattered along the gastrointestinal tract and produce hormones that dynamically link metabolism and appetite to rates of nutrient absorption. Enteroendocrine L-cells produce several hormones, including Glucagon-like peptide-1 (GLP-1), which enhances insulin secretion and satiety. EECs detect luminal contents by G-protein coupled receptors like the G-protein coupled bile acid receptor GPBAR-1, or the free fatty acid receptor FFA1. Goldspink et al. identify the electrophysiological and second messenger responses to FFA1 and GPBAR1 activation in single L-cells using intestinal organoids from transgenic mouse models.|
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Objective: The aim of this study was to investigate the electrical properties of ileal Glucagon-like peptide 1 (GLP-1) secreting L-cells using murine organoid cultures and the electrophysiological and intracellular signaling pathways recruited following activation of the Gαq-coupled free fatty acid receptors FFA1 and Gαs-coupled bile acid receptors GPBAR1.
Methods: Experiments were performed using ileal organoids generated from mice transgenically expressing fluorescent reporters (Epac2-camps and GCaMP3) under control of the proglucagon promoter. Electrophysiology and single cell imaging were performed on identified L-cells in organoids, and GLP-1 secretion from cultured organoids was measured by immunoassay.
Results: The FFA1 ligand TAK-875 triggered L-cell electrical activity, increased intracellular calcium, and activated a depolarizing current that was blocked by the TRPC3 inhibitor Pyr3. TAK-875 triggered GLP-1 secretion was Pyr3 sensitive, suggesting that the TRPC3 channel links FFA1 activation to calcium elevation and GLP-1 release in L-cells. GPBAR1 agonist triggered PKA-dependent L-type Ca2+ current activation and action potential firing in L-cells. The combination of TAK-875 and a GPBAR1 agonist triggered synergistic calcium elevation and GLP-1 secretory responses.
Conclusions: FFA1 and GPBAR1 activation individually increased electrical activity in L-cells by recruiting pathways that include activation of TRPC3 and L-type voltage-gated Ca2+ channels. Synergy between the pathways activated downstream of these receptors was observed both at the level of Ca2+ elevation and GLP-1 secretion.[Hide abstract]
|CART neurons in the Arc and LHA exert differential controls on energy homeostasisThe cocaine- and amphetamine-regulated transcript (CART) is a neuropeptide involved in the regulation of appetite control, maintenance of body weight, reward and addiction, psychostimulant effects, and neuroendocrine functions. CART is highly expressed in hypothalamic areas important for energy homeostasis regulation, including the arcuate nucleus (Arc) and lateral hypothalamic area (LHA). Lau et al. used CART-deficient mice and reintroduced CART in a neuron-specific manner. They demonstrate that CART exerts a catabolic influence in the Arc, but an anabolic influence in the LHA.
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Objective: The cocaine- and amphetamine-regulated transcript (CART) codes for a pivotal neuropeptide important in the control of appetite and energy homeostasis. However, limited understanding exists for the defined effector sites underlying CART function, as discrepant effects of central CART administration have been reported.
Methods: By combining Cart-cre knock-in mice with a Cart adeno-associated viral vector designed using the flip-excision switch (AAV-FLEX) technology, specific reintroduction or overexpression of CART selectively in CART neurons in the arcuate nucleus (Arc) and lateral hypothalamic area (LHA), respectively, was achieved. The effects on energy homeostasis control were investigated.
Results: Here we show that CART neuron-specific reintroduction of CART into the Arc and LHA leads to distinct effects on energy homeostasis control. Specifically, CART reintroduction into the Arc of otherwise CART-deficient Cartcre/cre mice markedly decreased fat mass and body weight, whereas CART reintroduction into the LHA caused significant fat mass gain and lean mass loss, but overall unaltered body weight. The reduced adiposity in ArcCART;Cartcre/cre mice was associated with an increase in both energy expenditure and physical activity, along with significantly decreased Npy mRNA levels in the Arc but with no change in food consumption. Distinctively, the elevated fat mass in LHACART;Cartcre/cre mice was accompanied by diminished insulin responsiveness and glucose tolerance, greater spontaneous food intake, and reduced energy expenditure, which is consistent with the observed decrease of brown adipose tissue temperature. This is also in line with significantly reduced tyrosine hydroxylase (Th) and notably increased corticotropin-releasing hormone (Crh) mRNA expressions in the paraventricular nucleus (PVN).
Conclusions: Taken together, these results identify catabolic and anabolic effects of CART in the Arc and LHA, respectively, demonstrating for the first time the distinct and region-specific functions of CART in controlling feeding and energy homeostasis.[Hide abstract]
|PGC-1α functions as a co-suppressor of XBP1s to regulate glucose metabolismPeroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1 α) plays key roles in the development of obesity, insulin resistance, and type 2 diabetes. As obesity and diabetes progress, metabolically crucial organs experience elevated ER stress, which is partly due to impaired X-box binding protein 1 spliced (XBP1s) function, leading to leptin and insulin resistance. Lee and colleagues reveal a novel function of PGC-1α as a suppressor of XBP1s function, suggesting that hepatic PGC-1α promotes gluconeogenesis through multiple pathways.|
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Objective: Peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) promotes hepatic gluconeogenesis by activating HNF4α and FoxO1. PGC-1α expression in the liver is highly elevated in obese and diabetic conditions, leading to increased hepatic glucose production. We previously showed that the spliced form of X-box binding protein 1 (XBP1s) suppresses FoxO1 activity and hepatic gluconeogenesis. The shared role of PGC-1α and XBP1s in regulating FoxO1 activity and gluconeogenesis led us to investigate the probable interaction between PGC-1α and XBP1s and its role in glucose metabolism.
Methods: We investigated the biochemical interaction between PGC-1α and XBP1s and examined the role of their interaction in glucose homeostasis using animal models.
Results: We show that PGC-1α interacts with XBP1s, which plays an anti-gluconeogenic role in the liver by suppressing FoxO1 activity. The physical interaction between PGC-1α and XBP1s leads to suppression of XBP1s activity rather than its activation. Upregulating PGC-1α expression in the liver of lean mice lessens XBP1s protein levels, and reducing PGC-1α levels in obese and diabetic mouse liver restores XBP1s protein induction.
Conclusions: Our findings reveal a novel function of PGC-1α as a suppressor of XBP1s function, suggesting that hepatic PGC-1α promotes gluconeogenesis through multiple pathways as a co-activator for HNF4α and FoxO1 and also as a suppressor for anti-gluconeogenic transcription factor XBP1s.[Hide abstract]
|Dual role of PTP1B in the progression and reversion of non-alcoholic steatohepatitis Protein tyrosine phosphatase 1B (PTP1B) has emerged as a major negative regulator of insulin and leptin sensitivity. The study by González-Rodríguez, Valdecantos, et al. provides an experimental model evidencing the duality of PTP1B actions in the liver. Their results strongly suggest that during non-alcoholic steatohepatitis (NASH) progression, PTP1B restrains inflammation, whereas in NASH reversion, this phosphatase targets the proliferative responses mediated by hepatocyte growth factor receptor signaling in oval liver cells. This duality of PTP1B actions must be recognized for pharmacological purposes in chronic liver diseases such as NASH.|
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Objective: Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in Western countries. Protein tyrosine phosphatase 1B (PTP1B), a negative modulator of insulin and cytokine signaling, is a therapeutic target for type 2 diabetes and obesity. We investigated the impact of PTP1B deficiency during NAFLD, particularly in non-alcoholic steatohepatitis (NASH).
Methods: NASH features were evaluated in livers from wild-type (PTP1BWT) and PTP1B-deficient (PTP1BKO) mice fed methionine/choline-deficient diet (MCD) for 8 weeks. A recovery model was established by replacing MCD to chow diet (CHD) for 2–7 days. Non-parenchymal liver cells (NPCs) were analyzed by flow cytometry. Oval cells markers were measured in human and mouse livers with NASH, and in oval cells from PTP1BWT and PTP1BKO mice.
Results: PTP1BWT mice fed MCD for 8 weeks exhibited NASH, NPCs infiltration, and elevated Fgf21, Il6 and Il1b mRNAs. These parameters decreased after switching to CHD. PTP1B deficiency accelerated MCD-induced NASH. Conversely, after switching to CHD, PTP1BKO mice rapidly reverted NASH compared to PTP1BWT mice in parallel to the normalization of serum triglycerides (TG) levels. Among NPCs, a drop in cytotoxic natural killer T (NKT) subpopulation was detected in PTP1BKO livers during recovery, and in these conditions M2 macrophage markers were up-regulated. Oval cells markers (EpCAM and cytokeratin 19) significantly increased during NASH only in PTP1B-deficient livers. HGF-mediated signaling and proliferative capacity were enhanced in PTP1BKO oval cells. In NASH patients, oval cells markers were also elevated.
Conclusions: PTP1B elicits a dual role in NASH progression and reversion. Additionally, our results support a new role for PTP1B in oval cell proliferation during NAFLD.[Hide abstract]
|Why lipostatic set point systems are unlikely to evolve It is widely assumed that body fatness is regulated by a lipostatic regulatory system. By this model, a signal from the body reflecting the level of stored fat is compared to a set-point in the brain, and deviations of the body fat from the set-point result in compensatory responses. However, the molecular basis of the lipostatic set-point has never been discovered. It is thought that there are two main evolutionary drivers for a set point: having more fat will make it more likely to survive starvation, and having less fat will make it more likely to escape predators. Speakman argues that these supposed drivers are not constant in space and time, making it unlikely that a lipostatic set point evolves.|
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Objective: Body fatness is widely assumed to be regulated by a lipostatic set-point system, which has evolved in response to trade-offs in the risks of mortality. Increasing fatness makes the risk of starvation lower but increases the risk of predation. Yet other models are available. The aim of this work is to evaluate using mathematical modeling whether set-point systems are more likely to evolve than the alternatives.
Methods: I modeled the trade-off in mortality risks using a simple mathematical model, which generates an optimum level of fatness that is presumed to be the driver for the evolution of a set-point. I then mimicked the likely errors in this optimum level, that derive from the variation in the component parameters of the mortality curves using Markov Chain Monte Carlo (MCMC) simulation by Bayesian inference Using Gibbs Sampling (BUGS).
Results: The error propagation generated by the simulations showed that even very small errors in the model parameters were magnified enormously in the location of the optimum fatness level. If the model parameters had coefficients of variation of just 1% then the coefficient of variation in the optimum level of fatness was between 20 and 90%. In that situation, a set-point centered at the mathematical optimum from the component curves would be at the correct level of fatness that minimizes mortality, and hence maximizes fitness, on less than 8% of occasions.
Conclusions: Set-point regulation of body fatness is hence highly unlikely to evolve where there is any realistic level of variation in the parameters that define mortality risks. Using further MCMC modeling, I show that a dual-intervention point system is more likely to evolve. This mathematical simulation work has important implications for how we interpret molecular work concerning regulation of adiposity.[Hide abstract]
|Deficiency of leptin receptor disrupts hypothalamic circuits and causes weight increaseLeptin is an important adipokine regulating energy balance mainly through signaling in the hypothalamus. In the classic view, leptin is assumed to act mainly through leptin receptors (LepR) on hypothalamic neurons, but, more recently, the LepR has also been identified on glial cells. To investigate the significance of microglial leptin signaling, Gao, Vidal-Itriago et al. generated a mouse model with a specific LepR knockout in myeloid cells. These mice have higher body weight with hyperphagia. In the hypothalamus, pro-opiomelanocortin neuron numbers in the arcuate nucleus (ARC) and a-MSH projections from the ARC to the paraventricular nucleus (PVN) are decreased, which is accompanied by the presence of less ramified microglia with impaired phagocytic capacity in the PVN.|
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Objective: Leptin is a cytokine produced by adipose tissue that acts mainly on the hypothalamus to regulate appetite and energy homeostasis. Previous studies revealed that the leptin receptor is expressed not only in neurons, but also in glial cells. Microglia are resident immune cells in the brain that play an essential role in immune defense and neural network development. Previously we reported that microglial morphology and cytokine production are changed in the leptin receptor deficient db/db mouse, suggesting that leptin's central effects on metabolic control might involve signaling through microglia. In the current study, we aimed to uncover the role of leptin signaling in microglia in systemic metabolic control.
Methods: We generated a mouse model with leptin receptor deficiency, specifically in the myeloid cells, to determine the role of microglial leptin signaling in the development of metabolic disease and to investigate microglial functions.
Results: We discovered that these mice have increased body weight with hyperphagia. In the hypothalamus, pro-opiomelanocortin neuron numbers in the arcuate nucleus (ARC) and α-MSH projections from the ARC to the paraventricular nucleus (PVN) decreased, which was accompanied by the presence of less ramified microglia with impaired phagocytic capacity in the PVN.
Conclusions: Myeloid cell leptin receptor deficient mice partially replicate the db/db phenotype. Leptin signaling in hypothalamic microglia is important for microglial function and a correct formation of the hypothalamic neuronal circuit regulating metabolism.[Hide abstract]
|Optimal housing temperatures for mice to mimic the thermal environment of humansIn metabolic research, there is an increasing understanding that environmental temperature may dramatically affect the outcome of experiments. Under normal life conditions, humans display energy expenditure values of around 1.6-1.8 times basal metabolic rate (BMR). It is evidently of utmost importance for medically related research to ensure that experimental conditions in mice resemble human conditions. Fischer and colleagues experimentally test different conditions and find that already at thermoneutrality, daily energy expenditure of mice is about 1.6 times basal metabolic rate. Thus, thermoneutral temperatures remain the preferred method of modeling human conditions for metabolic research.|
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Objective: The laboratory mouse is presently the most common model for examining mechanisms of human physiology and disease. Housing temperatures can have a large impact on the outcome of such experiments and on their translatability to the human situation. Humans usually create for themselves a thermoneutral environment without cold stress, while laboratory mice under standard conditions (≈20° C) are under constant cold stress. In a well-cited, theoretical paper by Speakman and Keijer in Molecular Metabolism, it was argued that housing mice under close to standard conditions is the optimal way of modeling the human metabolic situation. This tenet was mainly based on the observation that humans usually display average metabolic rates of about 1.6 times basal metabolic rate. The extra heat thereby produced would also be expected to lead to a shift in the ‘lower critical temperature’ towards lower temperatures.
Methods: To examine these tenets experimentally, we performed high time-resolution indirect calorimetry at different environmental temperatures on mice acclimated to different housing temperatures.
Results: Based on the high time-resolution calorimetry analysis, we found that mice already under thermoneutral conditions display mean diurnal energy expenditure rates 1.8 times higher than basal metabolism, remarkably closely resembling the human situation. At any temperature below thermoneutrality, mice metabolism therefore exceeds the human equivalent: Mice under standard conditions display energy expenditure 3.1 times basal metabolism. The discrepancy to previous conclusions is probably attributable to earlier limitations in establishing true mouse basal metabolic rate, due to low time resolution. We also found that the fact that mean energy expenditure exceeds resting metabolic rate does not move the apparent thermoneutral zone (the lower critical temperature) downwards.
Conclusions: We show that housing mice at thermoneutrality is an advantageous step towards aligning mouse energy metabolism to human energy metabolism.[Hide abstract]