Les astrocytes et la détection hypothalamique du glucose (rôle métabolique et implication des connexines astrocytaires)

L'hypothalamus est fortement impliqué dans la régulation nerveuse de l'homéostasie énergétique. Il existe dans cette structure des neurones spécialisés (gluco-sensibles) qui détectent notamment l hyperglycémie puis déclenchent des réponses adaptées comme le maintien de la glycémie, en stimulant la sécrétion d insuline ou encore le rassasiement. Les astrocytes sont suspectés de participer à la détection neuronale du glucose. Dans l ensemble du cerveau, il existe un couplage métabolique entre astrocytes et neurones. Le lactate, issu de la métabolisation du glucose par les astrocytes, est transporté par les neurones par des transporteurs aux monocarboxylates (MCTs). De plus, il a récemment été montré que les jonctions gap (GJ), à l origine de la formation de réseaux au sein des astrocytes sont indispensables au passage du glucose de la circulation sanguine vers les neurones en activité. Ces GJ astrocytaires sont formées majoritairement de connexines 43 et 30 (Cxs).Mon travail de thèse s est orienté suivant deux axes, qui ont visé à étudier le rôle des astrocytes dans la détection hypothalamique du glucose et du lactate. Dans un premier temps, nous avons montré que le lactate, comme le glucose, est détecté au niveau central et induit une sécrétion d insuline. Dans un modèle de rat hyperglycémique pendant 48h (qui présente aussi une hyperlactatémie), nous avons montré que la détection du glucose et du lactate est altérée. Ces modifications ne sont pas dues à une variation de l expression protéique des MCTs astrocytaires ou neuronale de l hypothalamus.Dans un deuxième temps, nous nous sommes intéressés au rôle des Cxs astrocytaires. La Cx43 est très exprimée autour des micro-vaisseaux sanguins de l hypothalamus médio-basal (MBH), un site présentant de nombreux neurones gluco-sensibles. L expression de la Cx30 est plus diffuse dans cette structure. Nous montrons également que l expression protéique des Cxs astrocytaires varie très rapidement suite à des modifications du statut métabolique (jeûne, réalimentation, hyperglycémie). Afin d évaluer l implication de la Cx43 astrocytaire (majoritaire) dans la détection hypothalamique du glucose, nous avons inhibé son expression dans le MBH, in vivo, en injectant des siRNA permettant d inhiber la synthèse de cette protéine. L inhibition de la Cx43 (30% à 72h) induit une diminution de la prise alimentaire sans modification du poids, de la glycémie et de l insulinémie comparée aux témoins. Suite à l injection carotidienne de glucose (censée mimer une hyperglycémie), la sécrétion d insuline est fortement inhibée chez les animaux siCx43. De même, l effet satiétogène du glucose semble inhibé chez ces animaux lors de la réalimentation après un jeûne.Ces résultats montrent pour la première fois, de façon intégrée, l importance des connexines, et probablement des réseaux astrocytaires, lors de la détection hypothalamique du glucose. Ces nouvelles données renforcent l importance du rôle métabolique des astrocytes lors de fonctions neuronales précisesThe hypothalamus plays a pivotal role in the nervous control of glucose homeostasis. This area contains gluco-sensitive neurons. Some of them detect increases in glucose levels and regulate glucose homeostasis by stimulating insulin secretion or inhibiting food intake. It is widely accepted that astrocytes are metabolically coupled to neurons. Lactate, resulting from the metabolism of glucose by astrocytes, is transported via the monocarboxylate transporters (MCTs). In addition, gap junctions (GJ), that form networks within astrocytes, are essential to transfer glucose from the bloodstream to the active neurons. These astroglial GJ mainly consist of connexins 43 and 30 (Cxs).The aims of my thesis are twofold: first, to show that an intracarotid lactate injection toward the brain, as for glucose, triggers insulin secretion and, second, to investigate the role of astroglial Cxs.Our results demonstrate that lactate and glucose sensing are altered in 48h hyperglycemic rats (accompanied by high blood lactate level). These alterations are not due to changes in protein expression of astroglial or neuronal MCTs in the hypothalamus. We then show that Cx43 is highly expressed in astrocytic end-feet enwraping blood vessels, in medio-basal hypothalamus (MBH) where many gluco-sensitive neurons are present. The Cx30 expression is more diffuse in this structure. We also show that the protein expression of astroglial Cxs varies very rapidly due to changes in metabolic status (fasting, refeeding and hyperglycemia). To evaluate the involvement of astroglial Cx43 (the major isoform) in the hypothalamic glucose sensing, we silenced its expression in the MBH in vivo by injecting specific siRNA. A 30% diminution in protein levels (after 72h) induced a decrease in food intake without changes in weight, blood glucose and insulin levels compared to vehicle treated animals. The central response to glucose is drastically inhibited in terms of insulin secretion in siCx43 animals. Similarly, an intracarotid injection of glucose towards the brain does not reduce refeeding in siRNA treated animals.These results demonstrate for the first time in vivo, the importance of connexins and astroglial networks in hypothalamic glucose sensing mechanism. These new data reinforce the importance of the metabolic role of astrocytes in specific neuronal functionsDIJON-BU Doc.électronique (212319901) / SudocSudocFranceF

Prandial states modify the reactivity of the gustatory cortex using gustatory evoked potentials in humans

Previous functional Magnetic Resonance Imaging studies evaluated the role of satiety on cortical taste area activity and highlighted decreased activation in the orbito-frontal cortex when food was eaten until satiation. The modulation of orbito-frontal neurons (secondary taste area) by ad libitum food intake has been associated with the pleasantness of the food's flavor. The insula and frontal operculum (primary taste area) are also involved in reward processing. The aim was to compare human gustatory evoked potentials (GEP) recorded in the primary and secondary gustatory cortices in a fasted state with those after food intake. Fifteen healthy volunteers were enrolled in this observational study. In each of two sessions, two GEP recordings were performed (at 11:00 am and 1:30 pm) in response to sucrose gustatory stimulation, and a sucrose-gustatory threshold was determined. During one session, a standard lunch was provided between the two GEP recordings. During the other session, subjects had nothing to eat. Hunger sensation, wanting, liking, and the perception of the solution's intensity were evaluated with visual analog scales. GEP latencies measured in the Pz (p < 0.001), Cz (p < 0.01), Fz (p < 0.001) recordings (primary taste area) were longer after lunch than in the pre-prandial condition. Fp1 and Fp2 latencies (secondary taste area) tended to be longer after lunch, but the difference was not significant. No difference was observed for the sucrose-gustatory threshold regardless of the session and time. Modifications in the primary taste area activity during the post-prandial period occurred regardless of the nature of the food eaten and could represent the activity of the frontal operculum and insula, which was recently shown to be modulated by gut signals (GLP-1, CCK, ghrelin, or insulin) through vagal afferent neurons or metabolic changes of the internal milieu after nutrient absorption. This trial was registered at clinicalstrials.gov as NCT02472444

Tissue-selective estrogen complexes with bazedoxifene prevent metabolic dysfunction in female mice

Pairing the selective estrogen receptor modulator bazedoxifene (BZA) with estrogen as a tissue-selective estrogen complex (TSEC) is a novel menopausal therapy. We investigated estrogen, BZA and TSEC effects in preventing diabetisity in ovariectomized mice during high-fat feeding. Estrogen, BZA or TSEC prevented fat accumulation in adipose tissue, liver and skeletal muscle, and improved insulin resistance and glucose intolerance without stimulating uterine growth. Estrogen, BZA and TSEC improved energy homeostasis by increasing lipid oxidation and energy expenditure, and promoted insulin action by enhancing insulin-stimulated glucose disposal and suppressing hepatic glucose production. While estrogen improved metabolic homeostasis, at least partially, by increasing hepatic production of FGF21, BZA increased hepatic expression of Sirtuin1, PPARα and AMPK activity. The metabolic benefits of BZA were lost in estrogen receptor-α deficient mice. Thus, BZA alone or in TSEC produces metabolic signals of fasting and caloric restriction and improves energy and glucose homeostasis in female mice

Mitochondrial Dynamin-Related Protein 1 (DRP1) translocation in response to cerebral glucose is impaired in a rat model of early alteration in hypothalamic glucose sensing

OBJECTIVE: Hypothalamic glucose sensing (HGS) initiates insulin secretion (IS) via a vagal control, participating in energy homeostasis. This requires mitochondrial reactive oxygen species (mROS) signaling, dependent on mitochondrial fission, as shown by invalidation of the hypothalamic DRP1 protein. Here, our objectives were to determine whether a model with a HGS defect induced by a short, high fat-high sucrose (HFHS) diet in rats affected the fission machinery and mROS signaling within the mediobasal hypothalamus (MBH). METHODS: Rats fed a HFHS diet for 3 weeks were compared with animals fed a normal chow. Both in vitro (calcium imaging) and in vivo (vagal nerve activity recordings) experiments to measure the electrical activity of isolated MBH gluco-sensitive neurons in response to increased glucose level were performed. In parallel, insulin secretion to a direct glucose stimulus in isolated islets vs. insulin secretion resulting from brain glucose stimulation was evaluated. Intra-carotid glucose load-induced hypothalamic DRP1 translocation to mitochondria and mROS (H2O2) production were assessed in both groups. Finally, compound C was intracerebroventricularly injected to block the proposed AMPK-inhibited DRP1 translocation in the MBH to reverse the phenotype of HFHS fed animals. RESULTS: Rats fed a HFHS diet displayed a decreased HGS-induced IS. Responses of MBH neurons to glucose exhibited an alteration of their electrical activity, whereas glucose-induced insulin secretion in isolated islets was not affected. These MBH defects correlated with a decreased ROS signaling and glucose-induced translocation of the fission protein DRP1, as the vagal activity was altered. AMPK-induced inhibition of DRP1 translocation increased in this model, but its reversal through the injection of the compound C, an AMPK inhibitor, failed to restore HGS-induced IS. CONCLUSIONS: A hypothalamic alteration of DRP1-induced fission and mROS signaling in response to glucose was observed in HGS-induced IS of rats exposed to a 3 week HFHS diet. Early hypothalamic modifications of the neuronal activity could participate in a primary defect of the control of IS and ultimately, the development of diabetes.Rôle des connexines astrocytaires dans le mécanisme de détection hypothalamique du glucose : implication sur le contrôle nerveux du métabolisme énergétiqu

Diabetic β-Cells Can Achieve Self-Protection against Oxidative Stress through an Adaptive Up-Regulation of Their Antioxidant Defenses

Background Oxidative stress (OS), through excessive and/or chronic reactive oxygen species (ROS), is a mediator of diabetes-related damages in various tissues including pancreatic β-cells. Here, we have evaluated islet OS status and β-cell response to ROS using the GK/Par rat as a model of type 2 diabetes. Methodology/Principal Findings Localization of OS markers was performed on whole pancreases. Using islets isolated from 7-day-old or 2.5-month-old male GK/Par and Wistar control rats, 1) gene expression was analyzed by qRT-PCR; 2) insulin secretion rate was measured; 3) ROS accumulation and mitochondrial polarization were assessed by fluorescence methods; 4) antioxidant contents were quantified by HPLC. After diabetes onset, OS markers targeted mostly peri-islet vascular and inflammatory areas, and not islet cells. GK/Par islets revealed in fact protected against OS, because they maintained basal ROS accumulation similar or even lower than Wistar islets. Remarkably, GK/Par insulin secretion also exhibited strong resistance to the toxic effect of exogenous H2O2 or endogenous ROS exposure. Such adaptation was associated to both high glutathione content and overexpression (mRNA and/or protein levels) of a large set of genes encoding antioxidant proteins as well as UCP2. Finally, we showed that such a phenotype was not innate but spontaneously acquired after diabetes onset, as the result of an adaptive response to the diabetic environment. Conclusions The GK/Par model illustrates the effectiveness of adaptive response to OS by beta-cells to achieve self-tolerance. It remains to be determined to what extend such islet antioxidant defenses upregulation might contribute to GK/Par beta-cell secretory dysfunction

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Détection centrale de la glycémie et homéostasie énergétique

National audienc

Détection hypothalamique du glucose et homéostasie énergétique

Prestation et revueDétection hypothalamique du glucose et homéostasie énergétique. JFN 2018, Journées Francophones de Nutritio