Neuronal Conduction of Excitation without Action Potentials Based on Ceramide Production

International audienceBACKGROUND: Action potentials are the classic mechanism by which neurons convey a state of excitation throughout their length, leading, after synaptic transmission, to the activation of other neurons and consequently to network functioning. Using an in vitro integrated model, we found previously that peripheral networks in the autonomic nervous system can organise an unconventional regulatory reflex of the digestive tract motility without action potentials. METHODOLOGY/PRINCIPAL FINDINGS: In this report, we used combined neuropharmacological and biochemical approaches to elucidate some steps of the mechanism that conveys excitation along the nerves fibres without action potentials. This mechanism requires the production of ceramide in membrane lipid rafts, which triggers in the cytoplasm an increase in intracellular calcium concentration, followed by activation of a neuronal nitric oxide synthase leading to local production of nitric oxide, and then to guanosine cyclic monophosphate. This sequence of second messengers is activated in cascade from rafts to rafts to ensure conduction of the excitation along the nerve fibres. CONCLUSIONS/SIGNIFICANCE: Our results indicate that second messengers are involved in neuronal conduction of excitation without action potentials. This mechanism represents the first evidence-to our knowledge-that excitation is carried along nerves independently of electrical signals. This unexpected ceramide-based conduction of excitation without action potentials along the autonomic nerve fibres opens up new prospects in our understanding of neuronal functioning

Conduction nerveuse d’excitation sans potentiel d’action

National audienc

Cholesterol and Sphingomyelin-Containing Model Condensed Lipid Monolayers: Heterogeneities Involving Ordered Microdomains Assessed by Two Cholesterol Derivatives.

International audienceLipid monolayers are often considered as model membranes, but they are also the physiologic lipid part of the peripheral envelope of lipoproteins and cytosolic lipid bodies. However, their structural organization is still rather elusive, in particular when both cholesterol and sphingomyelin are present. To investigate such structural organization of hemimembranes, we measured, using alternative current voltammetry, the differential capacitance of condensed phosphatidylcholine-based monolayers as a function of applied potential, which is sensitive to their lipid composition and molecular arrangement. Especially, monolayers containing both sphingomyelin and cholesterol, at 15% w/w, presented specific characteristics of the differential capacitance versus potential curves recorded, which was indicative of specific interactions between these two lipid components. We then compared the behavior of two cholesterol derivatives (at 15% w/w), 21-methylpyrenyl-cholesterol (Pyr-met-Chol) and 22-nitrobenzoxadiazole-cholesterol (NBD-Chol), with that of cholesterol when present in model monolayers. Indeed, these two probes were chosen because of previous findings reporting opposite behaviors within bilayer membranes regarding their interaction with ordered lipids, with only Pyr-met-Chol mimicking cholesterol well. Remarkably, in monolayers containing sphingomyelin or not, Pyr-met-Chol and NBD-Chol presented contrasting behaviors, and Pyr-met-Chol mimicked cholesterol only in the presence of sphingomyelin. These two observations (i.e., optimal amounts of sphingomyelin and cholesterol, and the ability to discriminate between Pyr-met-Chol and NBD-Chol) can be interpreted by the existence of heterogeneities including ordered patches in sphingomyelin- and cholesterol-containing monolayers. Since such monolayer lipid arrangement shares some properties with the raft-type lipid microdomains well-described in sphingomyelin- and cholesterol-containing bilayer membranes, our data thus strongly suggest the existence of compact and ordered microdomains in model lipid monolayers

Hypercholesterolemia of obese mice with deletion of vascular adhesion protein-1 occurs without other atherosclerosis risk factor

Aim: Encoded by Aoc3 gene, Vascular Adhesion Protein-1 (VAP-1), also called semicarbazide-sensitive amine oxidase (SSAO), is a protein supporting leucocyte extravasation to inflammation sites and catalyzing the oxidation of primary amines. We previously observed that a genetically-modified mouse model lacking active VAP-1/SSAO is obese and hypercholesterolemic. Here, we further studied the alterations related to factors that increase or alleviate the risk of atherosclerosis.Methods: Body weight and glucose tolerance were determined in mice homozygous for a null mutation of Aoc3 (AOC3KO) and fed standard or high-fat diet (HFD). White adipose tissue (WAT) inflammation was assessed by immunohistological observations. Cholesterol trafficking was explored by determining plasma and tissue levels and key enzyme expression. Vascular reactivity and VAP-1/SSAO activity were assessed via hydrogen peroxide release, uric acid and nitrate/nitrite levels.Results: AOC3KO mice were devoid of VAP-1/SSAO protein and activity, while, in WT control, WAT was the richest anatomical location regarding the capacity to oxidize benzylamine. AOC3KO mice were obese but did not exhibit alteration of glucose tolerance or insulin secretion. The elevated plasma cholesterol of AOC3KO mice was further increased by HFD, with LDL cholesterol levels higher than in WT. An increased cholesteryl ester accumulation occurred in plasma, liver and WAT, with higher HMGCoA expression in WAT and slightly reduced SR-BI hepatic transporters. However, in AOC3KO mice, no sign of WAT inflammation was detected, while lower hydrogen peroxide release and higher nitrite levels were found in aorta and kidney.Conclusion: The obesity of AOC3KO mice occurred with hypercholesterolemia but without other atherosclerosis risk factors, such as worsened insulin sensitivity, WAT inflammation, increased oxidative stress and reduced nitric oxide availability

Transforming growth factor activity is a key determinant for the effect of estradiol on fatty streak deposit in hypercholesterolemic mice.

International audienceOBJECTIVE: Whereas estradiol prevents fatty streak deposit in immunocompetent apoE-/- or LDLr-/- mice, it is totally ineffective in immunodeficient mice, underlining the key role of immunoinflammation in this effect. In the present work, the role of several major pro- and antiinflammatory cytokines involved in the atheromatous process was evaluated in the effect of estradiol on fatty streak constitution. METHODS AND RESULTS: The preventive effect of estradiol was fully maintained in LDLr-/- mice grafted with bone marrow from either IFN-gamma or interleukin (IL)-12-deficient mice, showing that this beneficial effect was not mediated through a specific decrease in the production of these 2 proinflammatory cytokines. Furthermore, IL-10-/- apoE-/- mice remained protected by estradiol, excluding a significant contribution of this antiinflammatory cytokine. In contrast, the protective effect of estradiol was (1) associated with enhanced aortic expression of TGF-beta1 in apoE-/- mice during early steps of atherogenesis; (2) abolished and even reversed in apoE-/- mice administered with a neutralizing anti-TGF-beta antibody; (3) abolished in LDLr-/- mice grafted with bone marrow from Smad3-deficient mice. CONCLUSIONS: The status of the TGF-beta pathway crucially determines the antiatherogenic effect of estradiol in hypercholesterolemic mice, whereas neither IFN-gamma, IL-12, nor IL-10 are specifically involved in this protection

Fluorescent probes for detecting cholesterol-rich ordered membrane microdomains: entangled relationships between structural analogies in the membrane and functional homologies in the cell

This review addresses the question of fluorescent detection of ordered membrane (micro) domains in living (cultured) cells, with a "practical" point of view since the situation is much more complicated than for studying model membranes. We first briefly recall the bases of model membrane structural organization involving liquid-ordered and -disordered phases, and the main features of their counterparts in cell membranes that are the various microdomains. We then emphasize the utility of the fluorescent probes derived from cholesterol, and delineate the respective advantages, limitations and drawbacks of the existing ones. In particular, besides their intramembrane behavior, their relevant characteristics should integrate their different cellular fates for membrane turn-over, trafficking and metabolism, in order to evaluate and improve their efficiency for in-situ probing membrane microdomains in the cell physiology context. Finally, at the present stage, it appears that Bdp-Chol and Pyr-met-Chol display well complementary properties, allowing to use them in combination to improve the reliability of the current experimental approaches. But the field is still open, and there remains much work to perform in this research area

Use of H-1-NMR metabolomics to precise the function of the third glutamate dehydrogenase gene in Arabidopsis thaliana

It is now well established that the GS/GOGAT cycle is the major route for ammonium assimilation in higher plants. However, it has often been argued that other enzymes, such as glutamate dehydrogenase, have the capacity to assimilate ammonium, leading to the hypothesis that alternative ammonium assimilatory pathways could operate under particular physiological conditions. The GDH enzyme is encoded by two distinct genes, GDH1 and GDH2. A third gene, GDH3, potentially encoding GDH has recently been identified by in silico studies performed on Arabidopsis thaliana. In order to precise its function, the metabolic profile of gdh3 knock out mutants were compared to wild type plants using the 1H-NMR technique. 1H-NMR spectra coupled with principal component analysis and partial least square-discriminant analysis were applied to identify changes of the metabolic profiles. These experiments were performed on roots, leaves and stems. In the gdh3 mutant, metabolic variations were observed for carboxylic acids, amino acids and carbohydrates content

Respective contributions of intestinal Niemann-Pick C1-like 1 and scavenger receptor class B type I to cholesterol and tocopherol uptake: in vivo v. in vitro studies

International audienceThe intestinal absorption of cholesterol and lipid micronutrients such as vitamin E has been shown to share some common pathways. The present study aims to further compare the uptake of cholesterol ([H-3] cholesterol v. 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3-ol (NBD-cholesterol)) and tocopherol in Caco-2 TC-7 cells and in mouse intestine, with special focus on the respective roles of scavenger receptor class B type I (SR-BI) and Niemann-Pick C1-like 1 (NPC1L1). Conversely to NBD-cholesterol, the uptakes of [H-3] cholesterol and tocopherol by Caco-2 cells were impaired by both block lipid transport-1 and ezetimibe, which inhibit SR-BI and NPC1L1, respectively. These inhibitions occurred only when cholesterol or tocopherol was delivered to cells included in micelles that contained biliary acid and at least oleic acid as a lipid. In vivo, after 2 h of digestion in mice, the uptake of the two cholesterol analogues and of tocopherol all showed distinct patterns along the duodenum-jejunum axis. [H-3] Cholesterol uptake, which correlated closely to NPC1L1 mRNA expression in wild-type (wt) mice, was strongly inhibited by ezetimibe. Intestinal SR-BI overexpression did not change NPC1L1 expression and led to a significant increase in [H-3] cholesterol uptake in the distal jejunum. Conversely, neither ezetimibe treatment nor SR-BI overexpression had an effect on NBD-cholesterol uptake. However, in contrast with SR-BI mRNA expression, tocopherol absorption increased strongly up to the distal jejunum in wt mice where it was specifically inhibited by ezetimibe, and was increased in the proximal intestine of intestinal SR-BI-overexpressing mice. Thus, cholesterol and tocopherol uptakes share common pathways in cell culture models, but display different in vivo absorption patterns associated with distinct contributions of SR-BI and NPC1L1