Stochastic models and dynamic measures for the characterization of bistable circuits

During the last few years, a great deal of interest has risen concerning the applications of stochastic methods to several biochemical and biological phenomena. Phenomena like gene expression, cellular memory, bet-hedging strategy in bacterial growth and many others, cannot be described by continuous stochastic models due to their intrinsic discreteness and randomness. In this thesis I have used the Chemical Master Equation (CME) technique to modelize some feedback cycles and analyzing their properties, including experimental data. In the first part of this work, the effect of stochastic stability is discussed on a toy model of the genetic switch that triggers the cellular division, which malfunctioning is known to be one of the hallmarks of cancer. The second system I have worked on is the so-called futile cycle, a closed cycle of two enzymatic reactions that adds and removes a chemical compound, called phosphate group, to a specific substrate. I have thus investigated how adding noise to the enzyme (that is usually in the order of few hundred molecules) modifies the probability of observing a specific number of phosphorylated substrate molecules, and confirmed theoretical predictions with numerical simulations. In the third part the results of the study of a chain of multiple phosphorylation-dephosphorylation cycles will be presented. We will discuss an approximation method for the exact solution in the bidimensional case and the relationship that this method has with the thermodynamic properties of the system, which is an open system far from equilibrium.In the last section the agreement between the theoretical prediction of the total protein quantity in a mouse cells population and the observed quantity will be shown, measured via fluorescence microscopy

GABA(B) receptors as potential therapeutic targets

gamma-Aminobutyric acid-B (GABA(B)) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. To date the only GABA(B) drug on the market is the agonist baclofen (Lioresal((R))) that is used to treat severe spasticity of cerebral and spinal origin. In addition baclofen is effective in animal models for many central and peripheral disorders, but side-effects and the development of tolerance prohibited a more widespread use of this drug in man. Similarly GABA(B) antagonists show great therapeutic promise but their shortcomings, e.g. the lack of brain penetration or some proconvulsive potential, prevented clinical development. The cloning of GABA(B) receptors in 1997 revived interest in these receptors as drug targets. The long-awaited availability of the tools that were necessary to develop more selective and safer drugs stimulated an impressive activity in the field. The demonstration that GABA(B) receptors needed to heteromerize for function provided new insights into the structure of G-protein coupled receptors in general and enabled to identify allosteric GABA(B) drugs. Gene knockout mice revealed neuronal systems that are under tonic GABA(B) control and therefore best suited for therapeutic intervention. Significant advances were made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate (GHB), a drug of abuse. Here we provide and update on the molecular composition, the physiology and the pharmacology of GABA(B) receptors and discuss to what extent our current knowledge influences ongoing and future drug discovery efforts

Régulation par les monoamines de l'expression de la vasopressine dans l'hypothalamus neuroendocrine (étude cytophysiologique chez la souris Tg8, knock-out pour la monoamine oxydase A (MAO-A))

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Le contrôle hypothalamique de l'homéostasie énergétique (impact de l'environnement maternel et implication du CNTF)

Le maintien de l homéostasie énergétique est placé sous le contrôle de cytokines qui agissent dans le système nerveux central, notamment au niveau de l hypothalamus. En particulier, la leptine, cytokine produite par le tissu adipeux, diminue la prise alimentaire et stimule la perte de poids. L obésité est une épidémie mondiale qui progresse de façon alarmante, notamment chez les enfants et souvent associée à des pathologies sévères et des désordres endocriniens comme la résistance à la leptine ou à l insuline. Le CNTF (Ciliary Neurotrophic Factor) est une neurocytokine de la même famille que la leptine dont l un des principaux avantages est qu il stimule la perte de poids dans les cas de leptino-résistance en activant les mêmes voies de signalisation que la leptine (Benomar et al., 2009). Face à l épidémie mondiale d obésité,chez la population adulte comme enfantine, il apparaît nécessaire de décrypter les mécanismes impliqués dans la genèse de la maladie ainsi que les potentiels agents thérapeutiques.L objectif premier de ce travail de thèse a été de caractériser l impact d une alimentation maternelle hyperlipidique (HF) sur les capacités de contrôle de l homéostasie énergétique chez la descendance. En effet, le concept de programmation métabolique propose que des pertubations de l environnement périnatal puissent influencer durablement la descendance, la rendant plus susceptible de développer une obésité dans un contexte nutrionnellement riche.Des études menées au sein du laboratoire ont montré qu un régime maternel HF pouvait programmer l acquisition de la leptino-résistance chez la descendance à l âge adulte (Ferezou-Viala et al., 2007b). Nous avons donc testé la prédispostion de ces animaux à prendre du poids lorsqu ils étaient nourris avec un régime hypercalorique (P). Nos données ont montréqu étonnamment, le régime maternel HF protégeait la descendance contre le gain de poids induite par le régime P, induisait des modifications d expression des marqueurs de l homéostasie énergétique dans le foie et l hypothalamus, ainsi que de profondes réorganisations cytoarchitectoniques dans le noyau arqué. Plus précisément, le régime maternel HF était associé à une réorganisation de la couverture astrocytaire périvasculaire dans le noyau arqué de la descendance qui persistait à l âge adulte.Dans une seconde partie de la thèse, nous avons étudié les mécanismes d action du CNTF. En effet, notre équipe a récemment mis en évidence que le CNTF endogène pourrait jouer un rôle dans la régulation de l homéostasie énergétique. Les niveaux hypothalamiques de cette cytokine, présente dans les astrocytes et les neurones du noyau arqué, augmentent chez les animaux résistant à une alimentation hypercalorique. Cela pourrait suggérer un rôle protecteur du CNTF contre la prise de poids chez certains individus (Vacher et al., 2008). A ce jour, les mécanismes d action du CNTF restent cependant mal compris car ce dernier ne possède pas de peptide signal et n est donc pas sécrété selon des mécanismes d exocytose classiques. Partant du constat que le CNTF et ses sous-unités réceptrices étaient distribuées de façon similaire dans les cellules du noyau arqué, nous avons émis l hypothèse que le CNTF pourrait exercer une action intracellulaire sur les cellules de cette structure. Dans cette étude nous démontrons que le CNTF peut interagir directement avec ses récepteurs dans le noyau des neurones anorexigènes du noyau arqué, pour réguler leur activité transcriptionnelle. Ces données proposent ainsi un nouveau mécanisme à l action anorexigène du CNTFObesity is a major health disease which involves numerous metabolic disorders. Increasing evidence suggests that the risk of developing the pathology in adulthood may be influenced through inappropriate perinatal nutrition. In our study, we first investigated the impact of a maternal high-fat (HF) diet, which is known to induce hypothalamic leptin resistance in adult offspring (Férézou-Viala et al., 2007), to develop obesity in a rich diet environment (P diet). Our results showed that surprisingly, HF maternal diet protected offspring against body weight gain induced by P diet. In a second part of the thesis, we studied mechanisms of action of CNTF, a neurocytokine which could protect some people against body weight gain induced by a P diet (Vacher et al., 2008). Results of this study showed that CNTF ant its subunits receptors could translocate to the hypothalamic cell nucleus to induced POMC transcription.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Leptin but not Ciliary Neurotrophic Factor (CNTF) induces Phosphotyrosine Phosphatase-1B expression in human neuronal cells (SH-SY5Y): putative explanation of CNTF efficacy in leptin-resistant state

Growing evidences suggest that obesity is associated with hypothalamic leptin resistance, leading to the alteration of food intake control. Alternative treatment using ciliary neurotrophic factor (CNTF) has been suggested because CNTF exerts a leptin-like effect, even in leptin-resistant states, but the mechanisms by which CNTF maintains this effect are not yet understood. Both leptin and CNTF act in the hypothalamus through similar signaling pathways including janus kinase-2/signal transducer and activator of transcription (STAT)-3 pathway. To explore the differences and interactions between leptin and CNTF signaling pathways, differentiated human neuroblastoma cells (SH-SY5Y) were exposed to either leptin or CNTF and then challenged for each cytokine. Leptin pretreatment completely abolished leptin-dependent STAT-3 and ERK 1/2 phosphorylations without affecting CNTF action. The lack of cross-desensitization between leptin and CNTF signaling pathways occurred despite the induction of suppressor of cytokine signaling-3 in response to both cytokines. Interestingly, leptin as well as insulin induced the expression of phosphotyrosine phosphatase (PTP)-1B, whereas CNTF treatment did not affect its expression. In addition, acute leptin treatment but not CNTF induced PTP-1B expression in mouse hypothalamic arcuate nucleus. Furthermore, the overexpression of human PTP-1B in SH-SY5Y cells completely abolished leptin-and insulin-dependent janus kinase-2, STAT-3, and ERK 1/2 phosphorylations, but CNTF action was not altered. Collectively, our results suggest that PTP-1B constitutes a key divergent element between leptin/insulin and CNTF signaling pathways at the neuronal level, which may constitute a possible mechanism that explains the efficacy of CNTF in leptin-resistant states. (Endocrinology 150: 1182-1191, 2009

Altered hippocampal expression of calbindin-D-28k and calretinin in GABA(B(1))-deficient mice

Balb/c GABA(B(1))(-/-) mice develop complex epileptiform activity, including spontaneous and audiogenic generalized seizures, 6-8 weeks after birth. The neuronal systems involved in these epilepsies have not been identified yet. Because the hippocampus is critically involved in epileptiform activity, we now investigated whether this brain region exhibits seizure-related alterations. Using semi-quantitative immunohistochemistry, we studied the temporal and cellular hippocampal expression pattern of two seizure-sensitive calcium-binding proteins, calbindin-D-28k and calretinin, in GABA(B(1))(-/-) mice. One month after birth, before the onset of overt epileptiform activity, wild-type (WT) and GABA(B(1))(-/-) mice exhibit comparable expression profiles for the two calcium-binding proteins. Three months after birth, once the epileptic phenotype is established, we observe clear alterations in the expression of calcium-binding proteins in the dentate gyrus area. GABA(B(1))(-/-) mice exhibit a 50% decline in the staining intensity of calbindin-D-28k expressing neurons and a 70% increase in the number of calretinin-positive neurons when compared to WT littermates. Six months after birth, the down-regulation of calbindin-D-28k protein is even more pronounced, while the calretinin expression in GABA(B(1))(-/-) mice reverts to the pattern seen in WT littermates. Our data demonstrate that the absence of functional GABA(B) receptors causes epileptiform activity through a mechanism that crucially involves dentate gyrus granule cells, and that this pathological activity is accompanied by adaptive changes

Cerebral cortex and hippocampus respond differently after post-natal exposure to uranium

The central nervous system (CNS) is known to be sensitive to pollutants during its development. Uranium (U) is a heavy metal that occurs naturally in the environment as a component of the earth's crust, and populations may therefore be chronically exposed to U through drinking water and food. Previous studies have shown that the CNS is a target of U in rats exposed in adulthood. We assessed the effects of U on behavior and cholinergic system of rats exposed from birth for 10 weeks at 10 mg.L-1 or 40 mg.L-1. For behavioral analysis, the sleep/wake cycle (recorded by telemetry), the object recognition memory and the spatial working memory (Y-maze) were evaluated. Acetylcholine (ACh) and acetylcholinesterase (AChE) levels were evaluated in the entorhinal cortex and hippocampus. At 40 mg.L-1, U exposure impaired object recognition memory (-20%), but neither spatial working memory nor the sleep/wake cycle was impaired. A significant decrease was observed in both the ACh concentration (-14%) and AChE activity (-14%) in the entorhinal cortex, but not in the hippocampus. Any significant effect on behaviour and cholinergic system was observed at 10 mg U.L-1. These results demonstrate that early exposure to U during postnatal life induces a structure cerebral-dependant cholinergic response and modifies such memory process in rats. This exposure to U early in life could have potential delayed effects in adulthood