No description available.

The Hampton Roads Sanitation District (HRSD) operates thirteen treatment plants in the eastern Virginia area with a combined capacity of 231 million gallons per day (mgd). The Nansemond Treatment Plant (NTP) is one of the larger facilities, and is designed to treat 30 mgd using a 3-stage Virginia Initiative Process (VIP) biological nutrient removal (BNR) process. The majority of the influent is domestic, but there is also a large industrial contribution, particularly from a hog processing facility, landfill leachate, and significant loads from septage and grease deliveries (Bilyk et al, 2008). NTP is currently being upgraded to a 5-stage Bardenpho process to achieve improved total nitrogen (TN) removal. For several years starting in about 2001, NTP has experienced continuous and sporadic nitrification upsets that cannot be explained by plant operations events. Sporadic nitrification upsets are characterized by sharp increases in effluent ammonia and nitrite with decreases in nitrate concentrations due to reduced growth rates in bacteria. The result is reduced overall total nitrogen (TN) removal. Continuous inhibition is evidenced by a previous engineering report by Hazen and Sawyer, P.C. (2007), whereby it was suggested that the ammonia oxidizing bacteria (AOB) maximum specific growth rate (μmax) be reduced from 0.9 to 0.57 days-1. This has significant implications in terms of the required aeration volume for consistent nitrification at cold temperatures. The objective of this project was to determine whether the NTP influent wastewater does in fact exhibit inhibition to ammonia (AOB) and nitrite oxidizing bacteria (NOB), evaluated independently, and to determine the impact on polyphosphate accumulating organism activity (PAO). Because the historical operational experiences and data analysis suggested inhibited AOB and NOB activity, an investigation was initiated targeting the source of that inhibition. After conducting seventeen weeks of batch experiments the source of inhibition was not determined. Batch experiments however, did reveal other possible sources of inhibition including large amounts of chemical toilet waste received at NTP possibly containing quaternary ammonium compounds (QACs). Due to available blower capacity during construction it was planned that nitrification would not be maintained during the fall of 2009. In an effort to stop nitrification, the solids retention time (SRT) was purposely reduced over a period of about one month (as wastewater temperature cooled) until additional blower capacity was available. This provided an opportunity to study baseline nitrification kinetics and determine the potential for continuous inhibition through profile sampling. Simulation modeling of the profile sampling and plant data was performed with Biowin 3.1 (EnviroSim, Ltd.) as a means for comparison and to generate μmax values for AOB to compare with the original design μmax of 0.57-1. Profile sampling was conducted from the primary effluent to the secondary effluent with samples collected along the length of the BNR process. This was being done to address the following issues: • Conduct baseline sampling prior to a more detailed nitrification inhibition study estimated to begin in May 2010, which will include influent sampling and the operation of bench-scale sequencing batch reactors. This will be used to establish "normal" COD, nutrient and DO profiles though the VIP process without (and possibly with) the impact of inhibitory conditions, specifically with respect to N conversions and P release and uptake along the process. • Evaluate the potential for nitrite accumulation in the process and its potential effect on aerobic phosphate uptake by phosphorus accumulating organisms (PAOs). • Evaluate the impact of sporadic ferric chloride addition to the biological process as a means of preventing effluent TP exceedances. • Evaluate the design μmax to the actual observed μmax for AOB through simulation modeling. • Compare modeling and observed profile data for signs of any continuous nitrification inhibition. Experimental results from batch-rate testing confirmed the sporadically inhibitory nature of NTP primary effluent when combined with other stable nitrifying biomasses. Investigation into quaternary ammonium compounds (QACs) which were contained in the chemical toilet waste suggested that QACs at higher concentrations caused some inhibition of NOB activity, but no significant impact on AOB activity. Profile sampling demonstrated no signs of sporadic or continuous nitrification inhibition or impact of nitrite accumulation and ferric chloride addition on biological treatment processes. Modeling of the profile data generated similar profiles; however, there were slight variations as the model predicted nitrification to stop earlier than what was actually observed. From the modeling it was also determined that the maximum specific growth rate (μmax) of ammonia oxidizing bacteria (AOB) was in the range of 0.50 – 60 days-1. This supported batch and profile work that showed NTP PE exhibited some degree of continuous inhibition. Diurnal loadings however, were not accounted for in the modeling which could slightly underestimate the actual AOB μmax value. Several suspected inhibitors were eliminated as potential causes of inhibition, including waste from a hog processing facility, landfill leachate, the addition of ferric chloride, plant internal recycle streams, branches of the collection system, and chemical toilet disinfectants containing QACs. References Bilyk, K., Cubbage, L., Stone, A., Pitt, P., Dano, J., and Balzer, B. 2008. Unlocking the Mystery of Biological Phosphorus Removal Upsets and Inhibited Nitrification at a 30 mgd BNR Facility. Proceedings of the Water Environment Federation Technical Conference and Exposition, 2008. Hazen and Sawyer. 2007. Nansemond Treatment Plant Nutrient Reduction Improvement Technical Memorandum. / Master of Science

L’hippocampe joue un rôle central dans la navigation spatiale, la mémoire et l’organisation spatio-temporelle des souvenirs. Ces fonctions sont maintenues par la capacité du gyrus denté (GD) de séparation des patrons d'activité neuronales. Le GD est situé à l’entrée de la formation hippocampique où il reconnaît la présence de nouveaux motifs parmi la densité de signaux afférant arrivant par la voie entorhinale (voie perforante). Le codage parcimonieux est la marque distinctive du GD. Ce type de codage est le résultat de la faible excitabilité intrinsèque des cellules granulaires (CGs) en combinaison avec une inhibition locale prédominante. En particulier, l’inhibition de type « feedforward » ou circuit inhibiteur antérograde, est engagée par la voie perforante en même temps que les CGs. Ainsi les interneurones du circuit antérograde fournissent des signaux GABAergique aux CGs de manière presque simultanée qu’elles reçoivent les signaux glutamatergiques. Cette thèse est centrée sur l’étude des interactions entre ces signaux excitateurs de la voie entorhinale et les signaux inhibiteurs provenant des interneurones résidant dans le GD et ceci dans le contexte du codage parcimonieux et le patron de décharge en rafale caractéristique des cellules granulaires. Nous avons adressé les relations entre les projections entorhinales et le réseau inhibitoire antérograde du GD en faisant des enregistrements électrophysiologiques des CG pendant que la voie perforante est stimulée de manière électrique ou optogénétique. Nous avons découvert un nouvel mécanisme d’inhibition qui apparait à délais dans les CGs suite à une stimulation dans les fréquences gamma. Ce mécanisme induit une hyperpolarisation de longue durée (HLD) et d’une amplitude prononce. Cette longue hyperpolarisation est particulièrement prolongée et dépasse la durée d’autres types d’inhibition transitoire lente décrits chez les CGs. L’induction de HLD crée une fenêtre temporaire de faible excitabilité suite à laquelle le patron de décharge des CGs et l’intégration d’autres signaux excitateurs sont altérés de manière transitoire. Nous avons donc conclu que l’activité inhibitrice antérograde joue un rôle central dans les processus de codage dans le GD. Cependant, alors qu’il existe une multitude d’études décrivant les interneurones qui font partie de ce circuit inhibiteur, la question de comment ces cellules sont recrutées par la voie entorhinale reste quelque peu explorée. Pour apprendre plus à ce sujet, nous avons enregistré des interneurones résidant iii dans la couche moléculaire du GD tout en stimulant la voie perforante de manière optogénétique. Cette méthode de stimulation nous a permis d’induire la libération de glutamate endogène des terminales entorhinales et ainsi d’observer le recrutement purement synaptique d’interneurones. De manière surprenante, les résultats de cette expérience démontrent un faible taux d’activation des interneurones, accompagné d’un tout aussi faible nombre total de potentiels d’action émis en réponse à la stimulation même à haute fréquence. Ce constat semble contre-intuitif étant donné qu’en générale on assume qu’une forte activité inhibitrice est requise pour le maintien du codage parcimonieux. Tout de même, l’analyse des patrons de décharge des interneurones qui ont été activés a fait ressortir la prééminence de trois grands types: décharge précoce, retardée ou régulière par rapport le début des pulses lumineux. Les résultats obtenus durant cette thèse mettent la lumière sur l’important conséquences fonctionnelles des interactions synaptique et polysynaptique de nature transitoire dans les réseaux neuronaux. Nous aimerions aussi souligner l’effet prononcé de l’inhibition à court terme du type prolongée sur l’excitabilité des neurones et leurs capacités d’émettre des potentiels d’action. De plus que cet effet est encore plus prononcé dans le cas de HLD dont la durée dépasse souvent la seconde et altère l’intégration d’autres signaux arrivants simultanément. Donc on croit que les effets de HLD se traduisent au niveau du réseaux neuronal du GD comme une composante cruciale pour le codage parcimonieux. En effet, ce type de codage semble être la marque distinctive de cette région étant donné que nous avons aussi observé un faible niveau d’activation chez les interneurones. Cependant, le manque d’activité accrue du réseau inhibiteur antérograde peut être compensé par le maintien d’un gradient GABAergique constant à travers le GD via l’alternance des trois modes de décharges des interneurones. En conclusion, il semble que le codage parcimonieux dans le GD peut être préservé même en absence d’activité soutenue du réseau inhibiteur antérograde et ceci grâce à des mécanismes alternatives d’inhibition prolongée à court terme. / The hippocampus is implicated in spatial navigation, the generation and recall of memories, as well as their spatio-temporal organization. These functions are supported by the processes of pattern separation that occurs in the dentate gyrus (DG). Situated at the entry of the hippocampal formation, the DG is well placed to detect and sort novelty patterns amongst the high-density excitatory signals that arrive via the entorhinal cortex (EC). A hallmark of the DG is sparse encoding that is enabled by a combination of low intrinsic excitability of the principal cells and local inhibition. Feedforward inhibition (FFI) is recruited directly by the EC and simultaneously with the granule cells (GCs). Therefore, FFI provides fast GABA release and shapes input integration at the millisecond time scale. This thesis aimed to investigate the interplay of entorhinal excitatory signals with GCs and interneurons, from the FFI in the DG, in the framework of sparse encoding and GC’s characteristic burst firing. We addressed the long-range excitation – local inhibitory network interactions using electrophysiological recordings of GCs – while applying an electrical or optogenetic stimulation of the perforant path (PP) in the DG. We discovered and described a novel delayed-onset inhibitory post synaptic potential (IPSP) in GCs, following PP stimulation in the gamma frequency range. Most importantly, the IPSP was characterized by a large amplitude and prolonged decay, outlasting previously described slow inhibitory events in GCs. The long-lasting hyperpolarization (LLH) caused by the slow IPSPs generates a low excitability time window, alters the GCs firing pattern, and interferes with other stimuli that arrive simultaneously. FFI is therefore a key player in the computational processes that occurs in the DG. However, while many studies have been dedicated to the description of the various types of the interneurons from the FFI, the question of how these cells are synaptically recruited by the EC remains not entirely elucidated. We tackled this problem by recording from interneurons in the DG molecular layer during PP-specific optogenetic stimulation. Light-driven activation of the EC terminals enabled a purely synaptic recruitment of interneurons via endogenous glutamate release. We found that this method of stimulation recruits only a subset of interneurons. In addition, the total number of action potentials (AP) was surprisingly low even at high frequency stimulation. This result is counterintuitive, as strong and persistent inhibitory signals are assumed to restrict GC v activation and maintain sparseness. However, amongst the early firing interneurons, late and regular spiking patterns were clearly distinguishable. Interestingly, some interneurons expressed LLH similar to the GCs, arguing that it could be a commonly used mechanism for regulation of excitability across the hippocampal network. In summary, we show that slow inhibition can result in a prolonged hyperpolarization that significantly alters concurrent input’s integration. We believe that these interactions contribute to important computational processes such as sparse encoding. Interestingly, sparseness seems to be the hallmark of the DG, as we observed a rather low activation of the interneuron network as well. However, the alternating firing of ML-INs could compensate the lack of persistent activity by the continuous GABA release across the DG. Taken together these results offer an insight into a mechanism of feedforward inhibition serving as a sparse neural code generator in the DG.

Le contrôle du biofouling sur des surfaces inertes immergées ou en atmosphère humide est une nécessité dans le secteur marin, tant pour des raisons économiques qu’environnementales. La formation de biofilm microbien, étape préalable à la formation du biofouling, est souvent intrinsèquement liée chez les bactéries au système de communication “Quorum Sensing” (QS). Chez certaines bactéries Gram négatif, le QS est basé sur la perception de petites molécules diffusibles appelées N- Acyl Homosérine Lactones (AHLs). L’une des stratégies antifouling en voie de développement de nos jours repose sur l’inhibition du QS bactérien. L’objectif de cette thèse est d’utiliser certaines bactéries marines afin d’identifier des molécules anti-QS capables de perturber la formation de biofilm. Ce travail a donc porté sur la mise en évidence de molécules AHLs impliquées dans le QS chez certaines bactéries marines isolées de la rade de Toulon, l’étude de la modulation de certains phénotypes dont la formation du biofilm, par ces molécules et, la mise en place d’un test préliminaire d’inhibition du QS. Parmi les trois bactéries isolées de la rade de Toulon (TC8, TC14 et TC15) du genre Pseudoalteromonas, connues pour produire de nombreuses molécules actives, et testées pour leur capacité à sécréter des AHLs, seule Pseudoalteromonas sp. TC15 a produit la C12-HSL. P. ulvae TC14, capable de produire un biofilm conséquent et de la violacéine, ne produit aucune AHL. Afin d’évaluer la possibilité d’utiliser une bactérie marine comme outil de criblage anti-QS, interférant avec les AHLs et les conséquences sur son biofilm, des AHLs exogènes ont été testées sur la production de violacéine, la formation de biofilm et la mobilité de TC14. Certaines AHLs ont montré qu’elles pouvaient réguler la production de violacéine et la formation de biofilm chez TC14, suggérant l’existence d’un récepteur AHLs fonctionnel. Des tests préliminaires d’inhibition du QS ont été effectués avec des molécules commerciales et des analogues synthétiques. La 3-oxo-C6-HSL commerciale, ainsi que l’esculétine et la p- benzoquinone, connues pour interférer avec le QS bactérien, ont été capables d’inhiber la production de violacéine ainsi que la formation de biofilm de TC14 à des concentrations n’affectant pas sa croissance. Cette étude suggère donc que P. ulvae TC14 pourrait être utilisée comme un outil de recherche de molécules anti-QS en conditions proches de celles trouvées dans l’environnement marin, et ce dans le but d’être ultérieurement testées sur la formation de biofilm. L’objectif à plus long terme reste de trouver un moyen de limiter la formation du biofilm en utilisant des molécules non toxiques pour l’environnement. / The biofouling control on immersed inert surfaces or in moist atmosphere is a necessity in the marine sector for both economic and environmental reasons. Microbial biofilm formation, the initial step of biofouling development, is intrinsically linked to the communication system “Quorum sensing” (QS). In some Gram negative bacteria, QS is based on the perception of small diffusible signaling molecules called Acyl Homoserine Lactones (AHLs). The inhibition of bacterial QS is part of the different antifouling strategies currently developed. This present work focused on the detection of AHLs molecules involved in this communication system in bacteria isolated from Toulon harbor and the study of modulation of some phenotypes, including biofilm formation, by these molecules as well as the development of a preliminary anti-QS assay. Three marine bacteria isolated from Toulon harbor (TC8, TC14 and TC15), belonging to the Pseudoalteromonas genus, known to synthesize many active molecules, have been tested for their ability to produce AHLs. Only Pseudoalteromonas sp. TC15 produced the C12-HSL. P. ulvae TC14 a violacein-producing and biofilm-forming bacteria, did not secrete any AHLs. Few marine bacteria are used as an anti-QS screening tool, especially by interfering with AHLs with the goal of studying the consequences on biofilm formation. In order to evaluate the possibility to use TC14 with this purpose, exogenous AHLs were tested on the violacein production, the biofilm formation and the motility of TC14. Some AHLs were able to regulate violacein production and biofilm formation suggesting the presence of a functional AHLs receptor in TC14. Preliminary QS inhibition assays were performed with commercial molecules and synthetic analogues. The commercial 3-oxo-C6-HSL as well as esculetin and p-benzoquinone, known to interfere with bacterial QS, were able to inhibit QS and biofilm formation at a non-toxic concentration. Overall, this study suggests that the marine strain P. ulvae TC14 may be used as a tool for the detection of anti-QS molecules in conditions closed to the marine environment. These molecules may subsequently be tested on the biofilm formation of marine bacteria. The long term objective is to find a way to limit biofilm formation, using non-toxic molecules for the environment.

Object recognition in the visual cortex is based on a hierarchical architecture, in which specialized brain regions along the ventral pathway extract object features of increasing levels of complexity, accompanied by greater invariance in stimulus size, position, and orientation. Recent theoretical studies postulate a non-linear pooling function, such as the maximum (MAX) operation could be fundamental in achieving such invariance. In this paper, we are concerned with neurally plausible mechanisms that may be involved in realizing the MAX operation. Four canonical circuits are proposed, each based on neural mechanisms that have been previously discussed in the context of cortical processing. Through simulations and mathematical analysis, we examine the relative performance and robustness of these mechanisms. We derive experimentally verifiable predictions for each circuit and discuss their respective physiological considerations.

Thesis (Ph. D.)--Catholic University of America, 1940. / Bibliography: p. 42-43.

En raison des de l’augmentation de la pollution à l’échelle mondial du notamment aux gaz d'échappement des automobiles, de nouvelles réglementations d'émissions ont été mises en place depuis les années 1990. Ces règlementations ont conduit à une évolution des carburants traditionnels vers les biocarburants et à des systèmes de post-traitement des gaz d'échappement, notamment pour les moteurs diesel, de plus en plus sophistiqués. Ils comprennent à ce jour une série d'unités catalytiques, contenant un filtre à particules, un catalyseur d’oxydation pour traiter le monoxyde de carbone et les hydrocarbures et enfin un catalyseur pour réduire les oxydes d’azote. Dans ce travail de thèse, deux de ces systèmes catalytiques industriels ont été étudiés, les catalyseurs à l'oxydation diesel (DOC) et les catalyseurs de réduction catalytique sélective (SCR). Une attention particulière est accordée à leur performance en présence de composés métaux alcalins. En effet, les alcalins sont présent dans les biocarburants et lorsque le biodiesel est utilisé comme carburant de remplacement, ils vont donc être présents dans les gaz d’échappement et à priori dans les systèmes de post-traitement. / Due to global lean exhaust gas and new emission regulations, exhaust after-treatment systems of diesel engines are getting more and more sophisticated and comprise a series of catalytic units. In the present work, two of these catalytic systems were studied, Diesel Oxidation Catalysts (DOC) and Selective Catalytic Reduction (SCR) catalysts. Particular attention is paid to their performance in the presence of alkali compounds when bio-diesel is utilized as the alternative fuel.Firstly, this thesis focuses on the catalytic behavior of the Diesel Oxidation Catalyst using different aging characteristics of road mileage in order to improve the efficiency of an ammonia SCR system on an after-treatment line composed by a DOC + DPF + SCR. The studied catalyst is a commercial diesel oxidation catalyst (Pt/Pd/Al2O3) provided by Continental. Hydrothermal aging under different conditions on carrots of monolith were performed. Also studied in the monolith form over the commercial DOC, the influence of the addition of different alkali metal species (K and Na) on the commercial DOC through catalytic tests performed on this structured catalyst under multicomponent (C3H6 / CO / NO / NO2) co-feeding conditions was explored. Aiming at investigating the effects of the presence of different alkali metal species on the DOC at the level of active phase, homemade bimetallic DOC is prepared and then different alkali metal species incorporated. Finally, encouraged by the evident influence of alkali compounds on DOCs, their impacts on the downstream SCR catalysts are also studied in this thesis. The studied SCR catalyst is a commercial V2O5-based catalyst provided by UMICORE company.

Following exposure to consistent stimulus–stop mappings, response inhibition can become automatised with practice. What is learned is less clear, even though this has important theoretical and practical implications. The main contribution of this thesis is to investigate how stimulus-stop associations are acquired and the conditions under which they influence behaviour. To this end, this thesis addressed several outstanding issues concerning the associative architecture of stop learning, the role of expectancies, and the specificity of learning in inhibition tasks. Experiments 1-4 provide evidence that participants can acquire direct associations between specific stimuli and the stop goal without mediation via a single representation of the stop signal. However, these experiments also suggest that the influence of stimulus-stop associations on behaviour depends on top-down attentional settings: if participants begin to ignore the stop-associated stimuli, the effects of stop learning are diminished or eliminated entirely. Across eight experiments, this thesis provides evidence that participants generate expectancies during stop learning that are consistent with the stimulus-stop contingencies in play. However, Experiments 5-6 indicate that there may be some differences in the relationships between stimulus-stop expectancies and task performance under instructed and uninstructed conditions; stimulus-stop associations that are acquired via task instructions or via task practice have similar effects on behaviour, but seem to differ in how they trigger response slowing for the stop-associated items. Experiments 7-8 investigated the role of signal detection processes during the acquisition of stimulus-stop associations. To distinguish between stimulus-stop learning and stimulus-signal learning, the contingencies between specific stimuli and the stop goal and the contingencies between specific stimuli and the spatial location of the stop signal were independently manipulated. Although these experiments showed evidence of stop/go (goal) learning, there was no evidence that participants acquired the stimulus-signal associations. Across four experiments, this thesis investigated the specificity of stop learning. Experiments 9-10 compared the effects of training on behavioural performance in inhibition (go/no-go) and non-inhibition (two-choice) tasks. The results of these experiments revealed that learning in inhibition and non-inhibition tasks could arise through similar associative mechanisms, but suggest that the effects of training in these tasks could also depend on top-down response settings and general non-associative processes. Experiments 11-12 investigated the neural specificity of stop learning. These experiments also revealed similar effects of training across the go/no-go and two-choice tasks adding weight to the claim that training in inhibition tasks primarily influences task-general processes. Combined, the overall conclusion of this thesis is that bottom-up control can influence response inhibition but what is learned depends on top-down factors. It is therefore important to consider bottom-up factors and top-down factors as dependent, rather than independent, influences on response inhibition.

Tissue transglutaminase (TG2) is a calcium-dependent enzyme that natively catalyses the formation of isopeptidic bonds between protein- or peptide-bound glutamine and lysine residues. Physiologically, it is ubiquitously expressed in tissues, with roles in cellular differentiation, extracellular matrix stabilisation, and apoptosis, among others. However, its unregulated activity has been associated with various pathologies including fibrosis, cancer and celiac disease. Since most pathologies are associated with an increased transamidation activity, efforts have been directed towards the development of TG2 inhibitors. In this context, the work described in this thesis is centred on reversible inhibitors, building on recent work done within the Keillor group in two directions, namely localisation and potency. In a localisation-driven approach, we developed a photolabile derivative of a known reversible inhibitor, in order to form a covalent bond with the enzyme and determine the inhibitor’s binding site. In tandem, we optimised a protocol for the expression of TG2 incorporating ArgΔ10 and LysΔ8, amino acids that are 13C- and 15N-labelled to provide a mass shift of 10 and 8 Da, respectively, compared to the corresponding unlabelled amino acids. This “heavy” TG2 was developed as a tool for reference in the analysis of the tryptic digest of labelled protein. In a potency-driven approach, based on the observation that previous trans cinnamoyl inhibitor scaffolds were susceptible to nucleophilic attack by glutathione, we developed a bis(triazole) scaffold with reduced electrophilicity. The preparation of a small library of compounds showed that this scaffold demonstrates a preference for electron-withdrawing substituents, such as nitro groups. Continuing in a potency-driven approach, and inspired by work done in the identification of glutathione-resistant scaffolds, we studied a new alkynyl scaffold. While still susceptible to glutathione addition, these compounds showed a marked improvement in potency, with the lead compound having an IC50 of 930 nM and being established as a competitive inhibitor with a Ki of 420 nM, our most potent reversible inhibitor to date. Furthermore, this scaffold also produced an inhibitor lacking nitro groups (to limit eventual cellular toxicity), but maintaining good potency, with an IC50 value of 3.03 μM.

Les petites GTPases sont des régulateurs majeurs de nombreux processus cellulaires. La dérégulation de l’activation des petites GTPases est à l’origine de nombreuses maladies comme, entre autres, certains diabètes et cancers. In vivo, l’activation des petites GTPases se fait par des facteurs d’échange nucléotidiques (GEF), qui interagissent avec les GTPases à la périphérie des membranes cellulaires. Au delà d’un simple lieu de co-localisation, les membranes biologiques possèdent des propriétés physico-chimiques impactant directement l’activation des petites GTPases par les GEFs. Ce projet de thèse s’articule autour de trois axes, 1) proposer une stratégie expérimentale pour mesurer quantitativement les effets des membranes dans cette activation, 2) établir un modèle d’activation à la périphérie des membranes du GEF EPAC1, cible thérapeutique de maladies cardiaques 3) caractériser des petites molécules inhibitrices connues d’ArfGEF dans un contexte membranaire. Les résultats ont montré que les membranes modifiaient l’efficacité catalytique des GEFs, et questionnait leur spécificité vis à vis des petites GTPases. Les membranes apparaissent également comme de véritables actrices de l’activation d’EPAC1 en coopération avec l’AMPc. Ces effets pourraient être expliqués par une colocalisation entre GEFs et GTPases à la surface des membranes, l’induction d’un réarrangement conformationnel du GEF par les membranes, une modification de la diffusion latérale des GEF, ou encore une géométrie catalytiquement avantageuse du complexe GEF-GTPase-membrane. Enfin comprendre et expliciter l’implication des membranes dans cette activation amène à imaginer de nouvelles stratégies d’inhibition thérapeutique. / Small GTPases are major regulators of many cellular processes. Nucleotide exchange factors (GEF) activate small GTPases. Deregulation of the activation of small GTPases is at the origin of several diseases, such as certain diabetes and cancers. GTPases and GEFs interact together at the periphery of cell membranes. Beyond a simple place of co-localization, biological membranes have physicochemical properties directly impacting the activation of small GTPases by GEFs. This thesis project is based on three axes, 1) to propose an experimental strategy to quantitatively measure the effects of membranes in this activation 2) to establish a model of the activation at the periphery of membranes of the GEF EPAC1, a therapeutic target in heart diseases, 3) to characterize known ArfGEF inhibitory small molecules in a membrane context. The results showed that membranes modified GEF catalytic efficiency, and questioned their specificity towards small GTPases. The membranes also appear as partners for the activation of EPAC1 in cooperation with cAMP. These effects could be explained by a co-localization between GEF and GTPases on the membranes surfaces, a conformational rearrangement of the GEF induced by membranes, a modification of lateral diffusion of the GEF, or a catalytically advantageous geometry of the GEF-GTPase-membrane complex. Finally, understanding the involvement of membranes in this activation leads us to imagine new therapeutic inhibition strategies.