Le glutamate est le principal neurotransmetteur excitateur du système nerveux central des vertébrés, et le codage de l’information cérébrale repose en partie sur des modulations de l’amplitude et de la fréquence des transmissions synaptiques glutamatergiques. De ce fait, la résolution spatiale et temporelle de ces transmissions nécessite un contrôle fin de la présence de glutamate dans la fente synaptique. Cette durée de vie du glutamate dans les synapses dépend directement de l’action de transporteurs spécifiques exprimés à la surface des astrocytes, en particulier les transporteurs de type GLT-1, qui retirent le neurotransmetteur et permettent ainsi de « nettoyer » la fente synaptique avant la survenue d’un nouvel épisode de neurotransmission. / A classic understanding of neurotransmitter clearance at glutamatergic synapses is that, in order to ensure sufficient glutamate uptake on a fast timescale, it is necessary to have high numbers of glutamate transporters in the vicinity of release sites to compensate for their slow transport kinetics. Using a combination of single molecule imaging and electrophysiological approaches, we now challenge this view by first demonstrating that GLT-1 transporters are not static but highly mobile at the surface of astrocytes, and that their surface diffusion is dependent upon both neuronal and glial cell activities. In the vicinity of glutamate synapses, GLT-1 dynamics are strongly reduced favoring their retention within this strategic location. Remarkably, glutamate uncaging at synaptic sites instantaneously increases GLT-1 diffusion, displacing the glutamate-bound transporter away from this compartment. Functionally, impairment of the transporter lateral diffusion through an antibody-based surface cross linking, both in vitro and in vivo, significantly slows the kinetics of excitatory postsynaptic currents. Taken together, these data reveal the unexpected and major role of the astrocytic surface GLT-1 fast dynamics in shaping glutamatergic synaptic transmission.Keywords:

<p>A modified liquid phase epitaxy (LPE) technique, called lateral diffusion LPE (LDLPE), is invented for low cost and high efficiency solar cell applications. Potentially, LDLPE is able to produce single crystalline silicon wafers directly from the raw material, rather than cutting wafers from single crystalline silicon ingots, therefore reducing the cost by avoiding the cutting and polishing processes.</p> <p>By using a traditional LPE method, the silicon is epitaxially grown on the silicon substrate by cooling down the saturated silicon/indium alloy solution from a high temperature. The silicon precipitates on the substrate since its solubility in the indium solvent decreases during the cooling process. A SiO₂ mask is formed on the (111) substrate with 100µm wide opening windows as seedlines. Silicon is epitaxially grown on the seedline and forms thick epitaxial lateral overgrowth (ELO) layers on the oxide mask. The ELO layers are silicon strips with an aspect ratio of 1:1 (width: thickness), approximately. The strip grows both laterally in width and vertically in thickness.</p> <p>The concept of LDLPE is to intentionally block the silicon diffusion path from the top of the seedline, but leave the lateral diffusion path from the bulk indium melt to the seedline. Theoretically, by using the LDLPE method, the silicon strip should have a larger aspect ratio, because the laterally growth in width is allowed but the vertical growth in thickness is limited. In addition, single crystalline silicon wafers can be achieved if the strip grows continuously.</p> <p>A graphite slide boat is designed to place a plate over the seedline to block the diffusion path of silicon atoms from the top of the seedline. After one growth cycle, silicon strips grown by LDLPE are wider than LPE strips but have similar thicknesses. The aspect ratios are increased from 1:1 to a number larger than 2:1. A Monte-Carlo random walk model is used to simulate the change of LDLPE strip aspect ratio caused by placing a plate over the seedline.</p> <p>Wetting seedline by indium melt is very critical for a successful growth. Due to the small space between the plate and seedline and the surface tension of the indium melt, the indium melt cannot flow into the small space. A pre-wetting technique is used to fill the space prior to loading the graphite boat into the tube furnace and solve the wetting problem successfully.</p> <p>The structure of a LDLPE silicon strip is characterized by X-ray diffraction. The electrical properties are characterized by Hall Effect measurement and photoconductive decay measurement. LDLPE silicon strips are (111) orientated single crystal and are the same orientation as the substrate. For the growth temperature of 950°C, the LDLPE strip has an estimated effective minority carrier lifetime of 30.9µs. The experimental results demonstrate that LDLPE is feasible for photovoltaic application if continuous growth and scaling up can be achieved.</p> / Doctor of Philosophy (PhD)

Ce travail de thèse a été consacré à l'élaboration d'un nouveau type de membranes de nanofiltration efficaces avec des propriétés améliorées (flux élevé et rétention élevée, et de bonnes propriétés mécaniques) en déposant un revêtement assemblé couche-par-couche (LbL) sur des supports poreux. Après avoir systématiquement étudié le mécanisme de croissance des films assemblés couche par couche des polyélectrolytes choisis et la relation entre les structures de ces films et les performances des membranes résultant, nous avons identifié avec succès les meilleures structures multicouches pour la construction de membranes de nanofiltration de référence avec des performances optimales. En outre, en prenant avantage de la technique LbL, nous avons introduit une couche de diffusion latérale assemblée soit de nanofibrilles de cellulose ou de nanotubes de carbone, qui permet d’augmenter le flux de 30% tout en conservant la même rétention par rapport à la membrane de référence. En plus, les films assemblés à base de chitosan et nanofibrils de cellulose ont montré une forte résistance à la traction allant jusqu’à 450 MPa et un module d’Young jusqu’à 50 GPa. / This thesis work was devoted to the development of a novel and efficient nanofiltration membrane with improved properties (high flux and high retention, good mechanical strength) by coating Layer-by-Layer (LbL) assembled films onto porous membrane support. After having systematically studied the growth mechanism of LbL-assembled films of chosen polyelectrolytes and the relationship between the structures of these films and the membrane performance of the resulting NF membranes, we successfully identified the best multilayer structures for constructing nanofiltration membranes (NF) of reference with optimal membrane performance. Furthermore, taking advantages of the LbL-assembly, we successfully introduced LbL-assembled lateral diffusion layer that is made of either cellulose nanofibrils or carbon nanotubes, which in turn led to membranes with 30% higher flux. In addition, the LbL-assembled films of chitosan and cellulose nanofibrils showed surprisingly strong tensile strength of up to 450 MPa and a high Young modulus of up to 50 GPa.