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<p>Early Xenopus laevis development involves highly complex morphogenic movements. Two key movements are gastrulation, which establishes germ layer spatial arrangement, and neurulation, which results in the folding and closure of the neural tube. Multiple signaling pathways are involved in regulating cell adhesion, migration, shape and polarity during these processes to ensure normal development. Two of the most characterized pathways are the canonical and non-canonical Wnt pathways. However, the roles of all the individual molecules involved are not fully understood. In this thesis I provide initial characterization of the POZ-ZF transcription factor xZnf131. Znf131 is a transcriptional activator and its binding partner Kaiso negatively regulates this function. Since Znf131 and Kaiso display antagonistic roles and Kaiso mediates Wnt signaling and morphogenesis during Xenopus gastrulation and neurulation I hypothesize that xZnf131 is also required to regulate morphogenesis during these key developmental events.</p> <p>Like other POZ-ZF proteins, xZnf131 contains an amino-terminal POZ domain and a carboxy-terminal ZF domain comprised of five zinc fingers. xZnf131 is continuously expressed through early Xenopus development but was spatially localized to the dorsal and anterior structures of the embryo, notably the neural plate. Morpholino oligonucleotide (MO) knockdown of xZnf131 resulted in severe defects in notochord and neural plate formation, with abnormal cell morphology, typical of non-canonical Wnt misregulation. Interestingly, xZnf131 overexpression produced phenotypes very similar to xZnf131 knockdown suggesting that xZnf131 protein levels need to be tightly maintained to regulate the correct/normal morphogenic movements during Xenopus gastrulation and neurulation.</p> <p>Our findings indicate that xZnf131 plays a role in the morphogenic movements during Xenopus gastrulation and neurulation. Our data provides a useful foundation for future experiments to elucidate the biological mechanism of xZnf131 action during these key developmental processes.</p> / Master of Science (MSc)

Yes / In a previous study we identified an extensive gating network within the inwardly rectifying Kir1.1 (ROMK) channel by combining systematic scanning mutagenesis and functional analysis with structural models of the channel in the closed, pre-open and open states. This extensive network appeared to stabilize the open and pre-open states, but the network fragmented upon channel closure. In this study we have analyzed the gating kinetics of different mutations within key parts of this gating network. These results suggest that the structure of the transition state (TS), which connects the pre-open and closed states of the channel, more closely resembles the structure of the pre-open state. Furthermore, the G-loop, which occurs at the center of this extensive gating network, appears to become unstructured in the TS because mutations within this region have a 'catalytic' effect upon the channel gating kinetics. / Deutsche Forschungsgemeinschaft, the Wellcome Trust (083547/ Z/07/Z and 092970/Z/10/Z) and the British Heart Foundation (PG/09/016/ 26992).

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Les résultats présentés dans cette thèse précisent certains aspects de la fonction du cotransporteur Na+/glucose (SGLT1), une protéine transmembranaire qui utilise le gradient électrochimique favorable des ions Na+ afin d’accumuler le glucose à l’intérieur des cellules épithéliales de l’intestin grêle et du rein. Nous avons tout d’abord utilisé l’électrophysiologie à deux microélectrodes sur des ovocytes de xénope afin d’identifier les ions qui constituaient le courant de fuite de SGLT1, un courant mesuré en absence de glucose qui est découplé de la stoechiométrie stricte de 2 Na+/1 glucose caractérisant le cotransport. Nos résultats ont démontré que des cations comme le Li+, le K+ et le Cs+, qui n’interagissent que faiblement avec les sites de liaison de SGLT1 et ne permettent pas les conformations engendrées par la liaison du Na+, pouvaient néanmoins générer un courant de fuite d’amplitude comparable à celui mesuré en présence de Na+. Ceci suggère que le courant de fuite traverse SGLT1 en utilisant une voie de perméation différente de celle définie par les changements de conformation propres au cotransport Na+/glucose, possiblement similaire à celle empruntée par la perméabilité à l’eau passive. Dans un deuxième temps, nous avons cherché à estimer la vitesse des cycles de cotransport de SGLT1 à l’aide de la technique de la trappe ionique, selon laquelle le large bout d’une électrode sélective (~100 μm) est pressé contre la membrane plasmique d’un ovocyte et circonscrit ainsi un petit volume de solution extracellulaire que l’on nomme la trappe. Les variations de concentration ionique se produisant dans la trappe en conséquence de l’activité de SGLT1 nous ont permis de déduire que le cotransport Na+/glucose s’effectuait à un rythme d’environ 13 s-1 lorsque le potentiel membranaire était fixé à -155 mV. Suite à cela, nous nous sommes intéressés au développement d’un modèle cinétique de SGLT1. En se servant de l’algorithme du recuit simulé, nous avons construit un schéma cinétique à 7 états reproduisant de façon précise les courants du cotransporteur en fonction du Na+ et du glucose extracellulaire. Notre modèle prédit qu’en présence d’une concentration saturante de glucose, la réorientation dans la membrane de SGLT1 suivant le relâchement intracellulaire de ses substrats est l’étape qui limite la vitesse de cotransport. / The results presented in this thesis clarify certain functional aspects of the Na+/glucose cotransporter (SGLT1), a membrane protein which uses the downhill electrochemical gradient of Na+ ions to drive the accumulation of glucose in epithelial cells of the small intestine and the kidney. We first used two microelectrodes electrophysiology on Xenopus oocytes to indentify the ionic species mediating the leak current of SGLT1, a current measured in the absence of glucose that is uncoupled from the strict 2 Na+/1 glucose stoichiometry characterising cotransport. Our results showed that cations such as Li+, K+ and Cs+, which interact weakly with SGLT1 binding sites and are unable to generate the conformational changes that are triggered by Na+ binding, were however able to generate leak currents similar in amplitude to the one measured in the presence of Na+. This suggests that the leak current permeating through SGLT1 does so using a pathway that differs from the conformational changes associated with Na+/glucose cotransport. Moreover, it was found that the cationic leak and the passive water permeability could share a common pathway. We then sought to estimate the turnover rate of SGLT1 using the ion-trap technique, where a large tip ion-selective electrode (~100 μm) is pushed against the oocyte plasma membrane, thus enclosing a small volume of extracellular solution referred to as the trap. The variations in ionic concentration occurring in the trap as a consequence of SGLT1 activity made it possible to assess that the turnover rate of Na+/glucose cotransport was 13 s-1 when the membrane potential was clamped to -155 mV. As a last project, we focused our interest on the development of a kinetic model for SGLT1. Taking advantage of the simulated annealing algorithm, we constructed a 7-state kinetic scheme whose predictions accurately reproduced the currents of the cotransporter as a function of extracellular Na+ and glucose. According to our model, the rate limiting step of cotransport under a saturating glucose concentration is the reorientation of the empty carrier that follows the intracellular release of substrates.

The anatomy of the cranial peripheral nervous system has been studied for over a century, yet surprisingly little is known about how the nerves are guided to their targets. The study of the development of these nerves has important implications for our understanding of craniofacial anomalies and possible treatments for both injury and genetic disorders of nerve development such as Goldenhar-Gorlin syndrome. We have discovered that retinoic acid (RA) may play a role in the development of the trigeminal nerve. Inhibition of retinoic acid receptors (RAR) results in trigeminal nerves that become unbundled or defasciculated in the eye region. To further understand how RA is affecting trigeminal development we searched for genes downregulated in response to RAR inhibition by the inhibitor BMS-453 and have identified neurotrophin-3 (NT-3), activated leukocyte cell adhesion molecule (ALCAM) and Semaphorin 4B (Sema4B). We have analyzed the expression patterns of Sema4B and NT-3 by in situ hybridization and have found NT-3 expression in the eye and Sema4B in the embryonic target of the trigeminal nerve, lens of the eye and in the pharyngeal arches. ALCAM has been analyzed via qRT-PCR and its transcription is downregulated just prior to the observed defasciculation phenotype. The pattern of expression of these genes combined with known expression of NT-3 receptors allows us to suggest a model whereby RA signaling regulates Sema4B, ALCAM and NT-3, which support the survival, guidance and fasciculation of the trigeminal nerve. This work has the potential to better understanding of the complex nature of cranial nervous system development.