Thesis (Ph. D. in Neuroscience)--Vanderbilt University, Aug. 2006. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Minnesota, 1999. / Includes bibliographical references (leaves 240-248). Also available on the World Wide Web as a PDF file.

Un développement racinaire optimal est essentiel pour favoriser la croissance des plantes et leur permettre d’accéder à de meilleurs rendements. La plupart de nos connaissances concernant le contrôle moléculaire du développement racinaire a été acquise grâce à l’étude de la plante modèle dicotylédone Arabidopsis thaliana. Les racines de riz, plante modèle des monocotylédones, présentent une anatomie semblable à celle d’A.thaliana. Ces deux systèmes racinaires sont caractérisés par une organisation concentrique des tissus autour de la stèle. Entre l’endoderme et l’épiderme, le riz présente deux couches plus externes, appelées le sclérenchyme et l’exoderme, ainsi que plusieurs couches de cortex dans la zone centrale. Chacun de ces tissus possèdent des spécificités anatomiques et moléculaires qui leur confèrent des rôles divers dans le développement et la croissance de la racine. Le nombre de couches variable de cortex dépend du type de racines, du stade de développement de la plante et des conditions environnementales. Le tissu cortical a un rôle structurel, fonctionnel et adaptatif essentiel pour les racines de riz. La différenciation du cortex et de l’endoderme a été longuement étudiée chez A.thaliana durant la dernière décennie. Le gène SHORT-ROOT (SHR) a été identifié comme un gène clé dans leur formation. Un modèle moléculaire a été développé, où SHR est transcrit dans la stèle, et la protéine migre jusque l’endoderme où elle active la transcription du gène SCARECROW (SCR). Ensemble SHR et SCR induisent la division périclinale asymétrique d’une cellule initiale, permettant la formation des deux couches de tissus internes que sont l’endoderme et le cortex. Chez le riz, il existe des orthologues de SHR et SCR, qui sont apparus suite à des duplications, appelés OsSHR1 & OsSHR2 et OsSCR1 & OsSCR2. Au sein de cette thèse nous avons tenté d’identifier le rôle des gènes OsSHR1&2 dans la mise en place des différentes couches de cortex chez le riz. / Optimal root development is central for plants to reach maximum growth and yield. Most of our knowledge regarding genes involved in root development has been accumulated in the dicotyledon model plant Arabidopsis thaliana. Roots of rice, the monocotyledon model, present several extra ground tissue layers compared to A. thaliana. Between epidermis and endodermis, rice possesses two outer cell layers, exodermis and schlerenchyma, and a multilayered cortex. All these tissues have specific cell identity, anatomical and molecular adaptations related to their diverse roles in root growth and function. Variation in the number of cortex cell layers depends on the rice root type and cortex is one of the key tissues for rice adaptation to submergence and tolerance to other environmental stresses. Cortex and endodermis differentiation in A. thaliana has been extensively studied during the last 10 years. Thereby, SHORTROOT (SHR) gene in Arabidopsis thaliana has been identified as a key gene required for their formations. An elegant model was developed, where SHR is transcribed in stele tissue and its protein moves to the endodermis where it activates SCARECROW (SCR) transcription. Together, SHR and SCR induce periclinal division of the ground tissue initial separating cortex and endodermis cell layers. Cortex formation in rice represents an intriguing contrast to A. thaliana. Variations in the number of cortex cell layers can be observed between the root types and during rice development. There should be a control mechanism for the number of cortical cell layers, and SCR and SHR rice orthologs represent good candidates. Two duplications in rice led to apparition ortholog genes of AtSHR (OsSHR1 and OsSHR2) and we have addressed the question of their respective function in cortex formation in rice root.

Memory formation and recall depend on a complex circuit that includes the hippocampus and associated cortical regions. The goal of this thesis was to understand how two of the cortical connections, the medial entorhinal cortex (MEC) and medial prefrontal cortex (mPFC), influence spatial and nonspatial activity in the hippocampus. Cells in the MEC exhibit prominent spatially selective activity and have been hypothesized to drive place representation in the hippocampus. In Experiment 1 the MEC was transiently inactivated using the inhibitory opsin ArchaerhodopsinT (ArchT), and simultaneous recordings from CA1 were made as rats ran on an elliptical track. In response to MEC disruption some cells in the hippocampus shifted the preferred location of activity, some changed firing rate and others were unaffected. The new representation that developed following MEC disruption remained stable despite the fact that inhibition was transient. If the MEC is the source of spatial activity in the hippocampus the activity would be either time-locked to periods of inhibition or unstable throughout the period of inconsistent input. These results show that the MEC guides spatial representation in the hippocampus but does not directly drive spatial firing. The mPFC is generally thought to guide behavior in response to contextual elements. Experiment 2 examined the interaction between the mPFC and the hippocampus as rats performed a contextual discrimination task. Recordings were made in CA1, and the mPFC was disrupted using ArchT during the odor sampling phase of the discrimination. As animals perform this task neurons in the hippocampus respond to a conjunction of odor and location which indicates an association of what and where information in the hippocampus. Optogenetic disruption of the mPFC led to a decrease in nonspatial representation. Individual cells showed lower levels of odor selectivity, but there was no change in the level of spatial representation. This indicates that the mPFC is important for determining how the hippocampus represents nonspatial information but does not alter the spatial representation. The results are discussed within a model of memory formation that includes binding spatial and nonspatial information in the hippocampus.

Residual vision, or blindsight, following damage to the primary visual cortex was first identified almost a century ago. However, the mechanism and pathways underlying this ability, as well as the extent of visual function, remain unclear and are a continuing source of speculation. The work presented here goes some way to try to address these questions, investigating 18 patients with V1 damage and homonymous visual field loss acquired in adulthood. Six experimental chapters explore the extent and potential for visual function after V1 damage, and apply novel neuroimaging paradigms and techniques to try to uncover the mechanisms and pathways that might be involved. A combination of psychophysics, functional and structural MRI was used to investigate responses to blind field stimulation in the dorsal and ventral streams. In addition, diffusion MRI tractography was performed and related to psychophysical performance, so that the three main pathways implicated in blindsight could be evaluated. Lastly, a small rehabilitation study was carried out to assess the effect of training in the blind hemifield, and to investigate whether there is any transfer of learning between the dorsal and ventral visual streams. The results from this work reinforce the suggestion that blindsight may be more common than was first thought, and may extend across a number of characteristics involving both visual streams. It is also suggested that visual function need not be completely unconscious, but that certain salient stimuli can elicit both non-visual and crude visual experience. The use of parametric functional imaging paradigms has enabled a number of properties of non-striate inputs to the extrastriate cortex to be revealed. Together with tractography, this points to an important role for the ipsilateral lateral geniculate nucleus in blindsight function. It is hoped that future work will build upon this, and that it may be possible to target these residual pathways in the rehabilitation of patients with V1 damage.

Precise point-to-point connectivity is the basis of ordered maps of the visual field in the brain. One point in the visual field is represented at one locus in the dLGN and one locus in primary visual cortex. A fundamental problem in the development of most sensory systems is the creation of the topographic projections which underlie these maps. Mechanisms ranging from ordered ingrowth of fibres, through chemical guidance of axons to sculpting of the map from an early exuberant input have been proposed. However, we know little about how ordered maps are created beyond the first relay. What we do know is that a topological mismatch requires the exchange of neighbours in the geniculo-cortical projection and that manipulating the input to the primary relay can affect the geniculo-cortical topography. Taking advantage of the immaturity of the newborn hamster’s visual system, I studied the generation of an ordered map in primary visual cortex during the time of target innervation in normal and manipulated animals. I also investigated the patterning of neuronal activity prior to natural eye-opening. Paired injections of retrograde fluorescent tracers into visual cortex reveal that geniculate fibres are highly disordered at the time of invasion of the cortical plate. Topography in the geniculo-cortical projection emerges out of an unordered projection to area 17 in the first postnatal week. Furthermore, I show that manipulating the peripheral input can alter the topographic map which arises out of the early scatter. Removal of one eye at birth appears to slow the process of geniculo-cortical map formation ipsilateral to the remaining eye and at the end of the second postnatal week, a double projection between thalamus and cortex has formed. If retinal activity is blocked during this time, this double projection does not emerge. The results implicate retinal activity as the signal that induces the development of a different topographic order in the geniculo-cortical projection. It is generally believed that visual experience can influence development only after eye-opening. However, the final part of my thesis shows that neurons in the developing visual cortex of the ferret can not only be visually driven at least 10 days before natural eye-opening, but are also selective for differently oriented gratings presented <i>through the closed eye-lid</i>. Thus, visually-driven neuronal activity could influence development much earlier than previously assumed in many developmental studies.

Les hétérotopies nodulaires périventriculaires (HNP) sont des malformations du cortex cérébral caractérisées par la formation d’amas de neurones dans des parties inappropriées du cerveau. Elles peuvent être responsables d’une multitude de troubles tels qu’une hypotonie, un déficit intellectuel ou des épilepsies. Notre équipe a montré que le gène NEDD4L codant pour une E3 ubiquitine ligase était responsable d’HNP associées à des syndactilies. Des études cellulaires et une approche in utero nous ont permis de montrer une instabilité de la protéine mutante et des problèmes lors de la neurogénèse, le positionnement neuronal et la translocation terminale. Des études plus approfondies mettent en lumière le rôle critique de NEDD4L dans différentes voies de signalisation. En effet, alors que l’excès de protéine NEDD4L WT conduit à une dérégulation de DAB1 et de la voie mTORC1, l’instabilité des protéines mutantes conduit à une dérégulation des voies mTOC1 et AKT. L’ensemble de ces données permet de mieux comprendre le rôle critique de NEDD4L dans la régulation des voies mTOR et sa contribution dans le développement cortical. / Neurodevelopmental disorders with periventricular nodular heterotopia (PNH) are etiologically heterogeneous, and their genetic causes remain in many cases unknown. Here we show that missense mutations in the HECT domain of the E3 ubiquitin ligase NEDD4L lead to PNH associated with toes syndactyly, cleft palate and neurodevelopmental delay. Cellular and expression data showed a sensitivity of PNH-associated mutants to proteasome degradation. Moreover, in utero electroporation approach showed that PNH-related mutants and excess of wild type (WT) NEDD4L affect neurogenesis, neuronal positioning and terminal translocation. Further investigations, including rapamycin based experiments, revealed differential deregulation of pathways involved. Excess of WT NEDD4L leads to a disruption of Dab1 and mTORC1 pathways, while PNH-related mutations are associated with a deregulation of mTORC1 and AKT activities. Altogether, these data provide insights to better understand the critical role of NEDD4L in the regulation of mTOR pathways and their contributions in cortical development.

No description available.

Chez les plantes les protéines de la famille GRAS régissent un grand nombre de processus allant du développement racinaire à la transduction de signaux hormonaux. Deux protéines de cette famille de facteurs de transcription, SCR et SHR jouent un rôle essentiel dans le développement racinaire d’Arabidopsis thaliana en régulant la formation des tissus internes. Chez le riz, il existe deux co-orthologues putatifs pour ces deux gènes, OsSCR1, OsSCR2, OsSHR1 et OsSHR2. Nous avons caractérisé la fonction de trois membres de cette famille au cours du développement racinaire et aérien du riz. OsSHR1 et OsSHR2 sont impliqués dans la variation du nombre de couches de cortex et interfèrent aussi dans la formation des cellules bulliformes et du sclerenchyme dans les feuilles. OsSCR2 a un rôle majeur dans le contrôle du nombre de ramifications aériennes. Nos résultats ont aussi permis de démontrer qu’OsSCR2 et OsSHR2 ont des fonctions spécifiques au riz. / Summary to be coming

Le néocortex humain est particulièrement plissé. Des variations de plissement cortical ont été associées à certaines maladies neuro-développementales comme l’autisme ou la schizophrénie. Il est cependant difficile de savoir ce qu’impliquent ces différences de motif, au delà d’être des marqueurs de variations de la structure interne du cortex. Au cours du développement, les plis corticaux apparaissent chez l’humain pendant le dernier trimestre de gestation, simultanément avec le développement de sa connectivité. Aujourd’hui la communauté scientifique semble se retrouver sur des théories qui relient l’émergence des plis à l’organisation cytoarchitectonique et fonctionnelle du cortex. Nous avons développé une hypothèse de plissement prenant également en compte le stress mécanique engendré par la croissance des tissus ainsi que son rôle dans une possible rétro-action mécanique des plis sur l’organisation interne du cortex. Grâce à son développement en partie post-natal, le furet apparait comme un modèle animal idéal pour l’étude de l’organisation et du plissement cortical. A partir de données d’IRM structurales, nous avons segmenté et reconstruit en trois dimensions les surfaces piale et interne du cortex de 28 cerveaux de furets, afin de mener des analyses de surface et d’épaisseur corticale. Par une description anatomique précise du plissement du cortex du furet, basée sur l’étude de cartes de courbure, nous montrons la présence inattendue de plis déjà au 4ème jour après la naissance (P4). Par ailleurs, l’analyse longitudinale du plissement du cortex à partir d’algorithmes de recalage, suggère un gradient rostro-caudal dans la croissance de la surface corticale qui pourrait être impliqué dans la gyrification. En effet, les aires corticales les plus rostrales, plus grandes, et plus plissées à l’âge adulte, semblent croitre plus tôt et plus vite au cours du développement, en comparaison avec les régions caudales plus petites, et moins plissées chez l’adulte. L’analyse de l’épaisseur corticale révèle une corrélation entre l’épaisseur du cortex et la localisation des gyri et sulci. Cependant cette corrélation n’apparait qu’à partir du 8ème jour après la naissance, soit après la formation des plis à P4. L’ensemble de nos résultats suggère donc un possible effet mécanique du plissement du cortex sur son épaisseur, et donc potentiellement sur son organisation cytoarchitectonique et sa connectivité. / The human neocortex is highly folded. Its folding pattern has been associated with neurodevelopmental conditions, like autism or schizophrenia. It is still difficult, however, to understand what theses differences may imply, beyond being a marker of underlying variations in cortical organization. During development, folds appear during the last semester of gestation in human, synchronously with connective development. Nowadays, researchers recognize the existence of a relationship between the geometry of neocortical folds, and its cytoarchitectonic and functional organization. We have developed a hypothesis considering the role that mechanical stress can play in the determination of neocortical organization. The ferret appears as an ideal animal model to study the link between folding and cortical organization, as both processes in the ferret take place after birth. We segmented and reconstructed the pial and white matter cortical surfaces from 28 ferret brain structural MR images, and used them for surface and cortical thickness analysis. We highlight an unexpected significant presence of folds at postnatal day 4 (P4), on curvature maps of the pial surface. Besides, longitudinal analyses of the pial folding using meshes registration suggest the presence of a gradient in surface expansion that can be related to gyrification. The larger rostral areas — that turn to be more folded in adult brains — seem to expand earlier and faster than caudal regions with smaller surface area. In cortical thickness analysis, the correlation between pial surface curvature and thickness reveals a variation of cortical thickness in gyri and sulci. However, this correlation appears after postnatal day 8, thus after the emergence of folds around P4.Together, our results suggest a possible mechanical causal effect of folding on the thickening of the cortex and on its cytoarchitectonic and connective development.