Cette thèse de doctorat a porté sur l'étude de deux protéines membranaires d'origine animale appelées CCC8 (ou SLC12A9) et CCC9 (ou SLC12A8) qui appartiennent à la superfamille des cotransporteurs cation-chlore, mais dont le rôle n'est pas de transporter les ions Na+ et/ou K+ avec le Cl" à travers les surfaces cellulaires à l'instar des autres membres de la famille. En cours de caractérisation, nous avons trouvé que CCC8 et 9 pouvaient se comporter comme des transporteurs de polyamines à la surface cellulaire, ce qui nous a permis d'être les premiers à identifier de tels systèmes de transport chez les animaux. On en connaissait l'existence depuis longtemps, mais pour une raison imprécise, leur identité moléculaire était demeurée elusive. Il faut mentionner à cet effet que tous les CCCs font partie d'un groupe plus large de protéines appelé « transporteurs d'acides aminés-polyamines-organocations » (ou APC pour l'acronyme anglais amino acid-polyamine-organocation carriers). L'importance de nos résultats réside dans le rôle que jouent les polyamines dans la prolifération et la differentiation cellulaires, et ce, dans des conditions normales autant que pathologiques. Entre autres, il semble exister un lien important entre le métabolisme intracellulaire des polyamines et la cancérogenèse de même qu'entre le gène CCC9 et une maladie appelée psoriasis vulgaris.

L’hypertension artérielle affecte 40 % des Canadiens et Canadiennes âgés de 56 à 65 ans et correspond à un facteur de risque important pour le développement de maladies cardiovasculaires. Les causes de l’hypertension artérielle sont multifactorielles et le plus souvent difficiles à circonscrire. Elles incluent des facteurs génétiques, comme le dérèglement de certains systèmes de transport ionique dans le rein, et des facteurs environnementaux, comme l’ingestion excessive de sodium par la diète, l’abus d’alcool, la sédentarité, etc. La réabsorption du sodium filtré par le rein est effectuée par des protéines de transport spécialisées. Parmi celles-ci, le cotransporteur Na-K-Cl de type 2 (NKCC2), exprimé uniquement dans l’anse ascendante large de Henle (AAH) du néphron, assure la réabsorption d’environ 20 % du sodium. Ce transporteur est inhibé par les diurétiques de l’anse qui sont utilisés en clinique pour traiter certaines formes d’hypertension. Un changement de l’activité de cette protéine, soit intrinsèque ou lié à celui de certaines enzymes qui agissent sur NKCC2, a aussi été associé à des désordres de la pression artérielle. NKCC2 existe sous trois variants principaux qui sont produits par l’épissage alternatif de l’exon 4. Ces variants d’épissage, nommés NKCC2A, NKCC2B et NKCC2F, sont identiques les uns aux autres à l’exception du segment transmembranaire deux et de la boucle intracellulaire adjacente. Malgré tout, ils ont des caractéristiques, des localisations et des rôles différents le long de l’AAH. NKCC2 est impliqué dans la régulation du volume cellulaire puisqu’il est activé en condition de stress hypertonique. Cette activation serait médiée (du moins en partie) par certains isoformes des kinases with no lysine (WNK) dont WNK1 et WNK3. Toutefois, l’effet de ces kinases sur chaque isoforme n’est pas connu et les mécanismes provoquant l’activation du cotransporteur en réponse au stress hypertonique sont peu définis. Une meilleure connaissance à ce sujet nous permettrait de mieux comprendre comment NKCC2 est régulé et de savoir de quelle manière il pourrait être modulé pour en contrôler l’activité dans un but thérapeutique éventuel. Les objectifs de cette thèse étaient comme suit : 1) déterminer les mécanismes par lesquels les kinases WNK1 et WNK3 régulent chacun des variants de NKCC2 lors du stress hypertonique (chapitre 1) et 2) identifier les résidus de NKCC2 qui permettent la réponse au stress hypertonique et la régulation différentielle par les kinases WNK (chapitre 2). Le modèle des ovocytes de Xenopus laevis a été utilisé à cette fin. Dans le chapitre 1, nous avons montré que le stress hypertonique produisait son effet en augmentant l’abondance de NKCC2A et NKCC2B à la surface cellulaire et que cet effet était mimé par WNK3, mais pas par WNK1. De plus, nous avons montré que WNK3 augmentait le recyclage à la membrane des transporteurs endocytés alors que le stress hypertonique ne produisait pas cette réponse. Enfin, NKCC2F s’est révélé peu sensible au stress hypertonique et à WNK3, suggérant que des résidus lui étant uniques dans l’exon 4 contribuaient à cette réponse différentielle. Dans le chapitre 2, nous nous sommes intéressés aux rôles des résidus divergents entre les variants pour déterminer si l’exon 4 jouait un rôle dans les réponses observées. Par des études de mutagénèse dirigée, nous avons mis en évidence que les résidus en position 230 et 238 avaient un impact sur le trafic cellulaire de NKCC2. En outre, nous avons constaté que les résidus de NKCC2F à ces positions avaient pour effet de favoriser la rétention du transporteur à la membrane cellulaire. En somme, l’ensemble de ces travaux permettent de mieux comprendre les mécanismes de régulation du cotransporteur NKCC2 par le stress hypertonique et par la voie des kinases WNK. À la partie variable de NKCC2, l’exon 4, nous avons identifié un nouveau rôle qui est de participer à la régulation du trafic cellulaire du transporteur. Grâce à cette connaissance, nous saurons désormais que des stratégies d’intervention pour contrôler l’activité de NKCC2 pourraient miser sur la modification du nombre de transporteurs au site d’expression. / High blood pressure affects 40% of Canadians aged 56 to 65 and is a major risk factor for cardiovascular diseases. The causes of high blood pressure are multifactorial and are often difficult to circumscribe. They include genetic factors, such as abnormalities in the function of renal ion transporters, and environmental factors, such as excessive dietary sodium intake, alcohol abuse, sedentary lifestyle, etc. In the kidney, the ultrafiltered NaCl load is reabsorbed by specialized ion transport systems. Of these systems, the Na-K-Cl cotransporter type 2 (NKCC2), is confined to the thick ascending loop of Henle (TALH) of the nephron where it reabsorbs approximatively 20% of the ultrafiltered NaCl load. This transporter is inhibited by loop diuretics that are used clinically to treat certain types of hypertension. A change in the activity of NKCC2, either intrinsic or secondary to the effect of regulatory enzymes, has also been associated with blood pressure disorders. NKCC2 exists as three main variants that are produced through the alternative splicing of exon 4. These splice variants, named NKCC2A, NKCC2B, and NKCC2F, are identical to each other except for the residue composition of transmembrane segment 2 and the following connecting segment. Yet, they have different characteristics and roles along the TALH. NKCC2 is involved in cell volume regulation as it is activated by cell shrinkage. This activation is mediated (at least in part) by certain isoforms of the with no lysine (WNK) kinases including WNK1 and WNK3. However, the effect of these kinases on each of the splice variants is not known and the mechanisms that underlie the response to cell shrinkage are poorly defined. A better knowledge in these regards would allow us to better understand how NKCC2 is regulated and how it could be acted upon optimally towards maximal clinical benefits. The objectives of this thesis were as follows: 1) to determine the mechanisms by which WNK1 and WNK3 regulate each of the NKCC2 variant under hypertonic stress (Chapter 1) and 2) to identify residues in NKCC2 that sustain the response to cell shrinkage and differential regulation by the WNK kinases (chapter 2). The oocyte model of Xenopus laevis was used for this purpose. In Chapter 1, we showed that cell shrinkage produced its effect by increasing the abundance of NKCC2A and NKCC2B at the cell surface and that this effect was mimicked by WNK3, but not by WNK1. In addition, we showed that WNK3 increased membrane recycling of endocytosed transporters while cell shrinkage failed to produce such a response. Finally, NKCC2F was found to be insensitive to cell shrinkage and WNK3, suggesting that specific residues that are unique to this variant in exon 4 contributed to this differential response In Chapter 2, we looked at the roles of divergent residues between the variants to determine whether exon 4 played a role in the observed responses. Using mutagenic studies, we showed that residues at positions 230 and 238 played a major role in NKCC2 trafficking. In addition, we found that the residues of NKCC2F at these positions had the effect of promoting carrier retention at the cell membrane In sum, our findings have allowed us to better understand the mechanisms through which NKCC2 is regulated during cell shrinkage and by the WNK kinase-dependent pathway. Our findings have also allowed us to identify a new role for exon 4 in NKCC2 trafficking. With this gain of knowledge, we have found that strategies aimed at controlling the activity of NKCC2 could be based on altering the number of transporters at the cell surface.

Years of intensive tillage operations and heavy winter rainfalls have taken a toll on the soils in the Mississippi Delta causing compaction and nutrient losses due to runoff. The purpose of this research was to determine the effects of tillage systems and P and K fertilizer placement methods on corn and soybean production, as well as soil properties in the Mississippi Delta. The tillage treatments included conventional tillage, conventional tillage with subsoiling, and strip-tillage. The fertilizer placement treatments included a non-treated control, incorporated with tillage, and broadcast after tillage. Strip-tillage treatments reduced soil compaction and maintain nutrient levels while improving or maintaining yields in both crops. A more long term study is needed to determine if incorporating fertilizer with tillage will impact crop yields.

In response to increasing interest in greenhouse production and difficulties imposed by adverse environmental conditions in the southeastern United States, two studies were conducted with green and red-leaf lettuce cultivars. The first study analyzed effects on yield and quality of green and red-leaf lettuce subjected to increasing nutrient solution electroconductivity (EC) across three growing seasons. Results indicated that the interaction between season and cultivar had the greatest effect on growth, flavonoids and phenolics, and leaf mineral content. The second study analyzed effects on yield and quality of increasing solution potassium (K) in red Lollo lettuce at two temperatures. The results suggested that temperature is a stronger regulatory factor than increasing K in the determination of lettuce yield and quality. However, increasing K concentrations to 234.6 mg·L-1 results in higher concentrations of leaf mineral concentrations without compromising lettuce yield and quality.

Continuous population growth and rapid industrial advancement and development have paved the way for ever increasing environmental pollution. At present, water pollution is a serious global issue that threatens environmental sustainability. The contamination of aquatic bodies with potentially toxic organic and inorganic substances are the result of world-wide anthropogenic activities. These pollutants can have detrimental health consequences on humans and ecosystems. Over the past decades, techniques such as chemical precipitation, ion-exchange, adsorption, membrane filtration, and electrocoagulation-flocculation have been developed and employed for the treatment of drinking and wastewater. Among the currently available techniques, pollutant removal by adsorption is most promising due to its cost-effectiveness, simplicity in operation, environmental friendliness, and abundance of adsorbents. This study emphasized the utilization of biochar (BC), after appropriate surface modification, for the removal of potentially toxic contaminants. In the first study, a base activated biochar was synthesized by treating the biochar with potassium hydroxide (KOH) at 700 ℃ in a muffle furnace for 1 h. The resulting high surface area biochar (KOHBC) was used for the removal of Cr(VI), Pb(II) and Cd(II). In the second study, a biochar-supported polyaniline hybrid was synthesized for aqueous chromium and nitrate adsorption. Introduction of amine and imine groups to the biochar facilitated the removal of these contaminants. In the final study, a composite containing Fe-Ti oxide/biochar (Fe2TiO5/BC) was synthesized for sorptive removal of metal cations, oxy anions, inorganics, and organic contaminants from aqueous solutions. Additionally, this composite was used as a photocatalyst towards aqueous methylene blue (MB) degradation. The surface chemistry and composition of these adsorbents were examined by PZC SEM, TEM, XPS, FTIR, TGA, elemental analysis, and surface area measurements.

Thermal methods of high precision were used to determine the solid-liquid phase equilibria diagrams for the potassium-rubidium and the rubidium-cesium systems. Both form minima in the liquidus curves occurring at 307.00° K with 0.667 mole fraction rubidium in the potassium-rubidium system and 282.85° K with 0.530 mole fraction cesium in the rubidium-cesium system. In the potassium-rubidium system, the liquidus and solidus points are very close together, giving a very narrow temperature range for the two phase region. The freezing point minimum in the potassium-rubidium system occurs at 0.667 mole fraction rubidium, suggesting the possible formation of a KRb2 intermetallic compound.

Thermal methods were used, to determine with high precision, the solid-liquid phase equilibria diagrams for the sodium-rubidium and sodium-potassium-rubidium systems. In the sodium-rubidium system, the results differ greatly from those of earlier works, especially near the eutectic composition where differences in freezing points as large as 25°K occur. A detailed search was made for possible intermetallic compounds (especially Na2Rb). Neither slow temperature cycling in the temperature range where compound formation could occur nor extended periods of annealing just above the eutectic temperature produced any evidence for compound formation in the sodium-rubidium system. A 2:1 (sodium to rubidium) sample was subjected to 60,000 atmospheres pressure. Again, no evidence was obtained for solid compound formation. In the sodium-potassium-rubidium system, a partial phase diagram was obtained; however, there. is no evidence of the formation of NaKRb in this system.

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