Oxidative stress is a feature of many chronic neurodegenerative diseases as well as a contributing factor in acute disorders including stroke. Fork head class of transcription factors (Foxos) play a key role in promoting oxidative stress-induced apoptosis in neurons through the upregulation of a number of pro-apoptotic genes. Here I demonstrate that synaptic NMDA receptor activity not only promotes Foxos nuclear exclusion but also suppresses the expression of Foxo1 in a PI3K-dependent fashion. I also found that Foxo1 is in fact, a Foxo target gene and that it is subject to a feed-forward inhibition by synaptic activity, which is thought to result in longerterm suppression of Foxo downstream gene expression than previously thought. The nuclear factor (erythroid 2-related) factor 2 (Nrf2) is another transcription factor involved in oxidative stress and the key regulator of many genes, whose products form important intrinsic antioxidant systems. In the CNS, artificial activation of Nrf2 in astrocytes has been shown to protect nearby neurons from oxidative insults. However, the extent to which Nrf2 in astrocytes could respond to endogenous signals such as mild oxidative stress is less clear. The data presented herein, demonstrate for the first time that endogenous Nrf2 could be activated by mild oxidative stress and that this activation is restricted to astrocytes. Contrary to the established dogma, I found that mild oxidative stress induces the astrocytic Nrf2 pathway in a manner distinct from the classical Keap1 antagonism employed by prototypical Nrf2 inducers. The mechanism was found to involve direct regulation of Nrf2's transactivation properties. Overall these results advance our knowledge of the molecular mechanism(s) associated with the control of endogenous antioxidant defences by physiological signals.

Protein-bound 3,4-dihydroxyphenylalanine (PB-DOPA), a long-lived, redox-active product of protein oxidation, is capable of functioning as both a pro- and anti-oxidant. A number of in vitro and in vivo studies have demonstrated a toxic, non-toxic or even beneficial effect of free DOPA, however little investigation has examined the physiological activity of PB-DOPA. Furthermore, as free DOPA is currently the major treatment available for Parkinson?s disease, most studies have focused on the effect of DOPA within neurological cells or tissues, although the presence of PB-DOPA in other locations, for example within atherosclerotic plaques, suggests that broader research is needed to fully understand the physiological effects of both free and PB-DOPA. The hypothesis presented in this thesis is that under physiological conditions, when little redox active transition metal is available, PB-DOPA can function as a redox signalling molecule, triggering an enhancement of cellular antioxidant defences, with a potentially specific role in the regulation of defences targeted against protein oxidation. Physiological levels of PB-DOPA are very low, however the level on individual proteins can change to a proportionally large degree during oxidative stress, an appropriate property for a signalling molecule. In addition, remarkably elevated levels occur in some pathologies, including atherosclerosis. As an initial and commonly formed product of protein oxidation, PB-DOPA is well placed for a signalling role, promoting a significant up-regulation of antioxidant defences in the early stages of oxidative stress, before extensive damage has occurred. As an initiator of antioxidant defences, PB-DOPA would be potentially useful as a therapeutic for the treatment of diseases involving oxidative stress or the accumulation of oxidative damage. The main objective of this thesis was, therefore, to examine the effect of PB-DOPA on the cellular antioxidant defence system using monocytic and macrophage-like cells, key cells involved in the formation of atherosclerotic plaques. The incorporation of free DOPA into protein during protein synthesis, a process previously shown to occur both in vitro and in vivo, was used to generate PB-DOPA. Neither free nor PB-DOPA were found to be toxic to monocytic or macrophage-like cells in culture, but rather were both capable of protecting these cells from oxidative stress. Free DOPA was shown to be capable of directly scavenging radicals, a process that was thought to be in part responsible for the protection induced during oxidative stress. The presence of free and PB-DOPA up-regulated the activity of catalase and NAD(P)H:quinone oxidoreductase, two enzymatic antioxidants, however the activity of superoxide dismutase and the concentration of oxidised and reduced glutathione were not affected. Whilst it was thought that PB-DOPA would have a specific effect on the activity of antioxidant defences targeted against protein oxidation, proteolysis and bulk chaperone activity were not affected by a combination of free and PB-DOPA. Oxidatively-induced protein aggregation, however, was inhibited by the presence of free and PB-DOPA, suggesting that a more specific chaperone regulation may be taking place. The regulation of gene and protein expression was thought to be one possible mechanism by which PB-DOPA could function as a signalling molecule. To test this hypothesis, the effect of free and PB-DOPA on transcription factor activation and protein expression were investigated. Free and PB-DOPA did not induce the expression or activation of Nrf2, AP-1 or NFJB, three transcription factors thought to be involved in the expressional regulation of genes involved in the antioxidant defence system. However, the expression of a number of proteins, including antioxidants, chaperones and proteins involved in cell cycle progression, were regulated in monocytic and macrophage-like cells following the administration of free DOPA under conditions that resulted in either a high or low level of PB-DOPA generation. The regulated proteins differed between the two conditions, suggesting that the level of PB-DOPA may be a key factor in determining the specific defences targeted. The results presented in this thesis support the hypothesis that PB-DOPA can function as a signalling molecule, triggering an enhancement of cellular antioxidant defences, with a specific role in the regulation of the chaperone system, a key defence targeted against protein oxidation. This thesis may provide the basis for the potential use of free or PB-DOPA as a therapeutic for diseases known to involve oxidative stress or oxidative damage, however more research will be required to determine if the effects demonstrated in this thesis are also capable of occurring in vivo.

Le biofloc est un système d’élevage intensif avec un faible renouvellement d’eau ; ainsi se développe une population diversifiée de micro-organismes (micro-algues, zooplanctons et bactéries) associée à de la matière organique qui forme les floculats. Ces derniers jouent le double rôle de filtre biologique et de complément alimentaire. Cette thèse a pour objectif de mieux comprendre le fonctionnement de ce système d’élevage et ses interactions avec la crevette Litopenaeus stylirostris. Dans cet objectif général s’inscrit deux objectifs plus spécifiques : (i) mesurer les gains zootechniques apportés par l’élevage en biofloc (ii) étudier les interactions trophiques entre le milieu d’élevage et la crevette en lien avec les performances zootechniques. Nos résultats montrent des gains relatifs de l’élevage en biofloc aux niveaux de la survie, de la croissance, des performances de reproduction des femelles et de la qualité de leur progéniture. Ces meilleures performances s’expliquent par la contribution de la productivité naturelle, estimée entre 37 et 40%, dans l’alimentation de la crevette. Ce complément d’aliment, outre d’être toujours disponible dans le milieu d’élevage, apporte de l’énergie, des nutriments et des molécules bioactives. L’aliment naturel représente une source nutritive de lipides, particulièrement riche en phospholipides et en acides gras polyinsaturés, qui sont essentiels pour la reproduction et le développement des larves en phase de lécitotrophie ; ces lipides sont accumulés dans la glande digestive et les œufs des femelles élevées en biofloc. L’aliment naturel est également une source de glutathion, puissante molécule antioxydante, qui contribue au renforcement du système des défenses anti-radicalaires de la crevette et protège les lipides insaturés de la peroxydation, une cause du stress oxydant. Les bactéries sont prépondérantes dans la productivité naturelle d’un élevage en biofloc et contribuent donc à l’alimentation de la crevette. Aussi, dans le dernier volet de cette thèse, nous avons caractérisé la diversité taxonomique et l’abondance des bactéries du milieu d’élevage et montré son influence sur le microbiote intestinal des crevettes. De façon générale, nous observons une meilleure santé des animaux élevés en biofloc qui se traduit par une régulation positive des gènes impliqués dans l’immunité et les défenses anti-radicalaires après un stress au peroxyde d’hydrogène. Ainsi, les effets positifs de l’élevage en biofloc sur les survies, les croissances et la reproduction ont pour origine le complément d’aliment apporté par la productivité naturelle / Biofloc is an intensive rearing system with zero or minimal water exchange where a diverse population of microorganisms (microalgae, zooplankton and bacteria) develops in association with organic matter to form the floc particles. These particles play the double role of biological filter and dietary supplement. This dissertation aims to better understand the process of this rearing system and its interactions with the Litopenaeus stylirostris shrimp. Two specific objectives were integrated within the framework of this general objective: (i) to measure the production gain from biofloc rearing and (ii) to study the interaction between biofloc environment and shrimp and to assess its role on production performances of shrimps. Thus, our results show production gains of shrimp reared in biofloc in terms of survival, growth, reproductive performances and quality of larvae. This better performance can be explained by the contribution of natural productivity, estimated between 37 and 40%, in shrimp food. This food supplement, constantly available in the environment, provides energy, nutrients and bioactive molecules. The natural productivity represents a source of lipids, in particular of phospholipids and polyunsaturated fatty acids, which were essential for the reproduction and development of larvae during the lecitotrophic stage; these lipids were accumulated in digestive gland and eggs from females reared in biofloc. The natural food is also a glutathione source, a powerful antioxidant molecule, which contributes to strengthen antioxidant defense system of shrimps and protects lipids against peroxidation, a cause of oxidative stress. Bacteria were dominant in natural productivity of biofloc environment and contribute to shrimp food. Thus, in the last part of this dissertation, we characterized the taxonomic diversity and abundance of bacteria in biofloc environment and showed their influence on shrimp intestinal microbiota. Generally, we observed a better health of biofloc resulting in up-regulation of the studied genes involved in immunity and anti-radical defenses after oxidative stress with hydrogen peroxide. The positive effects of biofloc rearing on survival, growth and reproduction originate from food complement provided by natural productivity.