Chez l’Homme, les succès de la fécondation et d’un développement embryonnaire aboutissant à la naissance d’un enfant en bonne santé résident principalement dans la qualité des cellules reproductrices. Les dommages oxydants de l’ADN spermatique sont une cause majeure d’infertilité masculine. Afin de permettre une prise en charge thérapeutique optimale et adaptée, j’ai tout d’abord mis au point et validé un test diagnostique de l’oxydation de l’ADN spermatique par immunodétection du 8-hydroxy-2'-desoxyguanosine (8-OHdG), adduit majeur de l'oxydation nucléaire. Ce travail de thèse a déterminé, pour la première fois, un seuil d’oxydation de l’ADN spermatique en relation avec les paramètres conventionnels spermatiques. Dans un second temps, je me suis focalisée sur les atteintes de la chromatine et de l’ADN spermatique les plus fréquentes en cas d’infertilité masculine, à savoir les anomalies de condensation de la chromatine, la fragmentation et l’oxydation de l’ADN spermatique. Une corrélation entre l’oxydation de l’ADN, tout particulièrement la moyenne d’intensité de fluorescence, et le pourcentage de spermatozoïde fragmenté a été mise en évidence. Pour objectiver l’impact de ces dommages nucléaires spermatiques en pratique clinique, j’ai étudié, après cryopréservation, les effets bénéfiques d’une supplémentation en hypotaurine des milieux de sélection et de congélation/décongélation des échantillons. Une baisse de la cryocapacitation et du pourcentage de spermatozoïde fragmenté et décondensé ont été retrouvées ainsi qu’une amélioration de la vitalité et de la mobilité progressive spermatique. Enfin, comme le spermatozoïde a pour but ultime de participer à la genèse d’un nouvel individu, j’ai mis en évidence que la fragmentation et l’oxydation de l’ADN spermatique avaient un impact à des moments clés de la cinétique du développement embryonnaire précoce suite à une ICSI sans pour autant modifier l’obtention de blastocystes de bonne qualité. Ce travail de thèse a permis de mieux comprendre la physiopathologie de l’infertilité masculine et de mettre en évidence de nouveaux biomarqueurs spermatiques en lien avec un développement embryonnaire normal. / In humans, the success of fertilization and embryonic development leading to the birth of ahealthy child lies mainly in the quality of reproductive cells. Oxidative damage to sperm DNAis a major cause of male infertility. In order to provide optimal and appropriate therapeuticmanagement, I first developed and validated a diagnostic test for sperm DNA oxidation byimmunodetection of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a major adduct of nuclearoxidation. This thesis work determined, for the first time, a threshold for the oxidation ofsperm DNA in relation to conventional sperm parameters. In a second step, I focused on themost common chromatin and sperm DNA disorders in male infertility, namely chromatincondensation anomalies, sperm DNA fragmentation and oxidation. A correlation betweenDNA oxidation, particularly the mean fluorescence intensity, and the percentage offragmented sperm was found. To objectify the impact of this nuclear sperm damage inclinical practice, I studied, after cryopreservation, the beneficial effects of hypotaurinesupplementation to the selection and freeze/thaw media of seed samples. A decrease incryocapacitation and the percentage of fragmented and decondensed sperm has beenfound, as well as an improvement in sperm vitality and progressive mobility. Finally, sincethe ultimate goal of the sperm cells is to participate in the genesis of a new individual, I haveshown that the fragmentation and oxidation of sperm DNA has an impact at key moments inthe kinetics of early embryonic development following ICSI without modifying the obtainingof good quality blastocysts. This thesis work has led to a better understanding of thepathophysiology of male infertility and the identification of new sperm biomarkers related tonormal embryonic development.

In insects, the cuticle provides protection against physical injury and water loss, rigidness for muscle attachment and mechanical support, and flexibility in inter-segmental and joint areas for mobility. As most insects undergo metamorphosis, they need to shred off old cuticle and synthesize new cuticle to fit the body shape and size throughout their life cycles. The newly formed cuticle, mainly composed of cuticular proteins, chitin, and sclerotizing reagents, needs to be hardened through the crosslinks between cuticular proteins and sclerotizing reagents. This dissertation concerns the biochemical activities of several pyridoxal 5-phosphate (PLP)-dependent decarboxylases with most of them involved in insect cuticle hardening. Herein, we first present a detailed overview of topics in reactions and enzymes involved in insect cuticle hardening. Aspartate 1-decarboxylase (ADC) is at the center of this dissertation. beta-alanine, the product of ADC-catalyzed reaction from aspartate, is the component of an important sclerotizing reagent, N-beta-alanyldopamine; the levels of beta-alanine in insects regulate the concentrations of dopamine, therefore affecting insect sclerotization and tanning (collectively referred as cuticle hardening in this dissertation). Biochemical characterization of insect ADC has revealed that this enzyme has typical mammalian cysteine sulfinic acid decarboxylase (CSADC) activity, able to generate hypotaurine and taurine. The result throws lights on research in the physiological roles of insect ADC and the pathway of insect taurine biosynthesis. Cysteine was found to be  an inactivator of several PLP-dependent decarboxylases, such as ADC, glutamate decarboxylase (GAD) and CSADC. This study helps to understand symptoms associated with the abnormal cysteine concentrations in several neurodegenerative diseases. A mammalian enzyme, glutamate decarboxylase like-1 (GADL1), has been shown to have the same substrate usage as insect ADC does, potentially contributing to the biosynthesis of taurine and/or beta-alanine in mammalian species. Finally, the metabolic engineering work of L-3, 4-dihydroxyphenylalanine decarboxylase (DDC) and 3, 4-dihydroxylphenylacetaldehyde (DHPAA) synthase has revealed that the reactions of these enzymes could be determined by a few conserved residues at their active site. As both enzymes have been implicated in the biosynthesis of sclerotizing reagents, it is of great scientific and practical importance to understand the similarity and difference in their reaction mechanisms. The results of this dissertation provide valuable biochemical information of ADC, DDC, DHPAA synthase, and GADL1, all of which are PLP-dependent decarboxylases. ADC, DDC, DHPAA synthase are important enzymes in insect cuticle hardening by contributing to the biosynthesis of sclerotizing reagents. Knowledge towards understanding of these enzymes will promote the comprehension of insect cuticle hardening and help scientists to search for ideal insecticide targets. The characterization of GADL1 lays groundwork for future research of its potential role in taurine and beta-alanine metabolism. / Ph. D.