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Orofacial anomalies make up about a third of the 120,000 birth defects each year in the United States. Children born with these abnormalities must undergo immense physical and emotional strain in order to correct the defects. In fact, about $697 million is spent every year surgically treating children with cleft lip and/or cleft palate (2011). In countries where surgery is not an option, this abnormality causes immense difficulties in eating, hearing, speech, and psychosocial development. The causes of cleft lip/palate are extremely complex. Genetics play a role in the anomaly; however, 95% of cleft palate cases are non-syndromic and likely due to other factors. Vitamin deficiencies, lack of folic acid intake during pregnancy, exposure to cigarette smoke, anticonvulsant drugs, alcohol, and inappropriate amounts of retinoic acid have all been correlated to incidence of cleft palate and other orofacial defects (Weingartner, Lotz et al. 2007). Xenopus laevis, and the closely related Xenopus tropicalis, are excellent model systems for orofacial development studies. The ease of embryo collection and manipulation, in addition to the conservation of DNA sequence between the two species, makes them ideal for studying developmental processes. Further, tissue specific experiments are extremely feasible due to the size of Xenopus oocytes (approximately 1000 times larger than a human egg!), and their ability to develop outside of the mother (Lindeman, Winata et al. 2010; Liu 2011). Here, I show that molecules from both the folic acid and retinoic acid pathways are highly expressed in the developing face. I have found that inhibition of key enzymes that regulate these pathways induces similar orofacial malformations, including median clefts that extend into the developing palate. Further, disruption of these pathways induces severe abnormalities in the formation of the cartilages of the jaws and face. Thus, both folic acid and retinoic acid are key signaling molecules that regulate proper formation of the orofacial region.

Among the most prevalent and devastating types of human birth defects are those affecting the mouth and face, such as orofacial clefts. Children with malformed orofacial structures undergo multiple surgeries throughout their lifetime and struggle with facial disfigurements, speech, hearing, and eating problems. Therefore, facilitating new research in cranio- and orofacial development is paramount to prevention and treatment of these types of birth defects in humans. Xenopus laevis has emerged as a new tool for dissecting the mechanisms governing facial development. Thus, molecular analyses accompanied by quantitative assessment of morphological changes during orofacial development of this species could be very powerful for understanding how these defects arise. In this dissertation, I present such a study. I first establish a quantitative protocol to describe size and shape changes in facial morphology of wild-type Xenopus embryos. I then utilize this method on embryos in which retinoic acid signaling or folate metabolism have been disrupted to correlate morphological changes with their underlying mechanisms. Finally, I demonstrate the utility of Xenopus as a system for chemical genomics to uncover other regulators of orofacial development.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Because of their possible therapeutic potential stem cells are one of the most promising fields of study in biology. The aim of this thesis was to study the migration potential of testicular stem cells of Xenopus tropicalis, whose culture was established from the testes of juvenile males in the laboratory of my supervisor. Due to external fertilization and embryonic development Xenopus represents an ideal model organism for transplantation and microinjection experiments in-vivo. Transplantation of vital marked (PKH26) testicular stem cells into blastula and peritoneum of tadpoles showed their wide migratory potential including intestine (entoderm), heart, pronephros, genital groove (mesoderm) and epidermis (ectoderm). Based on my experiments, I found that the ideal number of cells for transplantation ranges between 250-500 per tadpole. To further characterize the stem cells, I constructed a plasmid vector carrying a gene for a red fluorescent protein. This plasmid was then used for preparation of frogs with whole-body expression of Katushka RFP under control of CAG promoter. The next aim is to gain the RFP positive offspring by crossing the transgenic individuals with the wild type. Male offspring can be used for establishing culture of testicular stem cells stably expressing the reporter gene. In this way...

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L'objectif de cette thèse est de mieux comprendre la régulation de la phase M du cycle cellulaire. Nos expériences ont été effectuées dans des extraits acellulaires d’embryons de Xenopus laevis. Tout d'abord, nous montrons que le moment de l'entrée en phase M est précisément déterminé par un équilibre entre l'activité de la protéine kinase CDK1 et l’activité d’une protéine phosphatase sensible à l'acide okadaïque, PP2A. Nous montrons également le rôle de la protéine CDC6 dans la régulation de l'entrée dans la première phase M embryonnaire. En effet, CDC6 inhibe CDK1 et à travers cette action régule la dynamique de cette kinase lors de l'entrée et de la progression en phase M. Ces résultats mettent en évidence un nouveau contrôle qui précise le moment du clivage embryonnaire. Ce contrôle joue un rôle clé dans la coordination entre les mécanismes de régulation du cycle cellulaire et le programme de développement de l'embryon. / The aim of this thesis is to understand better the regulation of the M-phase of the cell cycle. Experiments were done in cell-free extracts of Xenopus laevis one-cell embryos. Firstly, we show that the timing of the M-phase entry is precisely determined by a balance between the activity of CDK1 kinase and okadaic acid sensitive phosphatase, mainly PP2A. Secondly, we show the role of CDC6 protein in regulation of the entry into the first embryonic M-phase. CDC6 inhibits CDK1 and through this action regulates the dynamic of this kinase upon M-phase entry and during M-phase progression. This mechanism discovered during my PhD allows controlling precisely the timing of embryonic cleavage. This control plays a key role in coordinating the cell cycle regulating machinery and the development program of the embryo.

<p>To ensure genomic integrity, dividing cells implement multiple checkpoint pathways during the course of the cell cycle. In response to DNA damage, cells may either halt the progression of the cycle (cell cycle arrest) or undergo apoptosis. This choice depends on the extent of damage and the cell's capacity for DNA repair. Cell cycle arrest induced by double-stranded DNA breaks relies on the activation of the ataxia-telangiectasia (ATM) protein kinase, which phosphorylates cell cycle effectors (e.g., Chk2 and p53) to inhibit cell cycle progression. ATM is an S/T-Q directed kinase that is critical for the cellular response to double-stranded DNA breaks. Following DNA damage, ATM is activated and recruited to sites of DNA damage by the MRN protein complex (Mre11-Rad50-Nbs1 proteins) where ATM phosphorylates multiple substrates to trigger a cell cycle arrest. In cancer cells, this regulation may be faulty and cell division may proceed even in the presence of damaged DNA. We show here that the RSK kinase, often elevated in cancers, can suppress DSB-induced ATM activation in both Xenopus egg extracts and human tumor cell lines. In analyzing each step in ATM activation, we have found that RSK disrupts the binding of the MRN complex to DSB DNA. RSK can directly phosphorylate the Mre11 protein at Ser 676 both in vitro and in intact cells and can thereby inhibit loading of Mre11 onto DSB DNA. Accordingly, mutation of Ser 676 to Ala can reverse inhibition of the DSB response by RSK. Collectively, these data point to Mre11 as an important locus of RSK-mediated checkpoint inhibition acting upstream of ATM activation.</p><p>The phosphorylation of Mre11 on Ser 676 is antagonized by phosphatases. Here, we screened for phosphatases that target this site and identified PP5 as a candidate. This finding is consistent with the fact that PP5 is required for the ATM-mediated DNA damage response, indicating that PP5 may promote DSB-induced, ATM-dependent DNA damage response by targeting Mre11 upstream of ATM.</p> / Dissertation

The healing process is an attractive topic in biology and medicine. There are two types of wound healing - in embryos and in adults. In the case of embryonic wound healing the whole process is simplified and accelerated. An inflammatory reaction typical for adult healing is eliminated and the wound is healed without scars. Better understanding of embryonic wound healing could lead to more effective treatment of injuries, burns and chronic wounds in human population. The studying of molecular mechanisms in the healing process is also meaningful in the context of understanding the cancer regulation. The wound healing could be compared with embryonic development in many ways. The body's axes are essential for a spatial activation of genes involved in development. Our hypotesis was that the axis are also important for wound healing. It has been suggested that the cell's polarity and motility depends whether the wound is vertical or horizontal. There are also connections between wound healing and cancer development. One of them is the degradation of the basement membrane (BM) and extracellular matrix (ECM). The process of degradation is catalyzed by the coordinated action of several classes of enzymes. Some of them - matrix metalloproteinases (MMP) and their role in embryonic wound healing have been...