Régulations transcriptionelles impliquant les hormones thyroïdiennes et les glucocorticoïdes à l'échelle du génome – TRIGGER

In multi-cellular organisms homeostasis is essential for life and requires integration of all physiological functions by regulatory and communication systems. Thyroid hormones (THs) are essential elements of these regulatory systems. They regulate diverse cellular processes from mitosis to apoptosis, from metabolism to growth and development. One striking developmental processes is amphibian metamorphosis, which is orchestrated by THs. Tadpole transformation is marked by dramatic TH-induced changes including de novo morphogenesis (limb), tissue remodelling (brain, intestine…) and organ resorption through apoptosis (tail). These changes involve cascades of gene regulation initiated by THs. However, gene regulatory programs involved in these complex developmental processes are poorly understood. THs do not act alone. Indeed, the diverse effects orchestrated by THs imply integration of other signals. Other major metamorphic players that has been identified in various historical and more recent approaches are glucocorticoids (GCs). GCs synergize with TH to induce metamorphosis. THs and GCs regulate gene expression through structurally similar receptors belonging to the nuclear receptor super family, respectively TH receptors (TRs) and GCs receptors (GRs). However, again GC signalling during metamorphosis is not sufficiently studied. Thus the overall aim of this project is to find the regulatory programs induced by THs and GCs during metamorphosis. A number of arguments make the Xenopus model the ideal vertebrate system for the challenge of dissecting genetic networks underlying TH and GC signalling in vivo in vertebrate systems. This model, which has been used traditionally to dissect developmental programs, as it is easy to manipulate, has recently become gained the added advantage of genetic analysis. Last but not least, metamorphosis has close and interesting parallels with the perinatal period in mammals. This developmental period is a critical phase for endocrine signalling. Indeed, both periods coincide with a peak of THs and GCs that have roles, notably in maturation of the nervous system, the respiratory system and the intestine. The characterisation of the regulatory programs induced by THs and GCs will help understand how the dysfunction of these two endocrine signals can have deleterious effects on human health. GC and TH associated diseases are common endocrine disorders worldwide. THs and GCs are frequently prescribed drugs. However, therapeutic use can lead to several adverse effects. Moreover, certain chemicals released in the environment have been shown to disturb THs and GCs signalling. Indeed, environmental effects or clinical treatment during the sensitive perinatal period affects birth outcomes and predicts health over the lifespan. What is more, early life conditions are now known to contribute to several later pathologies including public health priorities (mental diseases, cardiovascular diseases, obesity and cancer). Finally, as the dysfunctions can involve epigenetic modifications occurring during early life exposure, they can in certain case be transferred to the next generations. Understanding gene regulations controlled by TH and GC is a crucial step in the isolation of the potential molecular determinants involved in such diverse pathologies. Tissue-specific regulation provides a challenge for researchers and physicians. In this context, whole genome promoter, annotation and transcriptional states, are essential to understand fully gene regulation. Our first aim is to apply the ChIP-Seq (chromatin immunoprecipitation and sequencing) method to localise RNA polymerase II (Pol II) positions on Xenopus tropicalis genome. This novel methodology has dramatically changed how fundamental questions in biology are addressed. Assigning Pol II localisations to corresponding regulated genes will permit the identification of most of the TH and GC direct and indirect target genes and the position of their promoter(s). To further investigate the transcriptome of THs and GCs target genes, quantification and localisation of the expression levels of some pertinent promoters identified by Pol II ChIP will be done. The target genes will be chosen for their implication in cell fate determination or in pathological disorders but also as a function of their potential use as drug targets or biomarkers. The creation of transgenic Xenopus laevis lines carrying GR DNA binding sites controlling reporter gene will provide functional information on TH and GC control of target gene expression. The mapping of GR binding sites by ChIP using a anti GR antibody provide useful data and requires the development of an appropriate antibody. will also be a very useful result, but several tools are necessary. Finally, the extraction of pertinent results and their applications and diffusion to the scientific community will be carried out in the light of the impact of GCs and THs on human health and quality of life.