The Role and Regulation of the Transcription Factor SP5 in <i>Hydra</i> Patterning

The Wnt/b-catenin signaling pathway is highly conserved in all metazoans, involved in numerous developmental processes and regulating stem cell proliferation in adulthood. Disruption of the Wnt/b-catenin signaling pathway induces a range of abnormalities in embryonic development as well as important adult pathologies such as cancer. In Hydra, the Wnt/b-catenin signaling pathway plays a key role in the apical organizer that maintains apical patterning and allows the development of new heads. The objective of this PhD project is to better understand the regulation of the Wnt/b-catenin pathway during Hydra regeneration and maintenance of the apical organizer. We designed strategies to understand the dialogue between Wnt/b-catenin signaling and Sp5 in the maintenance of the apical organizer in adult animals and in the formation of a new organizer during developmental processes such as budding and regeneration. We first identified the transcription factor Sp5 as an inhibitor of the apical organizing center in Hydra, and show that this gene fulfills the five conditions necessary for an apical inhibitor. Briefly, this Sp5 gene is (1) predominantly expressed in the head, with a graded expression profile from apical to basal; (2) up-regulated during apical regeneration; (3) its expression is induced by the Wnt/ b-catenin signaling pathway; (4) Sp5 inhibits the expression of Wnt3 and b-catenin, thereby inhibiting the activity of this signaling pathway; and (5) Sp5 restricts apical development. In the following chapter, we focused on the study of Sp5 regulation in vivo. We generated two transgenic lines, which constitutively express either in the epidermal layer or in the gastrodermal layer a unique construct, whose tandem structure allows the expression of the mCherry reporter gene under the control of the Hydra actin gene promoter on the one hand, and the eGFP reporter gene under the control of the Hydra Sp5 promoter on the other. This approach revealed that in the intact animal, especially in the apical organizer region, Sp5 has a distinct spatial regulation in the epidermis and in the gastrodermis. In conclusion, this study highlights the fine regulation of Sp5, a target gene of the Wnt/b-catenin signaling pathway, as well as the putative interactions between Wnt3 and Sp5 in the different cell layers of Hydra, either in intact or developing animals, or in animals subjected to pharmacological or genetic perturbations. It also highlights in the model organism of Hydraexpression patterns of Sp5 and Wnt3 that are different in the two modes of functioning of the apical organizer, homeostatic in adult animals on the one hand, dynamic because in the process of formation or newly formed on the other hand.