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Spatiotemporal Analysis of BMP Signaling in the Hepatopancreatic Endoderm

During mouse embryonic development, the pancreas originates from two separate outgrowths of the dorsal and ventral regions of foregut endoderm, which eventually fuse to generate one mature organ. The ventral foregut gives rise not only to the ventral pancreatic bud but also to the liver and the two fates are concomitantly specified between embryonic days E8.5-E9. Both hepatic and pancreatic endoderm domains are exposed to the same inducing factors, including BMP, TGF‐β and FGF, which are released from the surrounding tissues. Moreover, previous studies suggested that the ventral pancreas differentiates from bipotential progenitor cells, which can adopt either a pancreatic or a hepatic fate. In this thesis, I first present a comprehensive analysis of the temporal and spatial dynamics of BMP and TGF‐β signaling pathways in the ventral foregut region in the window of time between E8.0‐8.5. My results indicate that the BMP signaling is active in the future hepatic domains of the ventral foregut endoderm but silenced in the future pancreatic domain. By contrary, I find TGF-β signaling ubiquitously active throughout the foregut endoderm. My findings are in line with previous observations suggesting that basal levels of TGF-β activity are required for the proper development of both pancreatic and hepatic cell populations, whereas BMP signaling needs to be repressed for pancreas formation. Second, I generated a novel mouse model to study in vivo if BMP signaling exerts a role in the hepatic versus pancreatic fate decision during mouse embryogenesis and the window of time during which the endoderm is competent to respond to BMP. To activate in a spatiotemporally-controlled manner the BMP pathway, I generated a tetracycline (tet)-inducible mouse model. This is a binary system composed of the reverse tet-controlled transactivator (rtTA) inserted in the Prox1 locus and the operator sequences of the tet operon (tetO) driving the expression of a constitutively active form of the BMP receptor type IA (CA-Alk3). The Prox1 gene is the earliest marker of both pre-hepatic and pre-pancreatic endoderm from gastrulation onwards, enabling the spatial control of the CA-Alk3 transgene. The exposure to doxycycline allows temporally-controlled activation of the BMP pathway in Prox1-expressing cells of the double-transgenic mouse embryos. I performed functional characterization of the CA-Alk3 gain-of-function in vivo model at different time points of embryonic development. Preliminary results suggested that constitutive BMP activation leads to overall disorganization of the pancreatic tissue. Future studies are aimed at i. establishing the consequences of BMP activation in pancreas specification and ii. elucidating its role in pancreas morphogenesis in the mouse embryo.

BMP signaling; mouse; liver; pancreas; Tet-On system

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