All pancreatic endocrine cells, producing glucagon, insulin, somatostatin, or PP, differentiate from Pdx1+ progenitors that transiently express Neurogenin3. To understand whether the competence of pancreatic progenitors changes over time, we generated transgenic mice expressing a tamoxifen-inducible Ngn3 fusion protein under the control of the pdx1 promoter and backcrossed the transgene into the ngn3(-/-) background, devoid of endogenous endocrine cells. Early activation of Ngn3-ER(TM) almost exclusively induced glucagon+ cells, while depleting the pool of pancreas progenitors. As from E11.5, Pdx1+ progenitors became competent to differentiate into insulin+ and PP+ cells. Somatostatin+ cells were generated from E14.5, while the competence to make glucagon+ cells was dramatically decreased. Hence, pancreas progenitors, similar to retinal or cortical progenitors, go through competence states that each allow the generation of a subset of cell types. We further show that the progenitors acquire competence to generate late-born cells in a mechanism that is intrinsic to the epithelium.

During development, pancreatic endocrine cells are specified within the pancreatic epithelium. They subsequently delaminate out of the epithelium and cluster in the mesenchyme to form the islets of Langerhans. Neurogenin3 (Ngn3) is a transcription factor required for the differentiation of all endocrine cells and we investigated its role in their delamination. We observed in the mouse pancreas that most Ngn3-positive cells have lost contact with the lumen of the epithelium, showing that the delamination from the progenitor layer is initiated in endocrine progenitors. Subsequently, in both mouse and chick newly born endocrine cells at the periphery of the epithelium strongly decrease E-cadherin, break-down the basal lamina and cluster into islets of Langerhans. Repression of E-cadherin is sufficient to promote delamination from the epithelium. We further demonstrate that Ngn3 indirectly controls Snail2 protein expression post-transcriptionally to repress E-cadherin. In the chick embryo, Ngn3 independently controls epithelium delamination and differentiation programs. Developmental Dynamics 240:589-604,

The pancreas is a vital gland of exocrine and endocrine function. It is the target of two main affections: diabetes and pancreatic cancer. We describe the tissue interactions, signaling pathways and intracellular targets that are involved in the emergence of the pancreas primordium and its proliferation, morphogenesis and differentiation. It appears that several genes of developmental relevance have an adult function and are involved in pancreas affections. Embryological experimentation in animals contributed to provide candidate genes for human disease and holds promise for future treatments.

During development, the pancreatic endocrine cells are specified within the epithelium. They will subsequently delaminate and migrate out of the epithelium in order to form the islets.Neurogenin3 (Ngn3) is a bHLH transcription factor that is responsible for differentiation of all endocrine cell types, but whether or not it has a role in their migration is still an open question. By using the chicken embryo model organism, we found that differentiation and migration programs are two different processes that are induced by Ngn3 and that can be uncoupled. Therefore, we tried to unravel the mechanisms by which Ngn3 can induce endocrine cell migration. We found that, both in chick and mouse models, overexpression of Ngn3 induces a loss of apico-basal polarity, a breakdown of basal lamina, and more importantly, a loss of the epithelial marker E-cadherin (Ecad). We also found that this is not a direct effect mediated by E-boxes in the Ecad promoter. Therefore, we are currently trying to find targets of Ngn3 that could mediate repression of Ecad, focalizing on the zinc-finger transcription factors Snail and Slug. Moreover, we are also using a pancreatic explant culture method, developed in the laboratory of Pr. Jonathan Slack, that allows us to do time-lapse imaging. Taking advantage of our Pdx::Ngn3 ERTM transgenic line, we are following Ngn3 overexpressing cells to understand in a more physiological manner how they migrate in the developing pancreas.Recent studies have provided evidence that bHLH transcription factors such as Neurogenin (Ngn) and Mash1 regulate neuronal migration (Hand et al., 2005; Ge et al., 2006). Rnd2 has been found to mediate Ngn2 activity in cell migration in the cerebral cortex (Heng et al., 2008). However, the mechanisms by which Mash1 regulates neuronal migration appear to be different and are still unknown.By screening putative Mash1 targets identified in expression microarray experiments, we found genes that regulate cell migration in other systems like RhoE/Rnd3. As its role in the developing nervous system has not yet been addressed, we have begun to study the role of Rnd3 in the radial migration of cortical neurons. Rnd3 knock-down in the dorsal telencephalon at E14.5 results in radial migration defects of cortical neurons and increases the fraction of dividing progenitors. These migration defects are due to a distinct function of Rnd3 in post-mitotic neurons. Moreover Rnd3 silencing affects the morphology of migrating neurons. Mash1 is required for Rnd3 expression in the telencephalon and conserved Mash1 binding sites (E-boxes) are located in noncoding sequences of Rnd3 gene. Some of these E-boxes are able to bind Mash1 suggesting that this proneural factor directly regulates Rnd3 expression in telencephalic neurons. These results demonstrate that the Mash1-Rnd3 pathway plays a critical role in the migration of projection neurons in the developing telencephalon. The expression, regulation and knockdown phenotypes of Rnd2 and Rnd3 also indicate that the two genes are part of different reg...