20-P007 Role of Neurogenin3 in the migration of pancreatic endocrine cells

Abstract

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...