Pancreas-specific deletion of beta-catenin reveals Wnt-dependent and Wnt-independent functions during development

Mutations and deregulation of adenomatous polyposis coli (APC) and beta-catenin are implicated in specific cancers of the pancreas, but the role of Wnt pathway in normal pancreas development and homeostasis is unknown [1-4]. This article reports a comprehensive investigation of the activity and the role of the Wnt pathway in pancreas organogenesis. We have used two reporter lines to monitor canonical Wnt pathway activity during development and after birth and demonstrate activity in endocrine cells and in the mesenchyme. We have specifically deleted the beta-catenin gene in the epithelium of the pancreas and duodenum by using Pdx1-Cre mice. In agreement with Wnt pathway activity in pancreatic endocrine cells, we find a reduction in endocrine islet numbers. Our study reveals that beta-catenin deletion also affects cells in which Wnt pathway activity is not detected. Indeed, beta-catenin mutant cells have a competitive disadvantage during development that also affects the exocrine compartment. Moreover, the conditional knockout (KO) mice develop acute edematous pancreatitis perinatally due to the disruption of the epithelial structure of acini. These effects are likely to be due to the function of beta-catenin at the membrane. Mice later recover from pancreatitis and regenerate normal pancreas and duodenal villi from the wild-type (wt) cells that escape beta-catenin deletion.

Ductal cells of the pancreas

Anne Grapin-Botton

Ductal cells of the pancreas form the epithelial lining of the branched tubes that deliver enzymes produced by pancreatic acinar cells into the duodenum. In addition, these cells secrete bicarbonate that neutralizes stomach acidity. During development, epithelium of endodermal origin evaginates from the future duodenum area and invades the mesenchyme to form a complex branched network. All endocrine, acinar and ductal cells arise from common precursors in this epithelial structure. Adult ductal cells share some similarities with embryonic primitive ducts and may retain the ability to generate endocrine cells in the adult. Based on challenged pancreas regeneration experiments, the adult ductal cells have been proposed to be pancreatic stem cells but their role in normal endocrine cell turnover has recently been challenged. Manipulating their ability to give rise to endocrine cells may open new avenues in the treatment of diabetes and therefore they have recently been under scrutiny. In addition, in the main form of pancreatic cancer, pancreas adenocarcinoma, tumor cells share similarities with ductal cells. The secrets of an appropriate therapy for this deadly cancer may thus reside in the biology of ductal cells. (C) 2004 Elsevier Ltd. All rights reserved.

2005

Role of Neurogenin3 in migration of pancreatic endocrine cells during development

Mathieu Gouzi

The pancreas is one of the major organs of the digestive tract. Its endocrine function, mainly regulating glucose blood level, is achieved by endocrine cells. These cells are organized in clusters, the so-called islets of Langerhans. During development, these cells are specified within the pancreatic epithelium, and 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 the 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 the 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, in both chick and mouse models, over-expression 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 (E-cad). It seems that this is not a direct effect mediated by direct binding to E-boxes in the E-cad promoter. Therefore, we are currently trying to find targets of Ngn3 that could mediate repression of E-cad, focusing on the zinc-finger transcription factors Snail1 and Snail2. Moreover, we also used a pancreatic explant culture method, developed in the laboratory of Pr. Jonathan Slack, which allows us to do time-lapse imaging and describe precisely the migration of endocrine cells in a system similar to in vivo conditions. Taking advantage of our Pdx1::Ngn3ERTM transgenic line, we followed Ngn3 misexpressing cells to understand in a more physiological manner how they migrate in the developing pancreas. We found that endocrine cell migrate at an average speed of 11.35 µm/h and migrate through an average length of 132.30 µm in 12 hours. We also found that endocrine cells migrate in the three compartments we defined: in the mesenchyme, inside the epithelium and at the edge of the epithelium. Their average velocity significantly changes depending on the compartment cells are migrating in. We found that the fastest cells are the ones migrating at the edge of the epithelium. RNA profiling comparing Ngn3 knockouts rescued or not by the Pdx1::Ngn3ERTM transgene was also used to identify the genes that function in endocrine cell migration downstream of Ngn3. We identified new Ngn3 target genes, some of which are implicated in migration such as the Doublecortin family.

EPFL2010

Key events of pancreas formation are triggered in gut endoderm by ectopic expression of pancreatic regulatory genes

Anne Grapin-Botton

The mechanisms by which the epithelium of the digestive tract and its associated glands are specified are largely unknown. One clue is that several transcription factors are expressed in specific regions of the endoderm prior to and during organogenesis. Pdx-1, far example, is expressed in the duodenum and pancreas and Pdx-1 inactivation results in an arrest of pancreatic development after buds formation. Similarly, ngn3 is transiently expressed in the developing pancreas and a knockout results in the absence of endocrine cells. This paper focuses on the question of whether these and other transcription factors, known to be necessary for pancreatic development, are also sufficient to drive a program of pancreatic organogenesis. Using in ovo electroporation of chick embryos, we show that ectopic expression of Pdx-1 or ngn3 causes cells to bud out of the epithelium like pancreatic progenitors. The Pdx-1-expressing cells extinguish markers for other nonpancreatic regions of the endoderm and initiate, but do not complete, pancreatic cytodifferentiation. Ectopic expression of ngn3 is sufficient to turn endodermal cells of any region into endocrine cells that form islets expressing glucagon and somatostatin in the mesenchyme. The results suggest that simple gene combinations could be used in stem cells to achieve specific endodermal tissue differentiation.

2001

Role of Neurogenin3 in migration of pancreatic endocrine cells during development

Mathieu Gouzi

EPFL2010