The Molecular Mechanisms that Establish Organ Domains Along the A-P Axis
Endoderm Patterning is Highly Conserved in Mouse and Chick
Figure 1. E7.5 mouse endoderm is not specified along the A-P axis.
Posterior (intestinal) mouse endoderm is induced to express Pdx1-lacZ (blue cells in chouse) when grafted into the pancreatic domain of the chick embryo. The top panel shows expression of the pancreatic marker Pdx1 in mouse. The middle panel is a control chick embryo, and the bottom panel shows expression of Pdx1-lacZ in posterior mouse endoderm that was grafted in to chick embryos.
Significant progress has been made in understanding how endoderm is first specified during gastrulation and how specific endoderm organs, once specified, undergo morphogenesis in the developing fetus. However, there is a profound gap in our understanding of how endoderm organ domains are initially established along the A-P axis of the developing gut tube, an obligatory step for endoderm organogenesis. For example, one fundamentally important question is how are some endoderm cells directed to form the pancreas whereas other cells are directed to form the liver?
Our lab uses several model organisms to investigate endoderm patterning including frogs, chick and mouse. The figure here shows a mouse to chick xenograft approach that we developed in the lab to investigate how endoderm are specified to adopt certain organ lineages along the A-P axis at e7.5 (Figure 1). In this experiment we demonstrated that mouse endoderm is still plastic at this stage, for example cells that would normally contribute to the large intestines are re-specified toward a pancreatic fate by grafting it into the pancreatic domain of a chick embryo. This result suggests that endoderm patterning is highly conserved between birds and mammals. In addition this is powerful assay to identify the genes and signals that regulate how endoderm cells are specified during organogenesis.
Relevant Publications/Review Articles
Moore-Scott BA, Opoka R, Kordich JJ, Lin S, Wells JM. Identification of molecular markers that are expressed in discrete anterior-posterior domains of the endoderm from the gastrula stage to mid-gestation. Dev Dyn 2007;236(7):1997-2003.
Wells JM, Melton DA. Early mouse endoderm is patterned by soluble factors from adjacent germ layers. Development 2000;127:1563-72.
The role of FGF signaling in establishing the foregut, midgut and hindgut domains
What are molecular mechanisms that specify A-P endoderm pattern at this stage in development? We have used both in vitro and in vivo analyses to identify that FGF4-mediated signaling plays a role in establishing gut tube domains in 2 ways. FGF4 represses foregut fate between the gastrula and early somite stages while at the same time promoting midgut and hindgut fate in a graded fashion. Currently, we are using chick and mouse embryos and human endoderm cultures (from differentiated ES cells) to identify how FGF signaling directing posterior endoderm patterning in vivo. We have recently begun to investigate how FGF pathways interact with other posterior pathways including the Wnt pathway (in collaboration with Aaron Zorn). Establishing these gene expression domains during gut tube patterning is critical for subsequent stages of endoderm organogenesis.
Relevant Publications
Moore-Scott BA, Opoka R, Kordich JJ, Lin S, Wells JM. Identification of molecular markers that are expressed in discrete anterior-posterior domains of the endoderm from the gastrula stage to mid-gestation. Dev Dyn 2007;236(7):1997-2003.
Serls AE, Doherty S, Parvatiyar P, Wells JM, Deutsch GH. Different thresholds of fibroblast growth factors pattern the ventral foregut into liver and lung. Development 2005;132:35-47.
Dessimoz J, Kordich JJ, Opoka R, Grapin-Botton A, Wells JM. FGF signaling is necessary for establishing gut tube domains along the anterior-posterior axis in vivo. Mech Dev 2006;123(1):42-55.
Pancreas Development
The pancreas is a vital, endodermally derived organ that regulates glucose homeostasis and digestion. In order to investigate molecular pathways underlying the development of the pancreas we used transcriptional profiling to identify the genes expressed during endocrine pancreas development. We focused our analyses on 4 biologically significant stages of pancreas development; pre-pancreatic endoderm, early pancreatic cells, endocrine progenitor cells, and Islets of Langerhan's. We have used Genespring and Resolver clustering analysis software to identify known and novel genes that are temporally regulated during endocrine development, as well as genes which spatially define these cells from their neighbors. These studies have identified signaling pathways that have not previously been implicated in pancreas development, such as the Wnt signaling pathway. We have subsequently knocked out the Wnt pathway and demonstrated that it is necessary for development of the exocrine pancreas in vivo.
The role of beta-catenin/Wnt signaling in pancreas development
The above studies demonstrated that components of the Wnt pathway are expressed during pancreas development. In particular, several Wnt receptors are expressed in the non-endocrine population of cells. To study the role of Wnt signaling in pancreas development, in collaboration with Andy Lowy at the University of Cincinnati (now at University of San Diego) the Wnt effector protein, beta-catenin was knocked out in the developing pancreas. Animals lacking beta-catenin in the developing pancreas have no remarkable defects in endocrine pancreas development, however exocrine development is profoundly impaired and by birth, exocrine cells are nearly absent, where as islets appear relatively normal. There is also evidence of ductal metaplasia. Microarray analyses of e14.5 and e16.5 pancreata lacking beta-catenin (in collaboration with Bruce Aronow, CCHMC) indicated that exocrine cells are specified, but exocrine progenitor cells fail to proliferate and expand and prematurely differentiate. These data indicate that beta catenin/Wnt signaling is required for expansion and maintenance of exocrine progenitor cells during development.
Relevant Publications
Wells JM, Esni F, Boivin GP, Aronow BJ, Stuart W, Combs C, Sklenka A, Leach SD, Lowy AM. Wnt/β-catenin is required for development of the exocrine pancreas. BMC Dev Biol 2007;7:4.
*Gu G, *Wells JM, Dombkowski D, Preffer F, Aronow B, Melton DA. Global analysis of gene regulatory pathways that function during development of the endocrine pancreas. Development 2004;131:165-79.
* These authors contributed equally and are listed alphabetically.
Wells JM. Genes expressed in the developing endocrine pancreas and their importance for stem cell and diabetes research. Diabetes Metab Res Rev 2003;19:191-201.
Also see the Endocrine Pancreas Consortium (http://www.cbil.upenn.edu/EPConDB/) web site for a data base of the genes expressed in the developing endocrine pancreas.
