Molecular regulation of gene expression in anterior mesendoderm of vertebrates

Abstract

The signaling pathways that regulate the fate of cells located in the anterior lateral (AL) region of the vertebrate embryo are not well understood. Mesodermal cells in this region will assume the heart muscle phenotype while adjacent endoderm gives rise to foregut tissues such as the liver. The AL endoderm supplies key signaling molecules to promote the survival and differentiation of the precardiac mesoderm. These AL endoderm factors are know to up-regulate transcription factors, such as Nkx2--5, that regulate cardiac genes. However, little is known about how the AL endoderm is patterned and the exact mechanism by which the cardiac transcription factors function within the mesoderm. Therefore, two projects were pursued to understand the developmental pathways that promote early heart development. One project looks at defining the mechanism by which the Nkx2 homeobox genes regulate cardiac gene expression in mouse embryonic stem cells. Unfortunately, this project was plagued with difficulties. Mouse ES cells were used as a model system to study cardiac differentiation. However, these cells were found to contain a potent genome protection mechanism that prevented the stable integration of transcription factors. This phenomenon is addressed and discussed within the thesis. The second project defines the role of retinoids in patterning the AL endoderm. In this study, the homeobox gene cHex, a gene required for hepatocyte development, was used as an AL endoderm marker. It was found that retinoids act directly on the cHex gene promoter to reduce its activity and restrict its expression domain to the AL endoderm.