The labs represented in this group are headed by the following individuals (labs wishing to have their research interests, recent publications, etc listed can send files to Tom Sargent (tsargent@nih.gov):
Yun-Bo Shi
Thyroid Hormone Regulation of Organogenesis and Roles of Extracellular Matrix
We have chosen the remodeling of the intestine during metamorphosis of the amphibian Xenopus laevis as a model to study organ development. The tadpole intestine is a simple tubular organ consisting of predominantly a single layer of primary epithelium. It is drastically remodeled during metamorphosis into a complex structure with multiple epithelial folds. This process involves cell death in the primary epithelium and proliferation and differentiation of various types of adult cells. Although complex, the entire process is controlled by thyroid hormone (TH). Thus, merely by adding TH to tadpole rearing water, or simply to medium of intestinal organ cultures, precocious metamorphosis can be induced.
We have recently isolated many TH response genes in the intestine. Our research focuses primarily on the regulation and function of these genes during intestinal remodeling. The first major area of our research concerns with the regulation mechanism of these genes by TH. This involves promoter analysis to identify cis-acting elements and trans-acting factors important for the regulation by TH. We are also interested in studying the effects of chromatin on transcriptional regulation by TR.
It is known that the extracellular matrix (ECM) plays important roles during organ development. Matrix metalloproteinases (MMPs) are important participants in the remodeling of the ECM during organogenesis. We have identified several MMP genes as TH response genes in the intestine. Of particular interest is the stromelysin-3 gene, whose expression during metamorphosis correlates with cell death in various organs and adult epithelial cell proliferation and differentiation in the intestine. We are interested in the spatial and temporal expression of stromelysin-3 protein and mRNA and its function in ECM modification and thus the role in intestinal remodeling. We also plan to investigate the roles of various ECM components and other MMPs during intestinal epithelial morphogenesis.
Our group is broadly interested in the mechanisms that control vertebrate development. We are focused on three main research problems: (1) Regulation of epidermal differentiation in the mouse (2) Function of Eph-class receptor tyrosine kinases in the Xenopus embryo and (3) Neural-specific cadherin expression and function in the zebrafish embryo.
For the mouse epidermis project, we use mis-expression and gene knock-out methods with transgenic mice to analyze the function of homeodomain genes in the skin of embryos and neonatal animals. We are particularly interested in members of the Distal-less class of homeobox genes. Mis-expression of one member of this family in the epidermis of transgenic mice results in severe disruption of skin development, probably by altering the timing of expression for epidermal proteins, such as filaggrin.
The receptor tyrosine kinase project deals primarily with two members of the Eph-class, one is called "Pagliaccio", and is a homolog of the SEK (mouse) Hek8 (human) and Cek8 (chick) genes. When experimentally up-regulated, Pagliaccio leads to a loss of cell adhesion between embryonic blastomeres, causing the embryo to dissociate. The biochemical, molecular and cell biological aspects of this phenotype are currently under study.
In the zebrafish, we are concentrating on a novel cadherin cloned in our lab. This gene, called VN-cadherin, is expressed in the ventral neural tube during early development, and in the brain of adult fish. Its function is unknown, but we hypothesize that VN-cad could play a role in axonal targeting and/or fasciculation in the fish. Mammalian homologs of this gene exist, but are poorly characterized.
The molecular basis of vertebrate embryogenesis in the amphibian Xenopus laevis and the zebrafish, Danio rerio, is the focus of the research in this group. After fertilization, the frog embryo establishes its future dorsoventral axis by generating a dorsal signal center, named the Nieuwkoop center. Signal transduction initiated by Wnt factors has been shown to be involved in Nieuwkoop center function. Current work, in collaboration with Xi He and H. Varmus, continues to probe the molecular mechanisms of initial axis determination, in particular the role of GSK-3 in this process.
With the beginning of gastrulation, dorsoventral polarity becomes overt, and cell movements and tissue differentiation ensue. This laboratory has studied the role of cell interactions and of the spatially restricted expression of transcription factor- encoding genes during gastrulation. A major focus is the study of the Xlim-1 gene which encodes a LIM class homeodomain protein that is expressed specifically in the dorsal mesoderm, the so-called Spemann organizer, during the gastrula stage. We have implicated the Xlim-1 protein in the stimulation of production of signals that induce neighboring cells to nervous tissue (neural induction) and modify ventral mesoderm to muscle and other dorsolateral tissues (dorsalization). In addition we study the transcriptional regulation of the Xlim-1 gene itself by growth factors like activin, and the regulation of the putative target gene goosecoid by the Xlim-1 protein (in collaboration with K. Cho). The homologous lim1 gene in the zebrafish has been characterized, and a search for lim1 mutants in this organism is being pursued in collaboration with M. Halpern.
Additional areas of interest concern the analysis of related LIM-homeobox genes, notably lim3 which is specifically expressed in the pituitary, pineal, and spinal motoneurons. Further, signalling molecules related to nodal have been isolated from the zebrafish, and their expression and possible function is being studied.