During animal development and in regenerating tissues, cells communicate with each other by releasing signaling molecules that can be recognized by specific receptors on neighboring cells. This cell-cell signaling is essential to regulate appropriate cell fate and growth. The research carried out in the Cadigan laboratory is concerned with the Wnt family of signaling proteins, which are known to play important roles in development and regeneration throughout the animal kingdom. Wnts are generally thought to act by activating gene expression in cells receiving the signal, however, the Cadigan laboratory has identified several genes that are directly repressed by Wnt signaling. These repressed targets have a different DNA sequence code in their regulatory regions compared with activated targets. The project will define this "repression code" in great detail, which will enable the Cadigan laboratory and other researchers to examine this underappreciated aspect of Wnt signaling in many important biological contexts.
This project will provide an excellent training opportunity for several graduate and undergraduate students working in the Cadigan lab, and students from underrepresented groups will be recruited to participate through programs run by the University of Michigan. Aspects of this project will also be incorporated into practical courses that Dr. Cadigan teaches for high school students visiting the University of Michigan campus and physicians entering research laboratories. In addition, a practical guide to characterizing targets that are repressed by Wnt signaling will be posted on an open resource web page for the Wnt community.
For animals to develop properly, cells need to communicate with each other, so that they make the correct choices of what type of cell to become. Cell-cell communication is also important for the maintance of adult tissues, and for wound repair. Surprisingly, many of these communication events are mediated by a handful of pathways, defined as groups of proteins that work together to deliver the signaling message from the cell surface to specific target genes in the nucleus. This project investigated the mechanisms by which one of these pathways, Wnt signaling, can both activate and repress target gene expression. We used the fruit fly Drosophila and the nematode C. elegans as model systems because they are simple animals with powerful genetics, allowing experiments to be performed more rapidly than in with mammalian models. Gene expression is controlled by proteins called transcription factors (TFs), and a particular type of TF, the TCF family, regulate Wnt target genes. The fly and worm target genes we have studied fall into two general classes. The first class followed the standard working model: they are repressed by TCF in unstimulated cells, and Wnt signaling converts TCF into an activator. The second class is regulated in the opposite manner, i.e., TCF activates their expression under basal conditions and Wnt signaling converts TCF into a repressor. We have demonstrated that a major factor influencing which class these targets fall into is the sequence of the DNA bound by TCF. To prove this, we altered the TCF binding sites of targets in each class, changing them into sites normally found in the other class. After this alteration, a target that was normally activated by the pathway was repressed by Wnt signaling. Similarly, changing the sites in a Wnt repressed target caused the target to be activated by Wnt signaling. During our characterization of both classes of targets, we discovered new aspects of Wnt biology. For example, in C. elegans, we discovered that Wnt signaling is required for proper defecation, which occurs rhythmically when nematodes have a constant source of food. This regulation occurs through activation of target gene expression by Wnt signaling in intestinal cells near the anus. We also found that Wnt mediated repression plays an important role in the regulation of cell fate in the fly larval lymph gland, the equivalent of the bone marrow in mammals. These findings may ultimately shed light on how this pathway controls target gene expression in higher animals.
