Experimental and computational analyses of tissue patterning in development

Tissue patterning is a crucial part of animal development. It is a process by which a uniform group of cells adopts various developmental fates upon reading extracellular cues, normally in the form of nonuniformly distributed molecules that act as ligands for various cell surface receptors. Tissue patterning is highly regulated and abnormal patterning may result in developmental defects. We used both experimental and computational techniques to study the regulation of tissue patterning during animal development, using the Drosophila egg follicular epithelium as our model system.;During Drosophila egg development, the follicular epithelium, the precursor of the mature eggshell, is patterned by the EGFR and the BMP/Dpp signaling pathways. While initial studies suggested that the two pathways act independently (EGFR signaling regulates dorsal-ventral patterning and Dpp signaling regulates anterior-posterior patterning), we showed that they, in fact, interact at multiple levels. The purpose of follicular epithelium patterning is to determine the expression domains of genes involved in eggshell formation. One of these genes, Broad, is expressed in a two dimensional pattern and is a known target of both pathways. Chapter 2 will discuss our use of genetic and biomolecular tools to construct a mechanism of Broad regulation by the EGFR and Dpp pathways. This highly non-linear regulatory network, involving multiple feedforward and feedback loops, emphasizes the high degree of interaction between the two pathways during follicular epithelium patterning.;One of the newly discovered regulatory mechanisms included in the above network is the regulation of Dpp receptor expression by EGFR signaling. While pathway regulation at the level of ligand distribution is commonly found and relatively well studied, regulation at the level of receptor expression is rarely encountered. Chapter 3 will discuss the use of mathematical modeling to explore the role of nonuniform expression of Dpp receptor in the patterning of the follicular epithelium. We concluded that patterned receptor is required but insufficient for proper patterning and suggested that diffusion range of the ligand plays a crucial part.;Lastly, advances in experimental techniques allow rapid discovery of new regulatory interactions and thus assembly of new regulatory networks. Since the non-linearity of these networks limits the use of genetic experiments to systematically analyze them, we proposed the application of modeling approaches to not only validate the consistency of suggested regulatory mechanisms, but also guide future experiments and summarize the rapidly increasing volume of biological data. In Chapter 4, we formulated a mechanistic model to describe our proposed regulation of Broad by EGFR and Dpp signaling and showed that the model was able to successfully recapitulate the wild type dynamics of the system, predict mutational defects and suggest novel regulatory mechanisms.