Characterization of EGFR signaling dynamics and enhancer coordination during Drosophila oogenesis

Abstract

Development is directed by temporal changes in the spatial patterns of gene expression. Although it is recognized that changes in gene expression can result from the dynamics of inductive signals and stage-specific use of different genomic regulatory sequences called enhancers, the mechanisms integrating these phenomena remain less understood. We explore this question in Drosophila oogenesis, at stages when the grk ligand secreted from the oocyte activates the Epidermal Growth Factor Receptor (EGFR) in the follicular epithelium to determine the cells that will form two dorsal respiratory appendages in the eggshell. This process depends on broad (br), a zinc-finger transcription factor required to specify the appendage-forming cell fate.

In the first part of this thesis, we demonstrate that different phases of EGFR signaling regulate different features of br expression; while in the second part, we show that br expression is regulated by two br enhancers whose superposition recapitulate the dynamic expression of the gene. The pattern of br is initially regulated by an early enhancer (brE)¸ which is expressed uniformly and downregulated in the dorsal domain. Subsequently, br is regulated by a late enhancer (brL), which is active exclusively in the appendage-forming cells. The EGFR pathway differentially regulates both enhancers, repressing brE and inducing brL in an incoherent feedforward motif, and in this way coordinates their complementary spatial patterns.

While our initial characterization suggested that the late enhancer controls br function in dorsal appendage formation, the function of the early enhancer was unclear. We demonstrate that the early enhancer is also essential for appendage formation, and explain this using a mechanism whereby the br protein produced by the early enhancer protects the late enhancer from repression resulting from premature amplification of EGFR signaling. Furthermore, we provide experimental data demonstrating that premature amplification of signaling likely results from misexpression of rhomboid, a serine protease that mediates autocrine feedback activation of EGFR.

In the final chapter of this thesis, we formulate and validate a computational model combining a transient inductive signal, feedback modulation of signaling, and multiple enhancers that supports the feasibility of our proposed mechanism. We conclude by comparing our modeling results with the experimental observations, and suggest further improvements to the model to better capture the dynamics of EGFR signaling.