Growth factors are important signaling proteins that control cell growth, survival, and differentiation in the developing and adult retina. The ultimate objective of the proposed research is to understand the molecular mechanisms that growth factors use to regulate cell behavior in the retina. We address this question by studying the differentiation of photoreceptor and cone cells in the Drosophila eye that are regulated by growth factor ligands for the Sevenless and EGF Receptor Tyrosine Kinases. This could provide important insights into how retinal cell differentiation is regulated, and how errors in growth factor signal transduction can lead to retinal diseases such as macular degeneration and retinitis pigmentosa. A paradox of growth factor signaling is how cells respond so specifically to a signal that appears to carry little informational content. For example, the Pros gene becomes transcriptionally activated as an immediate-early target of EGFR in a subset of cells (R7 and cone) that are activated by EGFR. A combinatorial mechanism restricts Pros enhancer responsiveness to EGFR activation. Other required inputs include two transcription factors (Lola, Lozenge) and a cell-cell signal mediated by the Notch Receptor. Pros transcription increases specifically in the R7 photoreceptor where it is required for its proper differentiation. The mouse homolog of Pros, Prox-1, is also required for proper development of the eye. The R7-specific transcriptional response requires activation of the Sevenless receptor, which induces expression of another immediate-early gene, Phyllopod. Phyllopod associates with a transcriptional repressor called Tramtrack, and this results in ubiquitination and degradation of Tramtrack protein. Tramtrack destruction may be the sole essential action of Sevenless to activate Pros. Our approach has been to characterize molecular interactions between proteins in the GFR/Sevenless pathways and a target gene, Pros. As such, Pros serves as an experimental model gene. We use a combination of genetic and biochemical experiments. The goal of the proposed research is to further characterize EGFR and Sevenless signaling by: 1) determining how other signals like Notch integrate with the EGFR signal to regulate Pros, 2) determining how EGFR and Sevenless exert different effects on Pros transcription, 3) characterizing a second model target gene, Phyllopod, as a way to compare and contrast with Pros regulation.
