Identification and characterization of transcriptional enhancers integrating Notch and other developmental signals

Abstract

Cell signaling pathways are frequently used in multiple tissue and stage-specific contexts during multicellular development. The integration of these signaling pathways by transcriptional enhancers controls the tissue specific gene expression necessary for proper development. Enhancers are segments of DNA that interpret developmental signals to produce patterns of gene expression. A set of operational rules defines how different enhancers targeted by the same signals interpret and act on these signals. Using the Drosophila model system, my thesis work focuses on determining the operational rules used by developmental enhancers that integrate the Notch signaling pathway with other pathways. The Notch signaling occurs between two adjacent cells and is used to pattern cell territories involved in cell fate determination. When membrane bound ligand interacts with the Notch receptor, the Notch Intercellular Domain (NICD) is cleaved and moves into the nucleus. In the nucleus the NICD interacts with the enhancer bound Suppressor of Hairless [Su(H)]. This interaction abrogates repression by Su(H) and works now to drive (activate) expression. I first used a computational approach to identify a set of candidate Notch-target enhancers. From this set I selected and studied one specific enhancer from the nab gene that integrates the Notch and Bone Morphogenetic Protein (BMP) signaling pathways with several wing specific selectors. The enhancer drives expression in the dorsal compartment of the developing wing imaginal disc. This selector-licensed integration of signaling pathways allows for the observed expression pattern. This nab enhancer is also a part of a cluster of enhancers that work together to drive the global nab expression pattern during development. Each of these enhancers drives the expected expression patterns as well as atypical expression patterns, which are silenced by adjacent enhancers. These results suggest that Notch targeted enhancers are involved in both tissue specific gene activation and gene silencing. v