Notch signaling and the patterning of Drosophila mesodermal segments

One of the first steps in embryonic mesodermal differentiation involves the allocation of cells to particular fates. In Drosophila, this process of subdivision requires regulation of the bHLH transcription factor Twist. During subdivision, Twist expression is modulated into stripes of low and high levels within each mesodermal segment. High Twist levels direct cells to the somatic muscle fate, whereas low levels are permissive for other mesodermal fates. My work demonstrates that Notch (N) signaling, mediated by the Suppressor of Hairless [Su(H)] transcription factor, represses Twist expression during subdivision and thus plays a critical role in patterning mesodermal segments. Genetics and promoter analysis revealed that N acts as a transcriptional switch on mesodermal target genes. Furthermore, data suggests N/Su(H) directly regulates twist, as well as indirectly regulates twist by activating proteins that repress Twist. I propose that N targets two distinct "Repressors of twist"---the Enhancer of split [E(spl)] complex proteins and the HLH protein Extra machrochaetae, which inhibits an activator of twist, the bHLH protein Daughterless. Hence, N signaling directs the activity of a network of bHLH transcriptional regulators.; My results also support the instructive and permissive N target gene model. Instructive gene transcription requires N to first alleviate Su(H)-mediated repression and then to serve as a coactivator for Su(H). In contrast, permissive gene activation requires N to solely alleviate Su(H)-mediated repression; other coactivators and/or transcriptional activators are necessary for transcription. Genetic data suggests that N acts permissively on the twist promoter and instructively on a repressor of twist promoter, such as E(spl).; Lastly, data indicate that the establishment of a modulated Twist pattern involves the integration of both N and Wingless (Wg) signaling. I hypothesize that N represses twist throughout the mesoderm, while Wg signaling and its immediate target, the Sloppy-paired transcription factor, antagonize N repression in presumptive high Twist domains.; Taken together, my findings underscore the complexity of N/Su(H) bHLH regulation in the Drosophila embryo. Furthermore, they suggest mechanisms for the analogous process of somite development in vertebrates.