PgmNr D1475: A novel Smad/Su(H)-target enhancer drives hedgehog expression in a signaling hole of the posterior wing disc compartment.

Authors:
Timothy Fuqua; Elizabeth Stroebele; Christian Noblett; Albert Erives


Institutes
Department of Biology, University of Iowa, Iowa City, IA, 52242-1324, USA.


Keyword: cis-regulatory logic

Abstract:

Developmental patterning involves dynamic integration of signals from the BMP, Notch, Wnt, Hedgehog and other signaling pathways. These signals are integrated by transcriptional enhancers in a manner that is customized for each target gene. To advance our understanding of this signal integration, we identified regulatory belts that are conserved across the Drosophila genus and contain binding sites for the Notch-signaling effector Su(H), the Dpp/BMP effectors Mad:Medea, and the LIM-homeodomain selector Apterous (Ap). We report identifying a novel wing imaginal disc enhancer for hedgehog driving an interesting expression pattern. While the hedgehog gene is normally expressed only in the posterior compartment of the wing imaginal disc, this novel enhancer, “hh-E”, drives twin spots of expression on either side of the Dpp expression stripe in the dorsal-proximal region of the columnar epithelial side of the wing disc. Of these twin spots of hh-E driven expression, one (ectopic) spot is located in the anterior compartment and the other (normal) spot is in the posterior compartment. While the hh-E expression pattern is consistent with a Dpp-readout licensed in part by Ap, it is not consistent with it being a canonical Notch/Su(H) target. To understand the role of the conserved Su(H) site in hh-E, we mutated this site and found that it functions as a dedicated repressor in the peripodial membrane of the wing disc. Mutated hh-E continues to drive twin spots of expression in the dorsal-proximal regions of both the anterior and posterior wing disc compartments. Thus, hh-E likely features a dedicated Su(H) repressor binding site that is refractive to Notch-induced activation. To understand what must be silencing the ectopic spot of anterior expression driven by hh-E, we “restored” anterior silencing in an expanded hh-DEF fragment. One of these flanking regions,“hh-F”, is another Su(H) binding site containing enhancer driving posterior-compartment specific expression in the distal regions of the wing disc, including the wing pouch. A combined hh-DEF enhancer with a mutated hh-F Su(H) site has attenuated wing pouch expression without diminishing the posterior spot of dorsal proximal expression driven by the embedded hh-E enhancer. In summary, the modular hh-DEF enhancers, which are different from the previously identified hh-ABC regulatory modules, are each responsible for various signaling holes in the wing disc and collectively piece together the full hedgehog expression throughout the posterior compartment. We conclude that some Su(H)-targeted enhancers may function independently of Notch signaling at unexpected loci and do so via additional unknown contextual cues present in these enhancers.



Flybase Genetic Index:
1. FlyBase gene symbol: Su(H); FBgn: FBgn0004837
2. FlyBase gene symbol: N; FBgn: FBgn0004647
3. FlyBase gene symbol: ap; FBgn: FBgn0267978
4. FlyBase gene symbol: dpp; FBgn: FBgn0000490
5. FlyBase gene symbol: Mad; FBgn: FBgn0011648
6. FlyBase gene symbol: Med; FBgn: FBgn0011655
7. FlyBase gene symbol: hh; FBgn: FBgn0004644