Decapentaplegic

Decapentaplegic (Dpp) is a key morphogen involved in the development of the fruit fly Drosophila melanogaster. It is known to be necessary for the correct patterning of the fifteen imaginal discs, which are tissues that will become limbs and other organs and structures in the adult fly. It has also been suggested that Dpp plays a role in regulating the growth and size of tissues. Flies with mutations in decapentaplegic fail to form these structures correctly, hence the name (decapenta-, fifteen, -plegic, paralysis). Dpp is the Drosophila homolog of the vertebrate bone morphogenetic proteins (BMPs), which are members of the TGF-β superfamily, a class of proteins that are often associated with their own specific signaling pathway. Studies of Dpp in Drosophila have led to greater understanding of the function and importance of their homologs in vertebrates like humans.

Dpp is a classic morphogen, which means that it is present in a spatial concentration gradient in the tissues where it is found, and its presence as a gradient gives it functional meaning in how it affects development. The most studied tissue in which Dpp is found is the wing. In the wing, Dpp is strongly expressed in a narrow stripe of cells down the middle of the wing where the tissue marks the border between the anterior and posterior sides. Dpp diffuses from this stripe towards the edges of the tissue, forming a gradient as expected of a morphogen.

Dpp, like its vertebrate homologs, is a signaling molecule. In Drosophila, the receptor for Dpp is formed by two proteins, Thickveins (Tkv) and Punt. Like Dpp itself, Tkv and Punt are highly similar to homologs in other species. When a cell receives a Dpp signal, the receptors are able to activate an intracellular protein called mothers against dpp (mad) by phosphorylation. The initial discovery of mad in Drosophila paved the way for later experiments that identified the responder to TGF-β signaling in vertebrates, called SMADs. Activated Mad is able to bind to DNA and act as a transcription factor to affect the expression of different genes in response to Dpp signaling. Genes activated by Dpp signaling include optomotor blind (omb) and spalt, and activity of these genes are often used as indicators of Dpp signaling in experiments. Another gene with a more complicated regulatory interaction with Dpp is brinker. Brinker is a transcription factor that represses the activation targets of Dpp, so in order to turn on these genes Dpp must repress brinker as well as activate the other targets.

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