Transcriptional up-regulation of the TGF-β intracellular signaling transducer Mad of Drosophila larvae in response to parasitic nematode infection.

The common fruit fly Drosophila melanogaster is an exceptional model for dissecting innate immunity. However, our knowledge on responses to parasitic nematode infections still lags behind. Recent studies have demonstrated that the well-conserved TGF-β signaling pathway participates in immune processes of the fly, including the anti-nematode response. To elucidate the molecular basis of TGF-β anti-nematode activity, we performed a transcript level analysis of different TGF-β signaling components following infection of D. melanogaster larvae with the nematode parasite Heterorhabditis gerrardi. We found no significant changes in the transcript level of most extracellular ligands in both bone morphogenic protein (BMP) and activin branches of the TGF-β signaling pathway between nematode-infected larvae and uninfected controls. However, extracellular ligand, Scw, and Type I receptor, Sax, in the BMP pathway as well as the Type I receptor, Babo, in the activin pathway were substantially up-regulated following H. gerrardi infection. Our results suggest that receptor up-regulation leads to transcriptional up-regulation of the intracellular component Mad in response to H. gerrardi following changes in gene expression of intracellular receptors of both TGF-β signaling branches. These findings identify the involvement of certain TGF-β signaling pathway components in the immune signal transduction of D. melanogaster larvae against parasitic nematodes .