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Evolutionarily conserved developmental pathways
A key question in developmental biology is how growth factor signals are integrated to generate pattern. This study investigated the integration of the Drosophila BMP and Wingless/GSK3 signaling pathways via phosphorylations of the transcription factor Mad, usually considered to be an agent acting solely in the BMP pathway. Wingless was found to regulate the phosphorylation of Mad by GSK3 in vivo. In epistatic experiments, the effects of Wingless on wing disc molecular markers (senseless, distalless and vestigial) were suppressed by depletion of Mad with RNAi. Wingless overexpression phenotypes, such as formation of ectopic wing margins, were induced by Mad GSK3 phosphorylation-resistant mutant protein. Unexpectedly, Mad phosphorylation by GSK3 and MAPK was found to occur in segmental patterns. Mad depletion or overexpression produced Wingless-like embryonic segmentation phenotypes. In Xenopus embryos, segmental border formation was disrupted by Smad8 depletion. The results show that Mad is required for Wingless signaling and for the integration of gradients of positional information (Eivers, 2009; full text of article).
Many developmental mechanisms have been conserved during evolution, but segmentation is one in which commonalities between Drosophila and the vertebrates have not been found. Segmentation in vertebrates relies on the cyclic oscillation of Notch pathway transcripts in the posterior paraxial mesoderm. In theory, Smad1/5/8 could provide an attractive regulator of the segmentation clock, because BMP signals have a duration of 12 hours in cultured cells, which can be extended by inhibiting GSK3. Wnt pathway genes cycle rhythmically in vertebrates, offering an interesting possibility for regulating Smad5/8 activity. Notch is required for segmentation in spiders, but not in Drosophila (Damen, 2007). Recently, it has been found that in the cockroach, an insect in which the segments are formed sequentially in a posterior growth zone (and not simultaneously as in Drosophila), stripes of Delta and Hairy mRNA (two genes of the Notch pathway) cycle rhythmically as in the vertebrates (Damen, 2007). This study has now found that Smad5/8 is required for the formation of segmental boundaries in Xenopus somites and that Mad is required for Drosophila segment patterning. However, the results do not establish whether similar molecular steps are affected in both organisms. The conservation of this unexpected conserved role for Mad/Smad is important from an Evo-Devo perspective because it suggests that the last common ancestor shared between Drosophila and vertebrates, Urbilateria, might have been segmented (Eivers, 2009).
These studies on Drosophila Mad have uncovered an unexpected role for Mad in the Wg signaling pathway. Mad/Smads are transcription factors that have low binding affinity for DNA and require other DNA binding proteins as co-factors in order to recognize the promoters and enhancers of hundreds of target genes. Future work will have to address how Mad or its partner Medea/Smad4 interact with proteins such as Armadillo/β-Catenin and Pangolin/Lef1 on Wnt-responsive promoters in Drosophila. The present study shows that Mad is required for Wg to signal, through its GSK3 phosphorylation sites, in a number of different in vivo assays. These include wing margin formation, sensory bristle induction in the wing, induction of the Wg induced gene senseless, the repression of neurogenic ectoderm, and segmental patterning. It is proposed that Mad serves as an integrator of patterning signals, which determine embryonic positional information. The finding that three major signaling pathways MAPK, Wnt/GSK3 and BMP are integrated at the level of Mad/Smad1/5/8 both in Drosophila and in the vertebrates has interesting implications for the evolution of animal forms through variations on an ancestral gene tool-kit (Eivers, 2009).
Damen, W. G. (2007). Evolutionary conservation and divergence of the segmentation process in arthropods. Dev. Dyn. 236: 13791391. PubMed Citation: 17440988
Eivers E., et al. (2009). Mad is required for wingless signaling in wing development and segment patterning in Drosophila. PLoS ONE 4: e6543. PubMed Citation: 19657393
date revised: 10 February 2012
Developmental Pathways conserved in Evolution
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