Integrins modulate Sog activity in the Drosophila wing

Morphogenesis of the Drosophila wing depends on a series of cell-cell and cell-extracellular matrix interactions. During pupal wing development, two secreted proteins, encoded by the short gastrulation (sog) and decapentaplegic (dpp) genes, vie to position wing veins in the center of broad provein territories. Expression of the Bmp4 homolog dpp in vein cells is counteracted by expression of the Bmp antagonist sog in intervein cells, which results in the formation of straight veins of precise width. We screened for genetic interactions between sog and genes encoding a variety of extracellular components and uncovered interactions between sog and myospheroid (mys), multiple edematous wing (mew) and scab (scb), which encode betaPS, alphaPS1 and alphaPS3 integrin subunits, respectively. Clonal analysis reveals that integrin mutations affect the trajectory of veins inside the provein domain and/or their width and that misexpression of sog can alter the behavior of cells in such clones. In addition, we show that a low molecular weight form of Sog protein binds to alphaPS1betaPS. We find that Sog can diffuse from its intervein site of production into adjacent provein domains, but only on the dorsal surface of the wing, where Sog interacts functionally with integrins. Finally, we show that Sog diffusion into provein regions and the reticular pattern of extracellular Sog distribution in wild-type wings requires mys and mew function. We propose that integrins act by binding and possibly regulating the activity/availability of different forms of Sog during pupal development through an adhesion independent mechanism.     

Context-dependent relationships between the BMPs gbb and dpp during development of the Drosophila wing imaginal disk

The Drosophila BMP5/6/7/8 homolog, glass bottom boat (gbb), has been shown to be involved in proliferation and vein patterning in the wing disk. To better understand the roles for gbb in wing development, as well as its relationship with the Drosophila BMP2/4 homolog decapentaplegic (dpp), we have used clonal analysis to define the functional foci of gbb during wing development. Our results show that gbb has both local and long-range functions in the disk that coincide both spatially and functionally with the established functions of dpp, suggesting that both BMPs contribute to the same processes during wing development. Indeed, comparison of the mutant phenotypes of dpp and gbb hypomorphs and null clones shows that both BMPs act locally along the longitudinal and cross veins to affect the process of vein promotion during pupal development, and long-range from a single focus along the A/P compartment boundary to affect the processes of disk proliferation and vein specification during larval development. Moreover, we show that duplications of dpp are able to rescue many of the phenotypes associated with gbb mutants and clones, indicating that the functions of gbb are at least partially redundant with those of dpp. While this relationship is similar to that described for dpp and the BMP screw (scw) in the embryo, we show that the mechanisms underlying both local and long-range functions of gbb and dpp in the wing are different. For the local foci, gbb function is confined to the regions of the veins that require the highest levels of dpp signaling, suggesting that gbb acts to augment dpp signaling in the same way as scw is proposed to do in the embryo. However, unlike scw-dependent signals in the embryo, these gbb signals are not transduced by the Type I receptor saxophone (sax), thus, the cooperativity between gbb and dpp is not achieved by signaling through distinct receptor complexes. For the long-range focus along the A/P compartment boundary, gbb function does not appear to affect the high point of the dpp gradient, but, rather, appears to be required for low points, which is the reciprocal of the relationship between dpp and scw in the embryo. Moreover, these functions of gbb also do not require the Type I receptor sax. Given these results, we conclude that the relationships between gbb and dpp in the wing disk represent novel paradigms for how multiple BMP ligands signal during development, and that signaling by multiple BMPs involves a variety of different inter-ligand relationships that depend on the developmental context in which they act.     

Decapentaplegic head capsule mutations disrupt novel peripodial expression controlling the morphogenesis of the Drosophila ventral head

Drosophila adult structures derive from imaginal discs, which are sacs with apposed epithelial sheets, the disc proper (DP) and the peripodial epithelium (PE). The Drosophila TGF-beta family member decapentaplegic (dpp) contributes to the development of adult structures through expression in all imaginal discs, driven by enhancers from the 3' cis-regulatory region of the gene. In the eye/antennal disc, there is 3' directed dpp expression in both the DP and PE associated with cell proliferation and eye formation. Here, we analyze a new class of dpp cis-regulatory mutations, which specifically disrupt a previously unknown region of dpp expression, controlled by enhancers in the 5' regulatory region of the gene and limited to the PE of eye/antennal discs. These are the first described Drosophila mutations that act by solely disrupting PE gene expression. The mutants display defects in the ventral adult head and alter peripodial but not DP expression of known dpp targets. However, apoptosis is observed in the underlying DP, suggesting that this peripodial dpp signaling source supports cell survival in the DP.     

Engrailed and polyhomeotic maintain posterior cell identity through cubitus-interruptus regulation

In Drosophila, the subdivision into compartments requires the expression of engrailed (en) and hedgehog (hh) in the posterior cells and of cubitus-interruptus (ci) in the anterior cells. Whereas posterior cells express hh, only anterior cells are competent to respond to the hh signal, because of the presence of ci expression in these cells. We show here that engrailed and polyhomeotic (ph), a member of the Polycomb Group (PcG) genes, act concomitantly to maintain the repression of ci in posterior compartments during development. Using chromatin immunoprecipitation (ChIP), we identified a 1 kb genomic fragment located 4 kb upstream of the ci coding region that is responsible for the regulation of ci. This genomic fragment is bound in vivo by both Polyhomeotic and Engrailed. In particular, we show that Engrailed is responsible for the establishment of ci repression early during embryonic development and is also required, along with Polyhomeotic, to maintain the repression of ci throughout development.