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A central theme during development and homeostasis is the generation of cell type-specific responses to the action of a limited number of extant signaling cascades triggered by extracellular ligands. The molecular mechanisms by which information from such signals are integrated in responding cells in a cell-type specific manner remain poorly understood. We have undertaken a detailed characterization of an enhancer that is regulated by DPP signaling and by the homeotic protein Labial and its partners, Extradenticle and Homothorax. The expression driven by this enhancer (lab550) and numerous deletions and point mutants thereof was studied in wild-type and mutant Drosophila embryos as well as in cultured cells. We find that the lab550 enhancer is composed of two elements, a Homeotic Response Element (HOMRE) and a DPP Response Element (DPPRE) that synergize. None of these two elements can reproduce the expression of lab550, either with regard to expression level or with regard to spatial restriction. The isolated DPPRE of lab550 responds extremely weakly to DPP. Interestingly, we found that the inducibility of this DPPRE is weak because it is tuned down by the action of a repressor element. This repressor element and an additional 50 bp element appear to be crucial for the cooperation of the HOMRE and the DPPRE, and might tightly link the DPP response to the homeotic input. The cooperation between the different elements of the enhancer leads to the segmentally restricted activity of lab550 in the endoderm and provides a mechanism to create specific responses to DPP signaling with the help of a HOX protein complex.

Differentiation of the Drosophila oocyte takes place in a cyst of 16 interconnected germ cells and is dependent on a network of microtubules that becomes polarized as differentiation progresses (polarization). We have investigated how the microtubule network polarizes using a GFP-tubulin construct that allows germ-cell microtubules to be visualized with greater sensitivity than in previous studies. Unexpectedly, microtubules are seen to associate with the fusome, an asymmetric germline-specific organelle, which elaborates as cysts form and undergoes complex changes during cyst polarization. This fusome-microtubule association occurs periodically during late interphases of cyst divisions and then continuously in 16-cell cysts that have entered meiotic prophase. As meiotic cysts move through the germarium, microtubule minus ends progressively focus towards the center of the fusome, as visualized using a NOD-lacZ marker. During this same period, discrete foci rich in gamma tubulin that very probably correspond to migrating cystocyte centrosomes also associate with the fusome, first on the fusome arms and then in its center, subsequently moving into the differentiating oocyte. The fusome is required for this complex process, because microtubule network organization and polarization are disrupted in hts(1) mutant cysts, which lack fusomes. Our results suggest that the fusome, a specialized membrane-skeletal structure, which arises in early germ cells, plays a crucial role in polarizing 16-cell cysts, at least in part by interacting with microtubules and centrosomes.

We report that DPP signaling is required for directed tracheal cell migration during Drosophila embryogenesis. The failure of tracheal cells to receive the DPP signal from adjacent dorsal and ventral cells results in the absence of dorsal and ventral migrations. Ectopic DPP signaling can reprogram cells in the center of the placode to adopt a dorsoventral migration behavior. The effects observed in response to ectopic DPP signaling are also observed upon the tracheal-specific expression of a constitutive active DPP type I receptor (TKV(Q253D)), indicating that the DPP signal is received and transmitted in tracheal cells to control their migration behavior. DPP signaling determines localized gene expression patterns in the developing tracheal placode, and is also required for the dorsal expression of the recently identified BRANCHLESS (BNL) guidance molecule, the ligand of the BREATHLESS (BTL) receptor. Thus, DPP plays a dual role during tracheal cell migration. It is required to control the dorsal expression of the BNL ligand; in addition, the DPP signal recruits groups of dorsal and ventral tracheal cells and programs them to migrate in dorsal and ventral directions.