Group cell migration is a highly coordinated process that is involved in a number of physiological events such as morphogenesis, wound healing and tumor metastasis. Unlike single cells, collectively moving cells are physically attached to each other and retain some degree of apical-basal polarity during the migratory phase. Although much is known about direction sensing, how polarity is regulated in multicellular movement remains unclear. Here we report the role of the protein kinase Pak3 in maintaining apical-basal polarity in migrating border cell clusters during Drosophila oogenesis. Pak3 is enriched in border cells and downregulation of its function impedes border cell movement. Time-lapse imaging suggests that Pak3 affects protrusive behavior of the border cell cluster, specifically regulating the stability and directionality of protrusions. Pak3 functions downstream of guidance receptor signaling to regulate the level and distribution of F-actin in migrating border cells. We also provide evidence that Pak3 genetically interacts with the lateral polarity marker Scribble and that it regulates JNK signaling in the moving border cells. Since Pak3 depletion results in mislocalization of several apical-basal polarity markers and overexpression of Jra rescues the polarity of the Pak3-depleted cluster, we propose that Pak3 functions through JNK signaling to modulate apical-basal polarity of the migrating border cell cluster. We also observe loss of apical-basal polarity in Rac1-depleted border cell clusters, suggesting that guidance receptor signaling functions through Rac GTPase and Pak3 to regulate the overall polarity of the cluster and mediate efficient collective movement of the border cells to the oocyte boundary.

Bone Morphogenetic Proteins (BMPs) signal by activating Smad transcription factors to control a number of decisions during animal development. In Drosophila, signaling by the BMP ligand Decapentaplegic (Dpp) involves the activity of brinker (brk) which, in most contexts, is repressed by Dpp. Brk encodes a transcription factor which represses BMP signaling output by antagonizing Smad-dependent target gene activation. Here, we study BMP-dependent gene regulation during Drosophila oogenesis by following the signal transmission from Dpp to its target broad (br), a gene with a crucial function in eggshell patterning. We identify regulatory sequences that account for expression of both brk and br, and connect these to the transcription factors of the pathway. We show that Dpp directly regulates brk transcription through Smad- and Schnurri (Shn)-dependent repression. Brk is epistatic to Dpp in br expression and activates br indirectly, through removal of a repressor, which is yet to be identified. Our work provides first cis-regulatory insights into transcriptional interpretation of BMP signaling in eggshell morphogenesis and defines a transcriptional cascade that connects Dpp to target gene regulation.

Intercellular signaling pathways activate transcription factors, which, along with tissue-specific co-factors, regulate expression of target genes. Responses to TGFβ/BMP signals are mediated by Smad proteins, which form complexes and accumulate in the nucleus to directly bind and regulate enhancers of BMP targets upon signaling. In Drosophila, gene activation by BMP signaling often requires, in addition to direct input by Smads, the signal-dependent removal of the transcriptional repressor Brk. Previous studies on enhancers of BMP-activated genes have defined a BMP-responsive motif, the AE, which integrates activatory and repressive input by the Smad complex and Brk, respectively. Here, we address whether sequence variations within the core AE sequences might endow the motif with additional properties accounting for qualitative and quantitative differences in BMP responses, including tissue specificity of transcriptional activation and differential sensitivity to Smad and Brk inputs. By analyzing and cross-comparing three distinct BMPresponsive enhancers from the genes wit and Dad in two different epithelia, the wing imaginal disc and the follicular epithelium, we demonstrate that differences in the AEs contribute neither to the observed tissue-restriction of BMP responses nor to differences in the utilization of the Smad and Brk branches for transcriptional activation. Rather, our results suggest that the cis-environment of the BMPresponse elements not only dictates tissue specificity but also differential sensitivity to the two BMP mediators.

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