This research program focuses on the role intercellular communication via peptide growth factors in animal embryonic development. Using the tools of genetics and molecular biology, our goal is to describe biochemical pathways through which growth factors act to modulate morphology, gene expression and cell behavior. The major focus is on the Transforming Growth Factor beta family and the Wnt pathway of secreted signaling molecules. Results are relevant to the use of growth factors as therapeutic agents in and of themselves, and as cytokines in the manufacture of cellular products. In addition, understanding the stability of differentiated cells and their capacity to be reprogrammed by their environment is a major concern in use of cellular therapies, particularly stem cells. Our work uses the fruitfly Drosophila melanogaster as a model system. All growth factors signaling pathways examined to date (TGF-beta, Wnt, FGF, EGF) are highly conserved between Drosophila and mammals. Major projects: a) Use of genetic interaction screens to identify components of TGF-beta signal transduction. The major TGF-beta family member in Drosophila, the product of the decapentaplegic (dpp) gene, is the TGF-beta family member best characterized by mutational analysis. Mutant phenotypes exist that reflect the many requirements for this growth factor during embryogenesis. I have discovered an adult viable mutation whose severity is closely linked to the functional level of dpp signaling. This mutation therefore serves as a barometer of signal transduction pathway activity and has been used to screen the Drosophila genome for loci that interact with dpp in this pathway. One of the screens used an existing bank of mutations created by P element transposon tagging to facilitate rapid recovery of identified loci. With the completion of the sequence of the Drosophila genome, we hope to be able to correlate interacting loci directly with putative protein coding regions. Some of the interacting mutations are in previously studied genes, such as transcription factors, cell cycle regulators and chromatin remodeling factors. Cell biological and genetic techniques will be employed to understand the nature of these interactions. b) Biology of the head capsule phenotype. The mutation used for the above described genetic screens alters the head capsule of the fly. Various external structures such as the eye and sensory organs are reduced, eliminated or duplicated. The adult head derives from paired epithelial sheets called imaginal discs that are elaborated during embryogenesis and then undergo final differentiation at the metamorphosis of the pupa. Imaginal discs have become a popular model system for studying signal transduction as they have complex pattern formation, but unlike the embryo, also undergo growth, allowing this aspect of signal transduction to be studied. We have initiated a study of the imaginal discs of the head capsule mutant. We predict that dpp expression is reduced in these discs and seek to identify how target genes are affected. We are particularly interested in potential feedback between the TGF-beta pathway and the Wnt pathway in this tissue, as the TGF-beta pathway is reported to promote eye development at the expense of head development, while the Wnt pathway does the opposite. c) Transcriptional control of a TGF-beta responsive gene. Dpp participates in the formation of the alimentary system of the fly larva, particularly in the morphogenesis of the midgut. Over the past 7 years I have described the factors responsible for dpp's expression in this tissue. The enhancer elements that control transcription are directly controlled by the HOX ortholog Ultrabithorax, a homeodomain-containing transcription factor. In addition, the homeodomain transcription factor extradenticle, homologous to the mammalian oncogene PBX, is required for enhancer function. We have recently shown that the WNT pathway controls transcriptional repression though the HMG-box factor Drosophila TCF. Many years ago I showed that dpp expression was autoregulated in this tissue and we are now examining sites within the enhancer for the recently described TGF-beta responsive transcription factors, the SMADS. We are also interested in understanding how multiple signal transduction pathways converge on a given enhancer element. We would like to know if this happens though synergy of transcription factors bound to DNA, or by a more complex mechanism of posttranscriptional modification of transcription factors via cross-talk between the two pathways.
