A gene expression screen for JAK/STAT and EGFR target genes during Drosophila melanogaster oogenesis

Abstract

The Janus kinase/Signal transducer and activator of transcription (JAK/STAT) and Epidermal growth factor receptor (EGFR) signaling pathways are conserved regulators of tissue patterning, morphogenesis, and other cell biological processes. The Drosophila melanogaster egg chamber is a useful system for studying how signaling regulates epithelial development. During egg chamber development, these signaling pathways are used at multiple points to determine the fates of the epithelial follicle cells. At mid-oogenesis, JAK/STAT and EGFR are both required to specify a population of cells called the posterior follicle cells (PFCs). The PFCs signal to the oocyte to establish the future embryonic axes. In this study, whole genome expression analysis was performed to identify genes activated in egg chambers by JAK/STAT and/or EGFR signaling. 317 genes were identified that are differentially expressed in egg chambers with ectopic JAK/STAT and EGFR pathway activity in the follicle cells. 69 of these candidates were tested for a role in axis establishment using RNAi knockdown in the follicle cells. We were not able to definitively identify any novel posterior signaling genes by this approach. However, we discovered that the signaling protein Sema-1b becomes enriched in the PFCs in response to JAK/STAT and EGFR signaling. Sema-1b RNAi produced a posterior signaling phenotype in our assay; however, an engineered null allele does not appear to cause oocyte polarity defects. We have also identified AdamTS-A as a novel transcriptional target of JAK/STAT signaling in the follicle cells that regulates egg chamber shape. AdamTS-A mRNA becomes enriched at the anterior and posterior poles of the egg chamber at stages 6-7. This mRNA expression pattern appears to be regulated by JAK/STAT signaling. We show that by manipulating AdamTS-A expression it is possible to alter egg chamber shape. AdamTS-A is therefore predicted to function as an effector for JAK/STAT signaling in regulating egg chamber elongation, since it is transcriptionally activated by JAK/STAT signaling and disrupting either JAK/STAT or AdamTS-A interferes with egg chamber elongation. We propose that AdamTS-A regulates egg chamber shape by remodeling the basement membrane at the poles of the egg chamber.