SIGNALING MECHANISMS IN FOLLICLE CELL FATE DETERMINATION

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Receptor tyrosine kinase (RTK) signaling is involved in germline-soma communication that is essential for germ cell maturation. It is also involved in a myriad of developmental processes, including limb development, cerebral development, organogenesis, etc. The key to such functional diversity is that RTK signaling can respond to many different extracellular signals and can elicit a wide variety of cellular responses. However, the mechanisms that confer signal specificity have remained largely unclear. To approach this problem, we ask three questions: 1) What specifies the different signals, ligands and/or receptor partners? 2) What transmits the different signals, different kinases and/or differently modified kinases? 3) Are there different gene regulation events other than modulation of transcription factor activities? Dorsoventral (D/V) patterning in Drosophila oogenesis provides an excellent model for resolving these questions. The patterning process is initiated by the transmission of a ligand from the oocyte to the anterior dorsal follicle cells, activating the epidermal growth factor receptor (Egfr) signaling pathway. We have identified a transcription factor, termed CF2, that plays a central role in this positional signal transduction process. We showed that CF2 is a negative regulator of the rhomboid (rho) gene that encodes an essential membrane-bound component of the dorsalizing pathway, and that expression of CF2 itself is negatively regulated by the activated Egfr. Our findings also implicate two important features in this pathway: (a) The D/V patterning involves a two-step signaling process-the initial Egfr signal, which represses CF2 and induces rho expression; and the subsequent EGFR+Rho signal, which determines the dorsal cell fates. We hypothesize that these two signals, differentiated by the absence or presence of Rho, require different ligands and/or elicit different secondary messengers. Therefore unraveling this two-step signaling event can help answer Questions 1 and 2 above. (b) CF2 expression is negatively regulated by the MAPK cascade through a novel mechanism involving cytoplasmic retention and proteasome-mediated proteolysis. Unraveling this mechanism can help answer Question 3 above. To explore these biological phenomena, we propose: Aim 1, to determine the mechanism of CF2 protein degradation mediated by Egfr signaling; Aim 2, to determine whether Rho induces changes in Egfr signaling cascade; and Aim 3, to screen for second-site suppressors of the CF2 gain-of-function allele.