Identification of Me31B from an in Vivo RNAi Screen as a Potential Regulator of Notch Signaling

The Drosophila somatic follicle cells are excellent for the study of cell-cycle regulation and cell differentiation. During oogenesis, the follicle cells go through three variations of cell cycle programs, the mitotic cycle, the endocycle and gene amplification. Notch signaling activation is required for the switch from the mitotic cycle to the endocycle (the M/E switch) and its downregulation is necessary for the switch from the endocycle to gene amplification (the E/A switch) in these cells. Recently, we have found that Broad, a zinc-finger transcription factor, is directly up-regulated by Notch signaling during the M/E switch in the follicle cells (Jia and Deng, unpublished data). During late oogenesis, Broad is also regulated by EGFR and Dpp pathways for chorionic appendage formation. To explore how these different signaling pathways regulate follicle cell differentiation and cell cycle switches, we performed an in vivo RNAi screen to examine the effect of induced knockdown of gene expression on Br expression during oogenesis. So far, 350 different RNAi lines have been screened and about 20 of them showed defects in either early or late Br expression in follicle cells. Knockdown of Me31B, a putative RNA helicase belonging to the DEAD-box family and component of ribonucleoprotein complex (RNP), resulted in disruption of the Br early expression pattern during the endocycle stages. In our studies with Drosophila have revealed that Hindsight and Cut, in follicle cells, Cut and Wg, in wing disc, are also regulated by me31B, suggesting a potential role of me31B in Notch signaling. In addition, here we report that me31B shows genetic interaction with Notch and acts upstream of the Notch signaling. Besides, experimental results also show that knockdown of me31B causes upregulation of Dl in follicle cells and cis-inhibition accordingly. Therefore, based on these findings I hypothesized that me31B potentially regulates Notch signaling by targeting Dl in follicle cells through miRNA pathway.