Characterization of the Drosophila Egfl7/8 ortholog during oogenesis
During animal development precise coordination and regulation of cell proliferation, differentiation and cell death is required for proper tissue organization. This is achieved through specific cell communication by intercellular signals. Cell death, for example, is a mechanism utilized by multicellular organisms for several developmental processes such as elimination of damaged cells or morphological shaping. Apoptosis can be induced by intrinsic signals generated within the cells or from extrinsic signals received from the surrounding environment. This work centered on the analysis of the mechanisms and signals that trigger apoptosis during Drosophila oogenesis. Drosophila ovaries are composed of approximately 16-20 ovarioles, each of which contains a series of egg chambers that are proceeding through the 14 stages of oogenesis moving from the germarium toward the oviduct. For the mature egg to be formed cell death has to occur at specific stages both in the germline (nurse cells) and in the somatic cells (follicle cells). However, whether this apoptosis is caused by intrinsic or extrinsic signals is not known. In addition to this developmentally controlled cell death, apoptosis can also be induced by environmental cues. Under starvation, for example, there is an increase of apoptosis at particular stages: in the germarium and at mid-oogenesis. Mid-oogenesis (stage8/9) is when vitellogenesis starts. At these stages the state of the egg chambers are checked in order to eliminate, through apoptosis of the germline, defective egg chambers. In starved flies, through activation of this “check-point”, oogenesis is blocked before vitellogeneis starts. It is not clear what are the signals that prevent apoptosis at mid-oogenesis in well-fed flies. In this study I have analyzed the function of a newly identified signaling molecule CG7447 during Drosophila oogenesis. My results indicate that CG7447 is required to prevent apoptosis at mid-oogenesis in well-fed flies. CG7447 RNA is only detectable in cells of the germarium, but not at later stages of oogenesis. When an HA tagged version of CG7447 (CG7447-HA) was expressed in the follicle cells of the germarium, this protein was found to be enriched in the oocyte during stages 2-8. These data suggest that CG7447 might be a secreted protein produced in the germarium, which is then secreted into the oocyte. To test the function of CG7447 during oogenesis, I generated a mutant allele. The mutation in CG7447 reduced female fertility. Mutant ovaries showed a block of egg chamber development at mid-oogenesis and this block correlated with apoptosis of the follicle cells. These mutant phenotypes could be reversed by expression of CG7447-HA, showing that these defects are due to the mutation in CG7447. Surprisingly, expression of CG7447-HA in the follicle cells only from stage 9 onward could restore fertility and normal oogenesis in a CG7447 mutant, indicating that CG7447 is required for follicle cell survival at later stages. Proper nutritional conditions are required to prevent apoptosis in the germline. Our data suggest that CG7447 is instead required to prevent apoptosis in the follicle cells. Thus, our analysis appears to have identified a novel signaling pathway that prevents survival of follicle cells in well-fed flies. Finally, our bioinformatic analysis showed that CG7447 is homologous to vertebrate EGF-like domain 7 (Egfl7) and EGF-like domain 8 (Egfl8) proteins. Importantly, expression of mouse Egfl7 or Egfl8 were able to confer normal oogenesis and fertility to CG7447 mutant flies. We therefore conclude that CG7447 is an evolutionary conserved protein and that CG7447 and Egfl7/8 share a common molecular function. CG7447 is a newly identified signaling molecule required during Drosophila oogenesis to promote the survival of follicle cells and to allow entry into vitellogenesis. Identification of the signaling cascade triggered by CG7447 will be important to more precisely understand its function during oogenesis. It may also help to reveal the molecular role of Egfl7/8 during vertebrate development.
Drosophila, Egfl7/8, oogenesis, oogenese
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