Overview of Drosophila Oogenesis and Axis Formation
Formation of both the anterior-posterior (AP) and dorsal-ventral (DV) axes requires inductive signals from specific somatic follicle cell (FC) populations. The oocyte nucleus and a microtubule organizing center (MTOC) are initially localized to the posterior of the oocyte. Prior to stage 7 of oogenesis, the posteriorly localized transforming growth factor α (TGF-α) like protein Gurken (Grk) binds to the Drosophila homologue of the epidermal growth factor receptor (Egfr), Torpedo (Top)/DER, in the adjacent uncommitted FCs. This activates a receptor tyrosine kinase signaling pathway to induce a posterior FC (PFC) fate. Once the Grk signal is received, an unknown signal is sent from the PFCs back to the oocyte to repolarize the microtubule (MT) cytoskeleton. The result is the disassembly of the posterior MTOC and the formation of a gradient of MTs from the anterior to the posterior of the oocyte with a concentration of minus ends now emanating from the anterior and lateral cortex of the oocyte. However, the mechanism governing this MT reorganization remains still unclear. This polarized MT network then directs the localization of different mRNAs in the oocyte, thereby defining the AP axis of the future embryo. For example, bicoid (bcd) mRNA is localized to the anterior of the oocyte, leading to a morphogenetic gradient of Bcd protein in the embryo. In contrast, oskar (osk) mRNA is localized to the posterior of the oocyte, where osk specifies the future germ cells. The repolarization of the oocyte cytoskeleton also directs the anterior movement of the oocyte nucleus and the relocalization of grk mRNA from the posterior to the thus defined dorsal-anterior corner. Once it has relocalized, Grk again signals through Egfr, but this time instructing the adjacent FCs to adopt dorsal fates leading to the secretion of special dorsal egg shell structures like the dorsal appendages.
Follicle Cell Patterning and Axis Formation
FCs are subject to a series of temporal and spatial regulation by various highly conserved signaling pathways. Patterning of the FCs along the AP axis by the Notch, JAK/STAT and Egfr pathways are essential for the PFCs to send the polarizing signal to the oocyte. In addition, activation of the Hippo pathway regulates PFC specification by cross-talking with Notch signaling. Components of the extracellular matrix (ECM) and proper apical basal polarity of the PFCs are also required for their inductive role in axis formation.
Around stage 6 of oogenesis, Delta (Dl) signals from the germline to activate its receptor Notch at the apical side of the entire follicular epithelium. Changes in the gene ex
At the poles, two pairs of polar cells secrete Unpaired (Upd) to the adjacent FCs, creating a gradient of JAK/STAT activity. Receiving different signaling levels according to the distance from the polar cells, the anterior cells differentiate into three types of anterior follicle cells (AFCs): border cells, stretched cells or centripetal cells.
Their equivalent group at the posterior would undergo this default AFC fate if the subsequent Egfr signaling is abolished, while in the wild-type, the anterior fate is masked by Egfr activity and the PFCs are induced instead. JAK/STAT is another permitting cue in conjugation with Notch required in the PFCs to respond to Egfr signaling, since mutants of JAK/STAT cause loss of PFC markers in a cell-autonomous fashion and mispolarization of the oocyte.
As described above, the Egfr pathway serves as the ultimate inductive signal for the PFCs. It is transduced through the MAPK cassette in the cells and enables the PFCs to send the theoretical polarizing signal back to the oocyte. Mutations in the genes required for grk localization and/or translation in the oocyte, such as vasa (vas) and spindle-E (spn-E), or the components of the MAPK pathway in the FCs, such as raf and mek, disrupt PFC specification and lead to defects in the oocyte polarity.
The Hippo pathway was identified in mammals as a tumor-suppressor pathway that controls organ sizes by regulating cell proliferation and apoptosis. In Drosophila oogenesis, the Hippo pathway controls PFC fate specification and thus oocyte polarization by interaction with Notch. Defective Notch signaling was only observed in PFCs mutant for Hippo components, but not in AFCs or lateral FCs. It was also suggested not to be directly involved in the polarizing signal from the PFCs, because forcing PFCs mutant for merlin (mer) , an upstream component of this pathway, to differentiate by removing Cut was sufficient to induce correct oocyte polarity
Correct apical-basal polarity of PFCs is necessary for the polarizing signal. PFCs mutant for α-Spectrin (α-Spec), encoding a key component of the apical membrane skeleton, causes oocyte polarity defects that disrupt apical-basal polarity and over proliferation. Interestingly, over proliferation is mostly observed in mutant FCs at the poles of the egg chamber, as in the cases of Hippo and other apical-basal polarity mutants. In addition, PFCs lacking ECM components Laminin A (LanA), Dlar or Dystroglycan (DG) also cause oocyte mispolarization to different extents without disrupting apical-basal polarity. ex