Proliferation, Growth, and Differentiation

Development of a multicellular organism requires regulation of cellular events that are highly orchestrated temporally and spatially. Although much research has been conducted on spatial regulation of cell differentiation and gene expression, relatively little is known about how basic cellular functions are regulated temporally. The follicle cells provide an excellent model system for study of the temporal regulation of cell proliferation and differentiation. In the course of oogenesis, the follicle cells undergo two important switches of cell-cycle program, first from the normal mitotic cell cycle to the specialized endocycle (in which cells duplicate their genomic DNA without dividing) and second from the endocycle to amplification of specific genomic regions. These switches are essential for supporting egg development and are coupled with other temporally and spatially regulated changes of differentiation and growth status. For example, at the first switch, follicle cells stop proliferation, grow in size, and begin expressing different fate markers. Spatially, these cells undergo different movements to reach their final destination. My lab has played a major role in identifying how expression of specific genes contributes to specific types of follicle-cell behavior.

Our work has led to a much greater understanding of the temporal and spatial patterns of the follicle cells. Our work revealed that the evolutionarily conserved Notch pathway has a unique and crucial role in temporal regulation of follicle-cell differentiation and proliferation. Notch signaling is activated by the germ-cell-expressed ligand Delta, which triggers the first of the two cell-cycle switches mentioned above. Several recent studies in my lab reveal that two transcription factor genes, hindsight and cut, are affected by Notch signaling during this first switch. Notch signaling turns on hindsight activity, which inhibits follicle cell proliferation by suppressing both cut and a proliferation-stimulating signaling pathway. Our studies established cut and hindsight as key linkers to connect Notch signaling and the cell-cycle machinery. More recently, we found that the precise timing of Notch activation is regulated by the microRNA pathway. We are currently in the process of determining how microRNAs Notch signaling and developmental timing in follicle cells.

Turning off a signaling system can be as important as turning it on. Recently, we found that turning off Notch signaling as well as activation of ecdysone receptor pathway are required for the second of the two switches. Our study was the first to show how developmental signals control the second switch in the Drosophila follicular system. It also has much wider significance in that it is among the few that demonstrate important developmental consequences after a signaling pathway is turned off in cells. Interestingly, we found that a microRNA, miR-7, plays a critical role in regulating this endocycle to gene amplification switch.

In addition, we found that Notch activation in follicle cells is spatially regulated by a tumor-suppressor pathway, the Hippo pathway. When this pathway is disrupted, Notch is strongly compromised in the posterior follicle cells, which are particularly important for sending signals to the oocyte that help establish its asymmetry. We also identified a novel bona fide component of the Hippo pathway, KIBRA. Precisely how Hippo and Notch signaling intertwine in the follicle cells is of great interest to us. We have some preliminary data showing the trafficking of Notch is compromised in Hippo pathway mutants. Further study on this topic will help us decipher the interaction of these two important and conserved pathways

Figure 1. Switch of cell-cycle programs and the Cut expression pattern in follicle cells. (A) During Drosophila oogenesis, somatically derived follicle cells undergo two cell-cycle switches: (1) mitotic cycle to endocycle switch and (2) endocycle to gene-amplification switch. From the germarium (G) to stage (S) 6, follicle cells undergo unsynchronized mitotic cycles. During stages 7 to 10A, these cells go through three rounds of endoreplication and thereafter switch to a localized amplification pattern characteristic of chorion gene amplification. (B) Cut expression (shown in red) begins in follicle cells in region 2b of the germarium. It persists in all follicle cells, including the polar cells and the interfollicular stalk cells, until about stage 6 and diminishes afterwards, concurrent with the first cell-cycle switch. PH3 (shown in green) was used to mark the M phase of the mitotic cycle. Between stages 7 and 10A of oogenesis, Cut expression ceases in all follicle cells except the polar cells. At about stage 10B, Cut expression resumes in the columnar follicle cells that surround the oocyte (From Sun and Deng, 2005).