The developing Drosophila follicular epithelium is an ideal system for identifying signals that control cellular behavior. First, available genetic tools and techniques for imaging or biochemistry enable the analysis of genetic mutations within individual cells. Second, roles for stem cells and polarization mechanisms in epithelium formation and maintenance are conserved in fly ovaries and vertebrate adult epithelia. Third, a new epithelium is generated to accompany each egg produced by an adult female fly (Fig.1) through a process that is mechanistically similar to mammalian epithelium formation. Finally, known signals that control follicular epithelium development also participate in epithelial formation in other systems. Our goal is to identify the conserved mechanisms that control critical decisions during epithelial development.
A striking example of the importance of environmental influence on cellular behavior is the control of stem cells by local environments, or niches. Stem cells are undifferentiated cells that can self-renew and generate the differentiated daughter cells that are necessary for tissue development and repair. Recent work has shown that stem cell interactions with the niche are critical for stem cell self-renewal and function. However, many of the niche-derived instructive signals and stem cell response pathway components are unknown. Our research focuses on two important questions. First, what are the molecular mechanisms that determine stem cell positioning within the niche? Second, how does positioning affect stem cell behavior?
We are exploring potential roles for integrin adhesion molecules and their extracellular matrix ligands in regulating interactions between epithelial stem cells and their niche in vivo. Our next goal is to dissect the signaling pathways that mediate integrin function to control stem cell anchoring and proliferation. Additionally, we are investigating potential roles for integrins and their ligands in the initial establishment of the epithelial stem cell niche. Top
The two daughter cells produced by an asymmetric stem cell division face critical developmental decisions. One daughter cell remains in the niche and retains stem cell identity whereas the daughter cell that leaves the niche differentiates in response to new environmental signals. The non-stem cell daughters of follicular epithelial stem cells, called pre-follicle cells, differentiate according to two separate programs, becoming either follicular epithelial cells or the stalk cells that separate adjacent follicles. The follicular epithelium arises from a mesenchymal to epithelial transition (MET). METs occur in several steps, including the specification of basal, lateral and apical membrane domains and maturation of cell-cell adhesion complexes. Our preliminary work suggests that epithelial stem cells have defined basal domains and work from other labs has demonstrated the importance of lateral adhesion for stem cell function. Although these cells have begun to polarize, they also retain some mesenchymal characteristics. This dual character gives them the flexibility to respond to environmental signals that promote invasive migration or further polarization. Some carcinoma cells also are partially polarized, a state that promotes their dissemination and establishment in secondary sites. These similarities suggest that the mechanisms that control epithelial character in the fly ovary may be relevant for understanding carcinoma progression.
The progression of partially polarized pre-follicle cells to fully polarized epithelia depends on apical domain specification. Only pre-follicle cells that directly contact germ cells form apical domains and continue the polarization program. This suggests that the adhesion molecules that mediate contact between germ cells and pre-follicle cells are the critical regulators of apical domain formation. We found that Src64, a Src family kinase (SFK), controls dynamic adhesion between germ cells and pre-follicle cells during epithelium formation (Fig.2) (O’Reilly, et al. Development 2006). Our next goal is to identify adhesion molecules and other signaling components that cooperate with Src64 in controlling the MET. We plan to identify these critical signals through genetic screening, biochemical analysis and using live imaging approaches. Already we have isolated 300 enhancers of Src64 mutant ovary defects that may be important for the MET. The cloning and characterization of these genes will contribute to our understanding of their roles in regulating the MET. Top
Regulated interactions between cortical adhesion complexes, the microtubule and actin cytoskeletons, and vesicle transport machinery are critical for the generation and maintenance of cellular polarity. Our results and others suggest that a region of the ring canal, called the outer rim, regulates the cytoskeletal dynamics and directional particle transport required for oocyte development. We are investigating the roles of specific adhesion complexes, microtubule and actin regulatory proteins, signaling molecules and endocytosis machinery proteins in outer rim regulation (Fig. 3). We also are using genetics, biochemistry and imaging to identify regulators of Src64 activity and to dissect the in vivo dynamic interactions that occur between outer rim components that are required to facilitate directional transport. Top
