Academic journal article Genetics

Novel Tools for Genetic Manipulation of Follicle Stem Cells in the Drosophila Ovary Reveal an Integrin-Dependent Transition from Quiescence to Proliferation

Academic journal article Genetics

Novel Tools for Genetic Manipulation of Follicle Stem Cells in the Drosophila Ovary Reveal an Integrin-Dependent Transition from Quiescence to Proliferation

Article excerpt

MANY regenerative tissues possess populations of self-renewing stem cells that are maintained throughout the lifetime of the organism to produce the differentiated daughter cells required for tissue function. Recent work has resulted in the identification of signals from the immediate microenvironment or niche that promote stem cell positioning and self-renewal. Less well understood are the stem cell regulatory signals produced by cells located outside the stem cell niche. For example, a relay of signals involving multiple tissues located far from the ovary results in production of insulin that directly influences proliferation of germ-line stem cells (GSCs) in the fly (Lafever and Drummond-Barbosa 2005; Geminard et al. 2009). In another case, nutritional changes control the production of growth factors by terminal filament and cap cells located at the apical tip of the Drosophila ovary (apical cells) that regulate proliferation of follicle stem cells (FSCs) located six to eight cell diameters to the posterior (Hartman et al. 2013).

Despite these advances, identification of critical stem cell control mechanisms is limited by our ability to precisely identify stem cell populations and their support cells in situ and to manipulate gene function in individual cell types that contribute to stem cell regulation. In flies, the Gal4-UAS system has been an extremely powerful tool for analysis of gene function in subsets of cells within a given tissue. Combined with extensive RNA interference (RNAi) libraries and classical genetic mutations, the Gal4-UAS system has made it possible to directly assess the effects of altering the activity of a single gene on cellular function in many developing tissues.

To enhance our ability to dissect molecular mechanisms of FSC regulation, we conducted a screen designed to identify Gal4 lines that are expressed in subpopulations of somatic cells within the stem cell compartment of the flyovary,calledthe germarium. Several populations of somatic cells in the germarium have been shown previously to be critical for stem cell regulation. Post-mitotic cap cells constitute the cellular niche for GSCs and provide the signals necessary for self-renewal, prevention of differentiation, and controlled orientation of cell division (Xie and Spradling 1998, 2000; Song and Xie 2002; Song et al. 2002; Kai and Spradling 2003; Losick et al. 2011). Terminal filament cells neighbor cap cells to the anterior and produce growth factors that influence GSC function (Forbes et al. 1996b; Xie and Spradling 1998; King and Lin 1999; King et al. 2001). Terminal filament and cap cells (collectively referred to as apical cells) also generate factors required for FSC proliferation (Forbes et al. 1996a, b; Zhang and Kalderon 2001; Song and Xie 2003; Kirilly et al. 2005; Hartman et al. 2013; Sahai-Hernandez and Nystul 2013). However, FSCs are separated from apical cells by six to eight somatic inner germarial sheath cells (IGS cells, also called escort cells) that provide additional growth factors for FSC control and adhesive signals that maintain FSC positioning (Song and Xie 2002; SahaiHernandez and Nystul 2013). Finally, FSC daughter cells, called follicle cells,influence FSC behavior from the posterior by creating gradients of signals that intersect with anterior signals produced by apical cells and escort cells (Vied et al. 2012).

Our goal was to identify new methods for marking individual somatic cell populations and tools for genetically manipulating gene function within them in order to probe previously unknown aspects of stem cell function. Here we identify new Gal4 lines that are expressed in apical cells, cap cells alone, IGS cells, follicle cells, and multiple somatic cell types. We further identify two new Gal4s that are particularly useful for manipulation of gene expression in FSCs. Using these new tools, we define the primary defect in FSCs that lack integrin function and define the role of integrin-mediated adhesion in FSC positioning, migration, and long-term self-renewal. …

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