Histone Demethylase LSD1 Restricts the Size of the Germline Stem Cell Niche in Drosophila Ovaries
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Specialized microenvironments called niches keep stem cells in an undifferentiated and self-renewing state by producing a variety of factors. The size and signaling output of niches must be finely tuned to ensure proper tissue homeostasis. I use the Drosophila female germline as an excellent model system to study niche development and function. Five to seven somatic cap cells form the ovarian stem cell niche and produce dpp, a BMP homolog necessary for the maintenance of germline stem cells (GSCs). Mutations in Lsd1, a histone demethylase exhibit GSC-like tumor formation. Clonal analysis, cell-type specific knock down and rescue experiments demonstrate that Lsd1 functions within the escort cells that reside immediately adjacent to cap cells (niche). Loss of Lsd1 causes the escort cells to adopt an intermediate fate expressing both escort cell and cap cell markers and enables them to function as ectopic niches for the expanded stem cell population. Temporally restricted gene knock-down experiments suggest that Lsd1 functions both during development, to specify EC fate, and in adulthood, to prevent ECs from forming ectopic niches independent of changes in cell fate. Lsd1 specifically functions to repress dpp, the niche signal in the adult germaria. I have identified engrailed as a direct target of Lsd1 by performing Chromatin Immunoprecipitation (ChIP-seq) analysis in the escort cells of the Drosophila ovary. Engrailed is expressed in the cap cells of wild type germaria and in Lsd1 mutants engrailed transcripts are misexpressed in the escort cells. Knocking down engrailed expression in the escort cells suppresses the Lsd1 mutant phenotype. Moreover, ectopic expression of engrailed in the escort cells displays a GSC-tumor phenotype. Furthermore, I have shown that Engrailed functions upstream of dpp, and activates its expression in the cap cell niche.