Functional Characterization Of DBre1/dSet1-dependent Tri-methylation Of Histone H3K4in Drosophila Oogenesis

Stem cells are biological cells found in all multicellular organisms, thatcan divide (through mitosis) and differentiate into diverse specialized celltypes and can self-renew to produce more stem cells. In Drosophila ovary,there are three types of stem cells in the germarium: germline stem cells(GSCs), follicle stem cells (FSCs) and escort stem cells (ESCs).TheDrosophila ovarian GSCs constantly experience self-renewal anddifferentiation, ensuring the female fertility throughout life. The conceptthat GSCs are controlled by particular microenvironment known as“niche” has been widely invoked. The balance between GSC self-renewaland differentiation is exquisitely regulated by the stem cell niche, thestem cells themselves and systemic factors. Increasing evidence hasshown that the GSC regulation also involves epigenetic mechanismsincluding chromatin remodeling and histone modification. Here, we findthat dBre1, an E3ubiquitin ligase, functions in controlling GSC self-renewal and germ cell differentiation via distinct mechanisms.Removal or knock down of dBre1function in the germline or somaticniche cell lineage leads to a gradual GSC loss and disruption of H3K4trimethylation in the Drosophila ovary. Further studies suggest that thedefective GSC maintenance could be attributable to failure of the stemcells in response to BMP signals from the niche and compromised celladhesion of GSCs to their niche. On the other hand, dBre1-RNAiexpression in escort cells causes a loss of H3K4trimethylation andaccumulation of spectrosome-containing single germ cells in thegermarium. Reducing dpp or dally levels suppresses the germ celldifferentiation defects, indicating that dBre1limits BMP signalingactivities for the differentiation control. Strikingly, all phenotypesobserved in dBre1mutant ovaries can be mimicked by RNAi-basedreduced expression of dSet1, a Drosophila H3K4trimethylase. Moreover,genetic studies reveal that dBre1interacts with dSet1in controlling GSCmaintenance and germ cell differentiation. Taken together, we identify adBre1/dSet1-dependent pathway for the H3K4methylation involved inthe GSC fate regulation in the Drosophila ovary.Beside that we also found that dSet1but not dBre1in niche control thetranscription of the dally, a glypican-encoding gene. This may be the oneof the mechanisms dSet1control GSCs self-renewal in niche.GSCs constantly undergo the asymmetric division by which one daughter cell remaining in contact with the GSC niche retains stem cellidentity, whereas the other is displaced away from the niche, acquiringcystoblast (CB) fate. The CBs further divide with incomplete cytokinesisto consecutively produce2-cell,4-cell,8-cell and16-cell germline cysts.Here we found that removal of dBre1in germline cysts, the cystdifferentiation is not disturbed.The intricately regulated differentiation of the somatic follicle celllineages into distinct subpopulations with specific functions plays anessential role in Drosophila egg development. The properly specifiedposterior follicle cells (PFC) signals is important to polarize the oocytealong the anterior-posterior (AP) and dorsal-ventral (DV) axes. Here weremoval of dBre1from posterior terminal of the egg chamber, the AP/DVaxis formation of the oocyte at mid-oogenesis is not disturbed. Our resultsshow that in the different developmental processes, the function of dBre1is distinct. Thus, this study may open another window for elucidatingfunction of dBre1and dSet1in controlling adult stem cell fate induction,maintenance and differentiation during development.