dSETDB1 and SU(VAR)3–9 Sequentially Function during Germline-Stem Cell Differentiation in Drosophila melanogaster

et al. (2008) dSETDB1 and SU(VAR)3-9 Sequentially Function during Germline-Stem Cell Differentiation in Drosophila melanogaster. PLoS ONE 3(5): e2234. doi:10.1371/journal.pone.0002234 dSETDB1 and SU(VAR)3-9 Sequentially Function during Germline-Stem Cell Differentiation in Drosophila melanogaster Jeongheon Yoon 0 Kyu-Sun Lee 0 Jung Sun Park 0 Kweon Yu 0 Sang-Gi Paik 0 Yong-Kook Kang 0 Axel Imhof, University of Munich and Center of Integrated Protein Science, Germany 0 1 Center for Regenerative Medicine, Korean Research Institute of Bioscience and Biotechnology (KRIBB) , Daejeon , Korea , 2 Department of Biology, Chungnam National University , Daejon , Korea Germline-stem cells (GSCs) produce gametes and are thus true ''immortal stem cells''. In Drosophila ovaries, GSCs divide asymmetrically to produce daughter GSCs and cystoblasts, and the latter differentiate into germline cysts. Here we show that the histone-lysine methyltransferase dSETDB1, located in pericentric heterochromatin, catalyzes H3-K9 trimethylation in GSCs and their immediate descendants. As germline cysts differentiate into egg chambers, the dSETDB1 function is gradually taken over by another H3-K9-specific methyltransferase, SU(VAR)3-9. Loss-of-function mutations in dsetdb1 or Su(var)3-9 abolish both H3K9me3 and heterochromatin protein-1 (HP1) signals from the anterior germarium and the developing egg chambers, respectively, and cause localization of H3K9me3 away from DNA-dense regions in most posterior germarium cells. These results indicate that dSETDB1 and SU(VAR)3-9 act together with distinct roles during oogenesis, with dsetdb1 being of particular importance due to its GSC-specific function and more severe mutant phenotype. - Drosophila oogenesis is a complex developmental process involving the coordinated differentiation of germline and somatic cells, and begins with asymmetric division of a single germline stem cell (GSC) [1,2]. This GSC is located at the tip of each ovariole in the germarium, which is a generative region that is divided into sub-regions such as region-1, -2a, -2b and -3. After each GSC division, the posterior daughter cell becomes a cystoblast, leaves region-1, undergoes four synchronous, incomplete divisions to form a 16-cell germline cyst [3,4], and steadily moves in a posterior direction through the germarium. Of the 16 interconnected cells, one cell develops into the oocyte whereas the other 15 develop into polyploid nurse cells [5]. This 16-cell cyst becomes surrounded by a monolayer of follicle cells and buds off from the posterior germarium to form an egg chamber [6,7], which ultimately gives rise to a single mature oocyte ready for fertilization. The germline cells, including the GSCs are the only population from which both parental epigenetic information and genetic information can be transferred to progeny. This indicates that, other than the known pluripotency [8], the germline cells possess another important property - an exceptional capacity for epigenetic modifications of the genome [9]. In fact, germ-cell development is associated with a dynamic process of epigenetic reprogramming, leading to re-construction of the whole genome-level epigenetic state [9,10,11,12,13]. The developmental significance of this has driven studies to investigate the epigenetic changes occurring in the germline cells. Therefore, germ-cell development is an excellent system to study how the epigenetic system involving DNA methylation and histone lysine methylation is erased, reestablished, and maintained in the germ cells at the genome-wide level. Histone-lysine methylation, which mainly occurs in the tails of histones H3 and H4, plays a pivotal role in cellular processes including heterochromatin formation, X-chromosome inactivation, and transcription regulation [14]. Lysine methylation is of particular interest because it can modulate the chromatin structure to a compacted state or a relaxed one, depending on which lysine residues are methylated. With regard to heterochromatin formation, histone H3 trimethylated at lysine 9 (H3K9me3) is enriched in pericentric heterochromatin and thereby recognized as typical of a heterochromatin marker [15,16,17,18,19]. Correct formation of heterochromatin is essential for chromosome stability and integrity, and is required for the proper segregation of chromosomes during mitosis [20] and the recombination events in fission yeast [21], which further demonstrates the biological significance of H3K9me3 that participates in heterochromatin formation. So far, several histone-lysine methyltransferases (HKMTases) with specificity to H3-K9 residues have been identified [14]. Some of them are implicated in germ-cell development. Male germ cells in mice lacking suv39h, which synthesizes H3K9me3 in pericentric heterochromatin, display severely impaired viability and chromosomal instability [22]. Mutant mice in which G9a is specifically inactivated in the germcell lineage exhibited a marked lo (...truncated)