Asymmetric chromosome strand inheritance during stem cell division in Drosophila

描述（由申请人提供）：成体干细胞具有产生新干细胞（自我更新）以及分化后代的能力（Knoblich，2008;莫里森和Kimble，2006）。在生物体的整个生命过程中，需要干细胞增殖并提供分化的细胞，同时避免重复细胞周期导致的DNA突变的潜在有害影响。已经假设干细胞可能通过在不对称细胞分裂期间保留较老的（“不朽的”）模板DNA链，从而将所有复制诱导的突变排除到分化的子代中来完成这一非凡的壮举（不朽链假说- ISH）（凯恩斯，1975）。此外，还提出了其他模型，其中干细胞出于不同原因（如保留表观遗传记忆）仅不对称地分离染色体模板DNA链的子集（Armakolas和Klar，2006）。最近，许多研究人员假设，在某些干细胞中典型保留的母体中心体可能被用作锚不朽DNA链的手段（Tajbakhsh，2008; Tajbakhsh and Gonzalez，2009）。然而，这些染色体不对称的机制和生物学相关性仍然难以捉摸。这主要是由于缺乏模型系统，其中染色体不对称性可以在其他不对称性，如细胞命运的背景下进行评估。本研究的目的是以果蝇睾丸为模型系统，研究干细胞不对称分裂过程中染色体链的分离。果蝇雄性生殖系干细胞（GSC）为研究潜在的染色体不对称性提供了独特的机会，因为它是唯一一个在细胞命运和中心体年龄中表现出定型不对称性的系统（Yamashita et al.，2003; Yamashita等人，2007年）。最近，我已经表明，果蝇睾丸中的雄性GSC不遵循永生链模型，尽管中心体不对称分离（Yadlapalli等人，2011年）。这表明GSC不是不对称地分离模板DNA链以维持其基因组完整性。然而，仍然存在GSC可能只不对称地分离染色体的一个子集的可能性，也许是为了保留表观遗传记忆。引人注目的是，在我的初步研究中，我发现GSCs优先遗传Y染色体的某条链，该链以卫星序列（AATAC）6的存在为标志，而其互补链（GTATT）6则由分化中的子代遗传。在这里，我建议研究GSC分裂期间染色体不对称分离的机制和生物学相关性。这项拟议中的研究可能会揭示干细胞如何通过不对称染色体分布调节细胞命运的新见解。此外，这些机制的表征将使我们能够理解干细胞群体如何精确地维持和调节以维持组织稳态，其失败可能导致人类病理学，包括癌症和年龄相关疾病。

DESCRIPTION (provided by applicant): Adult stem cells have the ability to produce new stem cells (self-renewal) as well as differentiated progeny (Knoblich, 2008; Morrison and Kimble, 2006). Throughout the life of an organism, stem cells are required to proliferate and supply differentiated cells while avoiding the potentially deleterious effects of DNA mutations resulting from repeated cell cycles. It has been hypothesized that stem cells might be accomplishing this remarkable feat by retaining older ("immortal") template DNA strands during asymmetric cell divisions, thereby excluding all replication-induced mutations into the differentiating daughters (Immortal Strand Hypothesis - ISH) (Cairns, 1975). In addition, other models have also been proposed in which stem cells asymmetrically segregate only a subset of chromosome template DNA strands for different reasons, such as retention of epigenetic memories (Armakolas and Klar, 2006). Recently, it was hypothesized by many researchers that the mother centrosome, which is stereotypically retained within some stem cells, might be used as a means to anchor the immortal DNA strands (Tajbakhsh, 2008; Tajbakhsh and Gonzalez, 2009). However, the mechanism and the biological relevance of these chromosome asymmetries remain elusive. This is primarily due to the lack of model systems in which chromosome asymmetries can be assessed in the context of other asymmetries, such as cell fate. The aim of this research proposal is to study chromosome strand segregation during asymmetric stem cell divisions using the Drosophila melanogaster testis as a model system. Drosophila male germline stem cells (GSCs) offer a unique opportunity to investigate potential chromosome asymmetry, as it is the only system where stereotypical asymmetry in both cell fate and centrosome age is demonstrated (Yamashita et al., 2003; Yamashita et al., 2007). Recently, I have shown that male GSCs in the Drosophila testis do not follow the immortal strand model, despite asymmetrically segregating centrosomes (Yadlapalli et al., 2011). This suggests that GSCs are not asymmetrically segregating the template DNA strands to maintain their genomic integrity. However, there still remains the possibility that GSCs might be asymmetrically segregating only a subset of chromosomes, perhaps to retain epigenetic memories. Strikingly, in my preliminary study I found that GSCs preferentially inherit a certain strand of Y chromosome that is marked by the presence of satellite sequence (AATAC)6, while its complementary strand (GTATT)6 is inherited by the differentiating daughter. Here, I propose to investigate the mechanism and the biological relevance of asymmetric chromosome segregation during GSC division. The proposed research may uncover novel insights into how stem cells might regulate cell fate through asymmetric chromosome distribution. Further, characterization of such mechanisms will allow us to understand how stem cell populations are precisely maintained and regulated to sustain tissue homeostasis, a failure of which might lead to human pathologies including cancers and age-related disorders.