S-Space College of Natural Sciences (자연과학대학) Dept. of Biological Sciences (생명과학부) Theses (Ph.D. / Sc.D._생명과학부)
Studies on the role of SRG3 for the stability of the SWI/SNF chromatin remodeling complex and the function of BAP1 during thymocyte development : SRG3에 의한 SWI/SNF 염색사 리모델링 복합체의 안정화 및 BAP1이 흉선세포 분화에 미치는 영향에 대한 연구
- 자연과학대학 생명과학부
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 생명과학부, 2015. 2. 성노현.
- T cells are major mediators of cell-mediated immune responses. T cell progenitors originating in the bone marrow enter the thymus, and then differentiate, undergo selection, and mature into functional T cells through a series of organized and complicated developmental stages. These multiple steps in T cell development are regulated by the transcriptional control of lineage-specific genes and by the sequential rearrangement of T cell receptor (TCR) genes. Switching the transcription of various genes, including Notch, GATA-3, E2A/HEB, Id proteins, c-Myb, ThPOK, Runx complex, and Ikaros family genes, on or off is critical for lineage commitment in T cells. In addition, the β-selection and positive selection, which depend on the successful rearrangement of TCR genes, are essential for the production of mature T cells. Chromatin status, which is regulated by histone modification enzymes and chromatin remodeling complex, ultimately determines whether gene expression is turned on or off.
Here, I studied the major components of the murine SWI/SNF complex. It was found that BRG1, SNF5, and BAF60a are targeted for ubiquitylation and degradation, but that SRG3 can protect them from degradation. Previous studies have shown that the SWI/SNF ATP-dependent chromatin remodeling complex is essential for the developmental transitions of thymocytes. In many studies, the stability of the SWI/SNF complex has been shown to be regulated by ubiquitylation. In particular, SRG3 stabilizes SNF5, BRG1 and BAF60a by attenuating their proteasomal degradation, suggesting its essential role in stabilization of the SWI/SNF complex. It was also found that CHFR, an E3 ubiquitin ligase and known tumor suppressor that plays an essential role in cell cycle control and tumorigenesis, interacts with and ubiquitylates BRG1, SNF5, and BAF60a of the SWI/SNF complex, targeting them for degradation through a proteasome-mediated pathway. Moreover, It was found that SRG3 stabilizes these components by blocking their interaction with CHFR.
Second, I investigated the function of Bap1, a deubiquitinase involved in the regulation of cell growth and proliferation, in thymocyte development. Bap1-deficient mice showed developmental retardation in embryo and expansion of the myeloid lineage. In this study, to gain insight into the contributions of Bap1 to T cell development, the mouse Bap1 gene was deleted cell specifically, using Lck-Cre-producing mouse strains. The total thymocyte number was significantly reduced in Bap1 conditional knockout mice. It was found that Bap1 plays a critical role in pre-TCR signaling and TCR-mediated T cell development and activation. Bap1-deficient T cells displayed the block from stage double negative (DN) 3 to DN4, and they exhibited reduced expression of TCRβ compared with controls. Thymic positive selection was also significantly decreased when Bap1 was knocked out in a TCR transgenic background. TCR-mediated signaling events, including calcium flux and extracellular signal-regulated kinase (ERK) activation, were impaired in Bap1-deficient mice. Additionally, peripheral T cells were not efficiently activated upon TCR stimulation. Taken together, these findings suggest that Bap1 is a critical component of T cell development and that it participates in TCR signaling events required for thymocyte activation.
From these results, I conclude that: 1) the SWI/SNF chromatin remodeling complex for ordinary T cell development is stabilized by SRG3 through the blockade of CHFR activity, and 2) Bap1 expression in pre-TCR and TCR signal transduction is critical for successful thymocyte development and maturation.