Refine
Has Fulltext
- yes (2)
Is part of the Bibliography
- yes (2)
Document Type
- Doctoral Thesis (2)
Language
- English (2)
Keywords
- Pluripotenz (2) (remove)
Institute
- Institut für Medizinische Strahlenkunde und Zellforschung (2) (remove)
Polycomb group (PcG) proteins are chromatin modifiers involved in heritable gene repression. Two main PcG complexes have been characterized: Polycomb repressive complex (PRC) 2 is involved in the initiation of gene silencing, whereas PRC1 participates in the stable maintenance of gene repression. Pcgf4 (Polycomb group protein, Bmi1) is one of the most studied PRC1 members with essential functions for embryonic development and adult stem cell self renewal. In embryonic stem cells (ES cells), Pcgf4 is poorly expressed while its paralogs (Pcgf1, Pcgf2, Pcgf3, Pcgf5 and Pcgf6) are expressed at higher levels. The relevance of the Pcgf paralog Pcgf6 for the maintenance of ESC pluripotency has not been addressed so far. My analyses revealed that Pcgf6 was the most expressed Pcgf paralog in undifferentiated ES cells. When ES cells differentiated, gene expression of Pcgf6 strongly declined. To investigate the functions of Pcgf6 in ES cells, we established a doxycycline (dox) inducible shRNA-targeted knockdown system according to publications by Seibler et al. (Seibler et al. 2005; Seibler et al. 2007). Following dox-induced knockdown (KD) of Pcgf6, we observed decreased ES cell colony formation. In parallel, gene expression of pluripotency markers Oct4, Nanog and Sox2 was reduced upon dox-treatment, wheras the expression of mesoderm genes such as T (Brachyury) were up-regulated. Further, microarray analysis revealed de-repression of several spermatogenesis-specic genes upon Pcgf6-KD, suggesting that Pcgf6 may play a role during spermatogenesis. Upon in vitro differentiation, Pcgf6-KD ES cells showed increased hemangioblast formation, paralleled by increased hematopoietic development. In summary, results of this study suggest that Pcgf6 is involved in maintaining ES cell identity by repressing lineage-specific gene expression in undifferentiated ES cells.
Stem cells with the particular potential to self renew and to differentiate into multiple cell lineages are fascinating cell types for basic and applied research. Pluripotent embryonic stem (ES) cells are derived from the inner cell mass (ICM) of preimplantation embryos. Upon differentiation ES cells can give rise to cells of ecto-, meso- and endoderm including germ cells. In contrast, multipotent adult stem cells are more restricted in their differentiation outcomes,they differentiate into cells of their tissue of origin. For example, hematopoietic stem cells (HSCs) that reside in hemogenic tissues such as the bone marrow (BM) differentiate into hemato-/lymphoid cell lineages. Upon differentiation of stem cells not the genome, but the epigenetic regulation changes. Differentiation-associated epigenetic changes generate cell types with distinct phenotypes and functions. For stem cell-based therapies it is important to deeper understand the relation between epigenome and cellular function. In the scope of this thesis I aimed to analyze cultures of differentiating stem cells with respect to gene expression, chromatin regulation and differentiation potential. For the analysis of global histone modification levels, which represent one mechanism for epigenetic regulation, fow cytometric protocols were established that allow single cell measurements. By applying this methodology decreased histone acetylation levels were shown in differentiated ES cell populations. In contrast, comparable histone acetylation levels were observed in differentiated and undifferentiated BM cells. In addition, I investigated effects of the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) on murine BM cells, comprising also HSCs. Upon TSA treatment the frequency of cells with in vitro and in vivo hematopoietic activity was increased, while lineage committed cells underwent apoptosis. Next, the loss of pluripotency was assessed in differentiating ES cell cultures. Using short-term in vitro differentiation protocols marker-based analyses and functional assays were performed.Functionally pluripotency was diminished after 2 days of differentiation as assessed by colony formation, embryoid body (EB) formation and cardiomyogenic differentiation approaches. In contrast, pluripotency marker expression was reduced at later time points. Further, the application of distinct differentiation systems (aggregation EB, clonal EB or monolayer (ML) culture) had an impact on the progression and homogeneity of differentiation cultures. To further study the end of pluripotency, differentiated ES cells were placed under ES cell culture conditions. The data suggest that 3 days differentiated ES cells had passed a point of no return and failed to regain Oct4-eGFP expression and that HDAC inhibitor treatment selectively killed differentiated ES cells. Finally, I aimed to study the effect of EED - a core subunit of the histone methylating Polycomb repressive complex 2 (PRC2) - on ES cell chromatin and function. ES cells lacking EED showed loss of histone H3 lysine 27 trimethylation (H3K27me3) accompanied by increased histone acetylation and reduced H3K9me3 levels. Despite typical ES cell morphology and pluripotency marker expression, EED knockout (KO) ES cells exhibited altered nuclear heterochromatin organization, delayed chromatin mobility and a failure in proper differentiation. Conclusively, my data provide insights into the epigenetic regulation of stem cells. Particularly, the results suggest that HDAC inhibitor treatment was detrimental for differentiated BM as well as for differentiated ES cells and that ES cells after 3 days of differentiation had lost pluripotency. Further, the data demonstrate that EED KO ES cells self renewed, exhibited morphology and pluripotency marker expression similar to wild type ES cells, but failed to differentiate. This indicates an important role of EED not only for undifferentiated but also for differentiating ES cells.