All ETDs from UAB

Advisory Committee Chair

Chenbei Chang

Document Type

Dissertation

Date of Award

2016

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Epigenetic factors control gene expression via modulating chromatin conformation. Current studies on epigenetics mainly focus on biochemical mechanisms or functions of epigenetic factors in cancer or neurobiology. The underlying mechanism for epigenetic regulation in early embryonic development, particularly neural and neural crest development, is not well understood. In this thesis, the model organism, Xenopus laevis, is used to study epigenetic factors – polycomb repressive complex 2 (PRC2) and heterochromatin protein 1 beta (HP1β) in neural and neural crest development. In chapter one, the processes of neural and neural crest development are described, and the concepts of epigenetic mechanisms, such as histone methylation, acetylation and DNA methylation, are introduced. The functions of the histone methyltransferase complex PRC2 and the methylated histone H3 lysine 9 binding protein HP1β are described in more detail. Finally, the advantages in using Xenopus as a model system and the tools to manipulate Xenopus embryos are highlighted. In chapter two, the role of PRC2 in neural crest development is examined. We show that the PRC2 core components Eed, Ezh2, and Suz12 are expressed in the neural crest cells. These components are required for neural crest markers expression. EZH2 has histone H3K27 methyltransferase activity. Knocking down of Ezh2 results in neural crest specification and migration defects. In this study, the direct interaction between EZH2 and Snail2 was also identified. Snail2 regulates EZH2 occupancy and histone H3K27 trimethylation levels at the promoter region of Snail2 direct target gene E-cadherin. These results suggest that PRC2 cooperates with Snail2 to regulate neural crest development. The focus of chapter three is on how HP1β regulates neural and neural crest development. We found that all three vertebrate HP1 paralogs, HP1α, β, and γ, were expressed in the neural and neural crest regions during neurulation. Antisense morpholino oligo (MO)-mediated knockdown of HP1β or HP1γ reduced the expression of the late neural marker and the neural crest markers. However, it did not affect the early neural marker or the neural plate border markers. Thus, HP1β and HP1γ appeared to regulate differentiation from precursors to more differentiated cell types during neural development. More in-depth investigation of HP1β suggested that this was indeed the case. HP1β knockdown expanded the expression domain of Oct25, a pluripotency-associated gene. Knockdown of Oct25 partially rescued the defects in HP1β morphant. These results suggest that HP1β promotes neural and neural crest differentiation at least partially via repression of the pluripotency-associated gene Oct25. Chapter four concludes the thesis by discussing the remaining, fascinating questions. We find that PRC2 works with the neural crest transcription factor Snail2 in chapter two. PRC2 might interact with other factors to regulate neural development. Identifying the factors that cooperate with PRC2 in neural development requires additional studies. Post-translational modification of HP1 affects its function and localization. We thus discuss the potential involvement of post-translational modifications of HP1 in neural and neural crest development. A previous study indicated that H3K27me3 increased HP1 binding to H3K9me2/3. However, it is unclear whether and how the PRC2 and the HP1 repressive systems crosstalk and whether they regulate neural and neural crest development via similar or distinct mechanisms. These issues need to be explored in detail in the future.

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