Understanding Multiple Independent Functions of the Tip60 Acetyltransferase in Embryonic Development

Abstract

Chromatin is a dynamic structure, and chromatin remodeling enzymes regulate chromatin structure to control gene expression and proper lineage specification. Tip60-p400 is a multi-subunit chromatin remodeling complex containing two biochemical activities: the Tip60 subunit is a lysine acetyltransferase (KAT) that targets histones and non-histone proteins, and p400 catalyzes ATP-dependent incorporation of histone variant H2AZ into chromatin. Both of these chromatin modifications have been widely studied with respect to gene expression, DNA damage repair, and apoptosis. Ablation of these catalytic subunits causes defects in normal embryonic development, ESC self-renewal, and gene expression. My goal has been to understand the multiple independent functions of Tip60-p400 acetyltransferase in ESC maintenance and embryonic development. I showed that Tip60 KAT function is dispensable for gene expression, chromatin accessibility, and ESC self-renewal, which is different from Tip60 knockdown phenotype. Interestingly, KAT deficient mutants exhibited defect in differentiation towards mesoderm and endoderm lineages. Consistent with this defect, I also observed gastrulation defect in mice lacking Tip60 KAT activity. Together, these data demonstrate that Tip60 KAT dependent function is only required during later stages of embryonic development, and it is dispensable for ESC self-renewal and pre-implantation development. Tip60 KAT contains four isoforms generated from alternative splicing, whose individual functions are poorly characterized. In the second part of this thesis, I investigated the developmental role of one of the isoforms of Tip60, called Tip55. Unlike Tip60 knockout mice, which lack all the isoforms and causes pre-implantation lethality, I found that ablation of Tip55 results in post-implantation lethality. I further found that loss of Tip55 causes defects in heart, and neural tube development, demonstrating the essential function of Tip55 isoform for organogenesis during embryonic development. Together, these studies have provided new insight into the functions of Tip60-p400 and the mechanisms by which this complex regulates gene expression, ESC pluripotency, and embryonic development. Furthermore, these studies set the stage for future work to identify how the catalytic and non-catalytic functions are directed to perform distinct regulatory functions, as well as how each Tip60 isoform individually contributes to formation of the mammalian body plan.