The role of the histone variant H2A.Z in early Xenopus laevis development

Abstract

The genome of eukaryotes is packaged into the small volume of the nucleus in an organised manner. This structure of DNA and associated proteins is called chromatin. The basic unit of chromatin is the nucleosome; an octomer of core histone proteins and associated DNA. Other proteins such as linker histones can also associate with the DNA or the core histones. The modular structure of chromatin allows for structural variation with functional consequences including activation or repression of transcription. Alterations can include post-translational modifications to histones, remodelling by multi-protein complexes, DNA methylation, and non-allelic variants of the canonical histones. Changes to chromatin structure have an important impact on all DNA processing events. This thesis investigated the histone variant H2A.Z, a variant of the canonical core histone H2A. H2A.Z is highly conserved and essential in a number of species suggesting it has a critical function. Preliminary work using the Xenopus laevis developmental model system had revealed that disruption of H2A.Z function resulted in defective embryo morphology consistent with disrupted gastrulation and mesoderm development (Ridgway et al., 2004a). This led to the following hypothesis: H2A.Z is important to gastrulation and mesodermal development in X laevis because it plays a developmental role. Temporal and spatial expression patterns of H2A.Z mRNA demonstrated in this study are consistent with a role in mesoderm development. Peak H2A.Z mRNA expression levels occur during gastrulation. H2A.Z mRNA is enriched in the marginal zone of the late blastula, involuting tissue in the gastrula and in notochord (a mesodermal tissue) in tailbud embryos. Significantly, maternal H2A.Z mRNA is enriched asymmetrically in dorsal cells of the early blastula before zygotic transcription, indicating that H2A.Z may play a role in determining polarity of the dorsal ventral axis. Two important processes for development were examined in this thesis: cell fate and cell movement. Determination of mRNA levels and localisations for a selection of mesodermal marker genes indicates that cell fate programs progress normally in embryos where H2A.Z function is disrupted. However, the localisation of mesoderm derived cells is perturbed suggesting cell movement is perturbed. Taken together these studies suggest the H2A.Z histone variant has a specific role in regulating cell mobility during early Xenopus laevis development.