Regulation and function of H2A.Z during the mid-blastula transition

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PROJECT SUMMARY Developmental progression from cleavage phase to gastrulation occurs during the mid-blastula transition (MBT). This transition coincides with several critical events, including maternal to zygotic transition, zygotic genome activation (ZGA), and stem cell formation. How the zygotic genome becomes activate and how proper developmental timing is regulated are critical unknowns in biology. Recent data in zebrafish and Drosophila indicate that these processes are highly sensitive to the levels a particular histone, the histone variant H2A.Z (H2Av in Drosophila). However, it remains largely unknown why altered H2A.Z levels disrupt MBT events and how the embryo ordinarily assures that the correct amount of H2A.Z is incorporated into chromatin. Preliminary observations indicate that in both species a subset of zygotic genes is prematurely activated when H2A.Z levels are elevated. A combination of genomics approaches and expression analysis of candidate genes will be used to determine the temporal pattern of gene activation in embryos with up- or downregulated nuclear H2A.Z levels. It will also be tested to what extent the observed changes are due to chromatin associated factors (in particular the pattern of H2A.Z distribution across the genome) and epigenetic marks. Use of the two distinct model systems will reveal to what extent the underlying mechanisms are conserved between vertebrates and insects. In Drosophila, it is known that H2A.Z levels in the nucleus can be regulated by the H2A.Z binding protein Jabba that sequesters H2A.Z in the cytoplasm, a process active during the time of the MBT. Preliminary studies have led to the hypothesis that a different H2A.Z-binding protein plays an analogous role in zebrafish. This hypothesis will be tested using live imaging, embryo injections, and structure-function analysis. Successful completion of this project will define the role of H2A.Z in controlling gene expression patterns and in cellular programming during one of the most crucial periods of development, the mid- blastula transition.