Unique Residues on the H2A.Z Containing Nucleosome Surface Are Important for Xenopus laevis Development

Abstract

Critical to vertebrate development is a complex program of events that establishes specialized tissues and organs from a single fertilized cell. Transitions in chromatin architecture, through alterations in its composition and modification markings, characterize early development. A variant of the H2A core histone, H2A.Z, is essential for development of both Drosophila and mice. We recently showed that H2A.Z is required for proper chromosome segregation. Whether H2A.Z has additional specific functions during early development remains unknown. Here we demonstrate that depletion of H2A.Z by RNA interference perturbs Xenopus laevis development at gastrulation leading to embryos with malformed, shortened trunks. Consistent with this result, whole embryo in situ hybridization indicates that endogenous expression of H2A.Z is highly enriched in the notochord. H2A.Z modifies the surface of a canonical nucleosome by creating an extended acidic patch and a metal ion-binding site stabilized by two histidine residues. To examine the significance of these specific surface regions in vivo, we investigated the consequences of overexpressing H2A.Z and mutant proteins during X. laevis development. Overexpression of H2A.Z slowed development following gastrulation. Altering the extended acidic patch of H2A.Z reversed this effect. Remarkably, modification of a single stabilizing histidine residue located on the exposed surface of an H2A.Z containing nucleosome was sufficient to disrupt normal trunk formation mimicking the effect observed by RNA interference. Taken together, these results argue that key determinants located on the surface of an H2A.Z nucleosome play an important specific role during embryonic patterning and provide a link between a chromatin structural modification and normal vertebrate development.

Changes in the nuclear deposition of histone H2A variants during pre-implantation development in mice

“…In mice, H2A.Zdeficient embryos develop normally until implantation (Faast et al, 2001). In Xenopus, the incorporation of H2A.Z into chromatin is not observed until gastrulation (Ridgway et al, 2004). In our study, we found that H2A.Z was re-expressed at detectable levels from the late blastocyst stage when differentiation takes place.…”

Reconciling the positive and negative roles of histone H2A.Z in gene transcription

“…46 Moreover in yeast, the deletion of either the Gal4 or Pho4 activators did not influence the ability of H2A.Z to be enriched at model target genes of these respective activators (Lemieux K and Gaudreau L, unpublished data). 47 Conversely, in human cells, results obtained by our and other laboratories suggest that targeting of H2A.Z, by virtue of the p400 or SRCAP such as Tetrahymena thermophila, 15 Xenopus laevis, 16,17 Drosophila melanogaster 18,19 and mice, 20 suggesting that it may provide distinct functions among different organisms. In budding yeast, loss of H2A.Z leads to defects in transcriptional activation, alterations in transcriptional silencing, and increased chromosome loss.…”

Temporal Regulation of Foregut Development by HTZ-1/H2A.Z and PHA-4/FoxA

“…Intriguingly, a genome-wide survey of Htz1 occupancy in yeast revealed a correlation between Htz1 and four transcription factors, one of which was the Fox homolog Fkh1 [5]. In Xenopus, endogenous H2A.Z is highly expressed in the notochord [50], a tissue that depends on the pha-4 orthologue FoxA2 for establishment [51][52][53]. These observations suggest that certain transcription factors, including forkhead proteins, may be acutely dependent on H2A.Z.…”