Suv4-20h Histone Methyltransferases Promote Neuroectodermal Differentiation by Silencing the Pluripotency-Associated Oct-25 Gene

Post-translational modifications (PTMs) of histones exert
fundamental roles in regulating gene expression. During
development, groups of PTMs are constrained by unknown mechanisms
into combinatorial patterns, which facilitate transitions from
uncommitted embryonic cells into differentiated somatic cell
lineages. Repressive histone modifications such as H3K9me3 or
H3K27me3 have been investigated in detail, but the role of H4K20me3
in development is currently unknown. Here we show that Xenopus
laevis Suv4-20h1 and h2 histone methyltransferases (HMTases) are
essential for induction and differentiation of the neuroectoderm.
Morpholino-mediated knockdown of the two HMTases leads to a
selective and specific downregulation of genes controlling neural
induction, thereby effectively blocking differentiation of the
neuroectoderm. Global transcriptome analysis supports the notion
that these effects arise from the transcriptional deregulation of
specific genes rather than widespread, pleiotropic effects.
Interestingly, morphant embryos fail to repress the Oct4-related
Xenopus gene Oct-25. We validate Oct-25 as a direct target of
xSu4-20h enzyme mediated gene repression, showing by chromatin
immunoprecipitaton that it is decorated with the H4K20me3 mark
downstream of the promoter in normal, but not in double-morphant,
embryos. Since knockdown of Oct-25 protein significantly rescues
the neural differentiation defect in xSuv4-20h double-morphant
embryos, we conclude that the epistatic relationship between
Suv4-20h enzymes and Oct-25 controls the transit from pluripotent
to differentiation-competent neural cells. Consistent with these
results in Xenopus, murine Suv4-20h1/h2 double-knockout embryonic
stem (DKO ES) cells exhibit increased Oct4 protein levels before
and during EB formation, and reveal a compromised and biased
capacity for in vitro differentiation, when compared to normal ES
cells. Together, these results suggest a regulatory mechanism,
conserved between amphibians and mammals, in which
H4K20me3-dependent restriction of specific POU-V genes directs cell
fate decisions, when embryonic cells exit the pluripotent state.