Somatic H1 histone accumulation and germ layer determination in amphibian embryos

Abstract

The induction of mesoderm and/or endoderm from prospective ectoderm and dorsalization of the marginal zone mesoderm may be linked to inhibition of cell cycling and DNA synthesis in early amphibian embryos. In turn, this may lead to reduction of somatic H1 histone accumulation. A greater number of cell cycles and rounds of DNA synthesis characterizes the induction of neural tissue. This is correlated with an increase of somatic H1 histone accumulation. The number of rounds of DNA replication may regulate the level of H1 histone accumulation and this may have a role in germ layer determination.

SSRP1-mediated histone H1 eviction promotes replication origin assembly and accelerated development

“…Phosphorylation of tyrosine 15 reflects the activation of Chk1 at MBT, which contributes to cell cycle progression regulation in post-MBT cycles 28 . Expression of somatic histone H1 proteins H1A and H1B, which are highly similar to histone H1.0, starts around stage 5 and rapidly increases at MBT 29 whereas SSRP1 protein levels, together with SPT16, decrease at this stage (Supplementary Fig. 4) consistent with previous MS analysis 29 .…”

“…Expression of somatic histone H1 proteins H1A and H1B, which are highly similar to histone H1.0, starts around stage 5 and rapidly increases at MBT 29 whereas SSRP1 protein levels, together with SPT16, decrease at this stage (Supplementary Fig. 4) consistent with previous MS analysis 29 . To test whether histone H1 proteins expression and decreasing levels of SSRP1 protein are determinant for the establishment of somatic replication mode, we injected Myc-tagged SSRP1 mRNA in fertilized eggs and monitored their development following the number of synchronous divisions-only for about 450 min from fertilization until MBT onset, which normally occurs between cycles 12 and 14 ( Fig.…”