Abstract
H3K9 methylation is an important histone modification that is correlated with gene transcription repression. The asymmetric H3K9 dimethylation (H3K9me2) pattern between paternal and maternal genomes is generated soon after fertilization. In the present study, we carefully determined the dynamics of H3K9me2 changes in mouse zygotes, and investigated the regulatory mechanisms. The results indicated that histone methyltransferase G9a, but not GLP, was involved in the regulation of asymmetric H3K9me2, and G9a was the methyltransferase that induced the appearance of H3K9me2 in the male pronucleus of the zygote treated with cycloheximide. We found that there were two distinct mechanisms that regulate H3K9me2 in the male pronucleus. Before 8 h of in vitro fertilization (IVF), a mechanism exists that inhibits the association of G9a with the H3K9 sites. After 10 h of IVF the inhibition of G9a activity depends on yet unknown novel protein(s) synthesis. The two mechanisms of transfer take place between 8–10 h of IVF, and the novel protein failed to inhibit G9a activity in time, resulting in the appearance of a low level de novo H3K9me2 in the male pronucleus.
Highlights
Epigenetic alterations are heritable changes that are not encoded by DNA sequences in a cell, and such changes can be delivered stably throughout development and cell proliferation
Our study carefully examined H3K9me[2] levels in different pronuclear stages of zygotes; we found that a low level de novo H3K9me[2] occurred in the male pronucleus at 10 h of in vitro fertilization (IVF), while de novo H3K9me[2] of zygotes treated with cycloheximide occurred at the same time, suggesting that there might be two distinct mechanisms that regulate the male pronucleus H3K9me[2] in mouse zygotes
We further investigated the effect of inhibition of protein synthesis, DNA replication and protein kinase on H3K9me[2] in mouse zygotes, as shown in (Fig. 1B)
Summary
Epigenetic alterations are heritable changes that are not encoded by DNA sequences in a cell, and such changes can be delivered stably throughout development and cell proliferation. G9a plays a catalytic role in H3K9me[2] activity at euchromatin regions and causes gene expression inhibition[16]. Another euchromatin area displays histone methyltransferase GLP that plays a role in silencing of E2F- and Myc-responsive genes in quiescent cells[15]. There are several kinds of epigenetic alterations in mouse zygotes, including asymmetry of DNA methylation and a series of histone modifications between male and female pronuclei[23]. Our study carefully examined H3K9me[2] levels in different pronuclear stages of zygotes; we found that a low level de novo H3K9me[2] occurred in the male pronucleus at 10 h of in vitro fertilization (IVF), while de novo H3K9me[2] of zygotes treated with cycloheximide occurred at the same time, suggesting that there might be two distinct mechanisms that regulate the male pronucleus H3K9me[2] in mouse zygotes
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