Peculiarities of the molecular composition of heterochromatin associated with pronucleoli in mouse embryos

Abstract

The nucleus of pre-implantation mammalian embryos is characterized by peculiar structural organization. At the initial stages of cleavage, the nucleus of the embryo contains the so-called nucleolus precursor bodies (NPBs) or pronucleoli rather than functionally active nucleoli. The NPBs are fibrillar electron-dense structures inactive in RNA synthesis. The vast majority of NPBs are surrounded by a ring-shaped zone of transcriptionally inactive heterochromatin. Intriguingly, these zones contain not only tri-methylated histone Н3K9me3 as an epigenetic mark of repressed chromatin but also acetylated histone H4K5ac, a well-known marker of active chromatin. Immunocytochemical data suggest that the molecular composition of this ‘ring heterochromatin’ in mouse embryos changes during the realization of embryonic genome activation events, as well as during artificial suppression of transcription. In zygotes, some factors of mRNA biogenesis including splicing factor SC35 (SRSF2) and basal transcription factor TFIID are detectable in the ring chromatin. At later stages of development, other nuclear proteins such as Y14, a core component of the exon-exon junction complex (EJC), as well as the proteins involved in chromatin remodeling (ATRX, Daxx) are also detectable in this area. A typical component of the ‘ring heterochromatin’ is actin. Anti-actin immunocytochemical labeling is most expressed at the two-cell cleavage stage after activation of the embryonic genome. Indicatively, the molecular composition of the ‘ring heterochromatin’ associated with different NPBs may differ significantly even in the same nucleus. This seems to reflect the functional heterogeneity of morphologically similar NPBs according to their competence to the process of nucleologenesis. Here, we discuss briefly some peculiarities of the molecular composition and possible functions of the NPB-associated heterochromatin in mouse early embryos.

Heterochromatin Morphodynamics in Late Oogenesis and Early Embryogenesis of Mammals

“…However, oocytes and zygotes are the cells significantly different in nature: the oocyte is a highly specialized cell, while the zygote is a totipotent cell. Nevertheless, the specific heterochromatin rings form in both late oocytes and zygotes soon after fertilization, contouring the specific nucleolus-related bodies—NLBs and NPBs [ 159 ].…”

“…It can be also mentioned that the NPB-associated heterochromatin rings, at least in the mouse, contain the marks of both transcriptionally inert and transcriptionally active chromatin. For example, H3K9me3 and H4K5ac—the representative marks of “repressed” and “active” chromatin, respectively— are detected in these areas [ 159 ] ( Figure 4 ), indicating a non-trivial bivalent status of the heterochromatin composing the karyosphere-like structures.…”