Biology Reference
In-Depth Information
2.3. Histone variants in heterochromatin in the embryo
The two gametes have a very different landscape of histone variants and chro-
matin compaction. For instance, the sperm is almost devoid of histones and is
instead packed by protamines, a histone-like protein (
Nonchev & Tsanev,
1990
). This replacement is important physically and physiologically because
it will help to package the paternal genome to fit into the sperm head for sub-
sequent transportation through the uterus andovidcut to reach the oocyte. The
DNAbound to protamines reaches 6-20 times more compact packaging com-
pared to the nucleosomal organization (
Jenkins & Carrell, 2012; Ward &
Coffey, 1991
). The protamination of the sperm DNA is tightly regulated
and is important for fertility (
Aoki, Liu, & Carrell, 2005; Balhorn, Reed, &
Tanphaichitr, 1988
), but it was also considered to result in the loss of any poten-
tial epigenetic information carried through the histones.However, the replace-
ment of histones is not complete and between 5% and 15% of histones
depending on the species remains bound to DNA (
Jenkins & Carrell, 2012;
Wykes&Krawetz, 2003
). This retention seems not tobe stochastic sincedevel-
opmental gene promoters that are important for embryonic development seem
to retain histones specifically (Brykczynska et al., 2010;
Hammoud et al., 2009
).
More interestingly, such promoters, which are silent during preimplantation
development, retain H3K27me3 in the sperm (
Hammoud et al., 2009
). This
result suggests that the nucleosomes retained in the sperm might act as trans-
mitters of silencing information for some genes during early development.
Additionally, testis-specific histone variants exist (
Moss, Challoner, &
Groudine, 1989; Trostle-Weige, Meistrich, Brock, & Nishioka, 1984; Witt,
Albig, & Doenecke, 1996
) and have been described to be posttranslationally
modified (
Lu et al., 2009
), although the function of these modifications has
not been addressed in the context of the zygotic reprogramming. Most inter-
estingly, constitutiveheterochromatic regions display a distinctive organization
in the sperm, with both telomeres and centromeres retaining histones andmost
likely some of their modifications (
Govin et al., 2007; Wykes & Krawetz,
2003
). In particular, testis-specific variants of H2A, H2AL1, H2AL2, and
TH2B are found in the pericentric chromatin to achieve a DNA packaging
structure that protects a
60 bp DNA fragment (
Govin et al., 2007
).
The oocyte also contains some specific histone variants but most likely
retains the traditional nucleosome packaging of the DNA (
Chang et al.,
2005
). The oocyte is thought to further contain a pool of histones that
are to be inherited by the zygote in order to replace the protamines of
the paternal genome upon fertilization. The mechanism by which the
