Mechanisms and Dynamics of Heterochromatin Formation During Mammalian Development: Closed Paths and Open Questions - Current Topics in Developmental Biology - page 32

and reverse strand was detected, but at different degrees and with different
temporal dynamics. Interestingly, the paternal pronucleus shows a signifi-
cantly higher transcriptional rate of major satellites compared to the maternal
one ( Probst et al., 2010; Puschendorf et al., 2008; Santenard et al., 2010 ).
A burst of transcription of the forward strand correlated temporally with
the formation of chromocenters. Importantly, by blocking the effect of major
satellite transcription using LNA gapmers, it was shown that there is a strand-
specific requirement for the remodeling of heterochromatin into a somatic
chromocenter configuration ( Probst et al., 2010 ). These data argue strongly
that transcription from the major satellites after fertilization is required for
heterochromatin remodeling and developmental progression.
Largely inspired by the work in S. pombe and Arabidopsis , work from our
lab has shown that the transcription of major satellites in the zygote is asso-
ciated with tethering of HP1 b and also with the spatial nuclear localization
typical of pericentromeric chromatin around the NLBs in the embryo
( Santenard et al., 2010 ). This further suggests that there might be a functional
link between spatial localization and silencing of pericentric domains after
fertilization. Although the mechanisms behind are still to be determined,
our data suggested that dsRNA from the major satellites can lead to the local-
ization of HP1 b to the highly condensed, DAPI-rich regions around the
NLBs in the mouse embryo. Interestingly, and in contrast to the S. pombe
model, it is the hinge region of HP1 b rather than its chromodomain that
seems to be the main determinant for HP1 b localization in embryonic het-
erochromatin, at least in the zygote and at the 2-cell stage. In agreement with
this, mutation of the chromodomain of HP1 b does not seem to elicit a major
defect in its subnuclear localization ( Santenard et al., 2010 ). This is in line
with the fact that the paternal chromatin lacks detectable H3K9me3. In
the embryo, silencing of pericentric heterochromatin occurs therefore in
the absence of any detectable levels of H3K9me3, and it is instead
H3K27me2/3 that seems to be a major player in this process ( Puschendorf
et al., 2008; Santenard et al., 2010 ).
Thus, it would seem that transcription of pericentromeric repeats also lies
at the heart of the formation of heterochromatic signatures de novo in mam-
mals. A number of questions remain, however, unanswered, mainly to
determine whether the downstream effectors of such transcription are the
components of the RNAi machinery or any related protein(s) and whether
the maintenance mechanism throughout the cell cycle depends also on this
transcription.