Introduction
During oocyte growth, maturation and zygote development, the morphology
and structure of the nucleus undergoes dynamic changes to adjust to
nuclear activities Oocyte growth is accompanied by active transcription
activity and chromatin displays a loose configuration. The oocyte
chromatin undergoes condensation at a later GV stage and becomes further
condensed during meiotic maturation. Before full oocyte growth, oocyte
chromatin is less condensed and not confined around the nucleolus, which
is termed the non-surrounded nucleolus (NSN) configuration. However,
when it comes to fully grown oocytes, the transcription of the oocytes
is terminated, and the chromatin shows a condensed peri-nucleolar ring,
which is termed surrounded nucleolus (SN) configuration (Liu et al.,
2009).
After fertilization, the maternal chromatin undergoes the reverse
process. The chromatin structure starts to loosen to form the
pronucleus, in order to allow the whole genome to re-initiate DNA
replication and thereafter transcription activity (Yu et al., 2016),
which is essential for further embryo development and successful zygotic
genome activation (ZGA) (Li et al., 2013).
Condensins play essential roles in chromatin condensation and chromosome
formation. In prophase, condensins mediate loop array formations. In
prometaphase, the combined action of condensin I and II forms hectically
arranged nested loop arrays (Wood et al., 2010). The knockdown of
condensing subunits, causes disordered chromosome structure and
incorrect segregation during cell division, resulting in abnormal
chromosome alignment, aneuploidy, and even apoptosis (Piazza et al.,
2013).
Structural Maintenance of Chromosomes 2 (SMC2) is the core component of
the condensin complex (Hirano, 2006a). It is essential for building
higher order chromosome structure. A loss of SMC2 in neural stem cells
results in loose chromosomes, nuclear accumulation of p53 and cell death
(Nishide & Hirano, 2014). Apart from the function in the organization
of the genetic material, SMC2 protein also plays an important role in
gene activity regulation. The depletion of SMC2 in chicken DT40 cells
caused a misregulation of gene expression during interphase (Zhang et
al., 2016).
The functions of SMC2 in meiosis is little understood. Injection of
anti-SMC2 antibody in vitro into meiotic mouse oocytes caused
severe defects in kinetochore orientation and chromosome segregation
(Lee et al., 2011). However, its participation in oocyte growth,
maturation and embryo development has never been studied in vivo ,
due to the early mortality of SMC2 knockout embryos (Nishide & Hirano,
2014); thus we used a loxp-Cre mouse model to specifically delete SMC2
in order to clarify its function in female reproduction. We demonstrated
that maternal SMC2 is absolutely required for oocyte chromosome
condensation and subsequent zygote development.