5051985 ). There is also a 50 % decline in the percentage of polyadenylated RNA during
meiotic maturation in Xenopus (Sagata et al. 1980 ). Poly(A) RNA reduction during
meiosis is not simply continuous mRNA decay in the absence of transcription but is
rather a result of induced and active mRNA degradation. This is evidenced by a
minimal overlap between transcripts degraded during meiotic maturation and
mRNAs that are intrinsically unstable in fully grown mouse oocytes (Su et al. 2007 ;
Puschendorf et al. 2006 ). Interestingly, mRNAs degraded during meiotic maturation
contradict the concept of an oocyte stockpiling resources for early development
because mRNAs degraded during meiotic maturation encode proteins with house-
keeping and basic metabolic functions, for example, mRNAs encoding essentially
all ribosomal proteins (Su et al. 2007 ).
One of the trans-acting factors facilitating mRNA destabilization is MSY2, an
abundant germ cell-specific RNA-binding protein that coats mRNAs and supports
global mRNA stability in mouse oocytes (Yu et al. 2004 ; Medvedev et al. 2008 ,
2011 ). MSY2 becomes phosphorylated during meiotic maturation, and, as a conse-
quence, maternal mRNAs become more accessible to the oocyte’s mRNA decay
machinery (Medvedev et al. 2008 ).
Another mechanism facilitating destabilization of maternal mRNAs is dor-
mancy of decapping (Ma et al. 2013a) and deadenylation (Ma et al. 2015 ). Dcp1a
and Dcp2 mRNAs (whose protein products form the decapping complex) are
maternal transcripts recruited during maturation via cis-acting CPE, thus provid-
ing an elegant mechanism for raising the oocyte’s capacity to degrade maternal
mRNA (Ma et al. 2013a). A similar observation was made for components of
CCR4 and PAN2/3 deadenylases (Ma et al. 2015 ). Inhibition of the maturation-
associated increase in DCP1A and DCP2 proteins prevents degradation of a large
population of maternal mRNAs and affects ZGA (Ma et al. 2013a). A similar
observation was made for inhibition of CCR4 deadenylase (Ma et al. 2015 ). These
data suggests that deadenylation and decapping during meiotic mRNA degrada-
tion are not redundant and could be uncoupled (Ma et al. 2013a). This would be
consistent with Xenopus oocytes where deadenylation also appears uncoupled
from decapping (Gillian- Daniel et al. 1998 ).
Taken together, the transition from mRNA stability to instability during mouse
oocyte maturation is consistent with the following model: mRNAs are relatively
stable during the growth phase because MSY2 binding confers mRNA stability,
while the activity of the mRNA degradation machinery is relatively low. Meiotic
maturation is associated with phosphorylation of MSY2, which makes mRNAs
more susceptible to degradation. In parallel, recruitment of dormant mRNAs encod-
ing components of decapping and deadenylase complexes increases the mRNA deg-
radation capacity of the maturing oocyte. That the increase in DCP1A, CNOT7, and
PAN2 occurs after extrusion of the first polar body suggests that degradation of the
bulk of maternal mRNAs during Phase I occurs late in meiotic maturation (Ma et al.
2013a, 2015 ) and presumably extends into Phase II.
One of the most puzzling questions concerning maternal mRNA degradation is
as follows: How is selectivity of mRNA degradation achieved? The classical model
proposes a combinatorial system of cis-acting sequence/structural motifs and
10 Clearance of Parental Products