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trans- acting factors, which would result in selective mRNA destabilization.
However, the mouse model offers a different view. Bioinformatics analysis of
mRNAs degraded during meiosis from Su et al. (Su et al. 2007 ) suggests that
degraded transcripts tend to have shorter 3′UTRs and degradation is not associated
with specific cis-acting 3′UTR motifs. In contrast, stable mRNAs have longer
3 ′UTRs and are enriched in cis-acting AU-rich motifs (Svoboda, unpublished).
Interestingly, AREs, which are a class of AU-rich motifs, can direct mRNA dead-
enylation without triggering ARE- mediated mRNA decay in Xenopus oocytes,
resulting in stabilization of deadenylated transcript until the midblastula transition
(Voeltz and Steitz 1998 ). The opposing effects of ARE and CPE on maternal
mRNA polyadenylation timing in Xenopus oocytes (Belloc et al. 2008 ) suggest
that AREs and ARE-binding proteins are key modulators, balancing the CPE-
mediated polyadenylation and translational activation. Mouse oocytes express a
number of ARE-binding proteins with roles during oocyte development and beyond
(Ramos et al. 2004 ; Chalupnikova et al. 2014 ).
Taken together, mouse transcriptome changes during meiotic maturation are
rather consistent with selective stabilization in an mRNA-destabilizing environ-
ment. In addition, increased RNA turnover could be also coupled with translation,
which would be consistent with the housekeeping nature of many of the degraded
mRNAs. While this model certainly needs further experimental support, it brings an
important concept to studies of maternal mRNA degradation—that degradation of
mRNAs does not need to be selective per se because selectivity may concern mRNA
stabilization. It remains to be examined to what extent is the selectivity of maternal
mRNA degradation during oocyte maturation driven by sequence-specific recruit-
ment of the degradation machinery and to what extent is it driven by the loss of
protection of a large portion of the transcriptome against degradation, e.g., protec-
tion conferred by RNA-binding proteins such as MSY2.
10.2.2.4 mRNA Degradation Induced by Fertilization
The mechanism of how fertilization induces degradation of mRNAs, which were
relatively stable during meiotic maturation, is poorly understood. One of the
known modifications of the mouse transcriptome after fertilization is an extensive
cytoplasmic polyadenylation. It is manifested as an apparent increase in the
poly(A) RNA content in the 1-cell embryo (Fig. 10.5). It is possible that this pro-
cess is linked not only to recruitment of dormant mRNAs after fertilization for
translation but also to increased turnover of maternal mRNAs. This idea is actu-
ally supported by the latest deep sequencing datasets from unfertilized eggs and
1-cell embryos (Abe et al. 2015 ; Xue et al. 2013 ). If the fertilization-induced
increase in mRNA polyadenylation correlates with increased mRNA decay, one
should simultaneously observe an apparent increase in mRNA abundance after
fertilization in deep-sequenced poly(A) RNA (Xue et al. 2013 ), while total RNA
deep sequencing would reveal reduced abundance of the same mRNA (Abe et al.
2015 ). We examined how frequent this scenario might be, and we have found that
P. Svoboda et al.