501the main pathway regulating mRNA decay (Yamashita et al. 2005 ). In specific
cases, mRNAs can be cleaved by an endonucleolytic activity (e.g., by RNA interfer-
ence (RNAi)). The final step in degradation of mRNAs with accessible 5′ or 3′ end
is exonucleolytic degradation by XRN1 or the exosome, respectively.
Maternal mRNAs can be divided into three groups with respect to their function
during OET:
- Translated mRNAs encoding proteins needed during oocyte growth but not later
(e.g., oocyte-specific transcription factors). Some of these maternal factors might
be even detrimental to OET and must be removed. - Translated mRNAs encoding proteins functioning during OET, i.e., during mei-
otic maturation and/or during early embryogenesis. This category includes pro-
teins with OET-specific roles as well as proteins with housekeeping function. - Dormant maternal mRNAs—stable untranslated mRNAs, which are recruited
for translation during meiotic maturation or early embryogenesis through cis-
acting cytoplasmic polyadenylation elements (CPEs), which are bound by trans-
acting CPE-binding proteins (CPEBs) (reviewed in Standart and Minshall 2008 ;
Radford et al. 2008 ). Uncoupling an mRNA from translation allows for its accu-
mulation and arrival of the encoded protein during transcriptional quiescence.
This category includes factors involved in the control of meiosis (e.g., Mos or
Cyclin B1 (Sheets et al. 1994 )) or regulating maternal mRNA degradation or
zygotic genome activation (ZGA) (e.g., Dcp1a, Dcp2, Cnot6l, Cnot7, Pan2, and
Lin28a in mice (Flemr et al. 2014 ; Ma et al. 2013a; Ma et al. 2015 )).
Maternal mRNA degradation is driven by the three major OET transitions:
resumption of meiosis (the traditionally recognized OET starting point), fertiliza-
tion, and zygotic genome activation (Fig. 10.4). Transcriptome analyses in the
mouse model suggest that each of these three transitions stimulates a wave of
mRNA degradation (Fig. 10.7). At the molecular level, each wave ostensibly
involves changes in the amount or activity of trans-acting factors regulating RNA
stability (i.e., RNA-degrading enzymes or RNA-binding proteins), whose combina-
tions determine changes in stabilities of individual mRNAs.
According to the phases of OET introduced in Sect. 10.1.3 (Fig. 10.4), we will
divide maternal mRNA degradation (and RNA degradation in general) into three
main phases according to developmental transitions initiating mRNA degradation:
Phase I (Sect. 10.2.2.3)—mRNA degradation initiated during meiotic maturation
Phase II (Sect. 10.2.2.4)—mRNA degradation induced by fertilization
Phase III (Sect. 10.2.2.5)—mRNA degradation induced by ZGA
Apart from the three stages mentioned above, we will discuss Phase 0
(Sect. 10.2.2.2), which concerns mRNAs whose degradation occurs before
resumption of meiosis. Since a steady-state mRNA level reflects the balance
between synthesis and turnover, decay of maternal mRNAs will commence with
termination of transcription of at the end of the growth phase (Bouniol-Baly
et al. 1999 ). Thus, maternal transcripts with a high turnover can decay even
before resumption of meiosis.
10 Clearance of Parental Products