ABSTRACT
Messenger RNAs in growing mouse oocytes that contain a short poly(A) tail (<90 residues) are much more stable compared to those with longer poly(A) tails. Those with about 150 residues are immediately translated150. Messenger RNA can be stored in the mouse oocyte for long periods during the growth phase with half-life times of approximately 28 days170. In Xenopus some messages without poly(A) tails are stable up to the mid-blastula transition171,172. Microinjection of RNAs containing long poly(A) tails (100-200 As) into mouse oocytes has shown that PARN reduces the length of the poly(A) tail of injected, foreign cytoplasmic mRNAs efficiently to about 20 to 50 adenosines at the 3¢UTR of the mRNA during oocyte growth. A message with a short poly(A) tail is not only more stable, but initiation of translation is repressed by the de-
adenylation173, as shown above. Thus, poly(A) tail removal is the initial and rate-limiting step in mRNA turnover that controls storage as well as decay. Deadenylation is the main mechanism responsible for translational silencing of maternal mRNAs during oocyte maturation and early development to cause translational repression or save them for their timed recruitment at a specific stage of maturation or development, depending on the 3¢UTRs172. Most mRNAs, especially those of housekeeping genes like actin, appear deadenylated at GVBD as shown in the mouse174, and this requires the activity of poly(A)-specific ribonuclease (PARN, or deadenylating nuclease, DAN)175. PARN is a member of the RNaseD family of RNA deadenylating nucleases. Interestingly, disturbances in default deadenylation and enhanced, untimely expression appear characteristic for bovine oocytes with low developmental potential176. Thus it
Figure 4.2 Messenger RNA sequences and RNA-binding proteins in regulation of circularization and initiation of translation of mRNAs during oocyte maturation and early development (modified from references 197, 224, and 246). (a): Modulation of expression of mRNAs by elements in RNA and binding of conserved proteins. CPEB and CPSF influence targeting of PAP to elongate the poly(A) tail for binding of PAPB. (b) Model of circularization of the mRNA. The N-terminal domain of elF4G adapter protein (hook-shaped gray structure, 4G) of the el4F complex (indicated by strippled line) is associated with the elFE cap binding protein (4E) to mediate binding to the PABP while the C-teminal domain of the elF4G protein of the elF4F complex is associated with the helicases elF4A (4A) and recruits the 40S ribosomal subunit with elF3 (F3) and the methyl tRNA with elF2 (triangle F2) to initiate RNA translation. (c) Upper part: masking of mRNAs with short poly(A) tail by binding to Maskin or related proteins to the CPEB, thus preventing tight interaction between CPEB and CPSF to position PAP/Gld2 poly(A) polymerase to elongate the poly(A) tail. Maskin phosphorylated by PKA when bound to elF4E excludes the elF4G protein from the complex and prevents ring formation by attachment to PABP. Lower part: upon resumption of maturation and activation of MPF/cdk1, Maskin is phosphorylated by cdk1 and this releases its interaction with elF4G (and possibly CPEB). Further phosphorylation by Eg2/aurora A kinase may facilitate centrosome attachment and regulation of microtubule length by maskin. Downstream from synthesis of Mos and activation of MAP kinases, phosphorylation of elF4E by kinases facilitates binding to the methylated cap mRNA at the 5¢UTR and interaction with ePABP in circularization. Phosphorylation of CPEB by Eg2/aurora A kinase may induce conformational changes that promote tight interaction between CPEB, CPSF, and symplekin to position the PAP/Gld2 poly(A) polymerase at the 3¢UTR to induce polyadenylation. In turn, this provides sites for attachment of embryonic PABP (ePAB) for ring formation and association of the ePAB with the elF4G adapter protein of the el4F initiation complex. For further explanation and references, see text
is feasible that overexpression of genes due to insufficient deadenylation in not fully developmentally competent oocytes such as derived from a suboptimal follicular environment might contribute to reduced quality.
