Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 158

embryos may be more reliant on maternal mRNAs than mouse in general,
since ZGA occurs at the 8-cell stage ( Cockburn and Rossant, 2010 ).
A recent study indicated that timely degradation of maternal transcripts
was critical for normal initiation of zygotic gene expression in individual
cells and for overall embryonic viability ( Wong et al., 2010 ). Since mamma-
lian blastomeres divide less synchronously than aquatic vertebrates, putative
localized maternal molecules could be involved in timing zygotic gene
expression and cleavage in early blastomeres. It will remain intriguing to
speculate on the existence of vegetally localized determinants in mammals
until they are either conclusively identified or conclusively ruled out.
4. FUNCTIONS OF LOCALIZED RNAs
The expression patterns of the first several localized mRNAs to be
identified immediately suggested possible functions in development.
Because of the current inability to perform targeted gene deletions in
Xenopus , loss-of-function studies have relied predominantly on maternal
mRNA depletion/interference experiments using antisense DNA oligos
and morpholino oligos ( Hulstrand et al., 2010 ). Localized maternal mRNAs
are particularly amenable to this technology since the RNAs can be
degraded by RNaseH-like activity in oocytes, effectively depleting the
maternal pool of that transcript. Isolated oocytes can also be cultured for long
periods, transitioned through meiosis in vitro and implanted into an egg-
laying host female, facilitating fertilization and ultimate assessment of the
embryonic phenotype ( Olson et al., 2012 ). While traditionally unique to
Xenopus , a comparable method has recently been developed in zebrafish
( Nair et al., 2013 ). It will be important to learn the extent that localized
RNAs have similar developmental roles in these two organisms.
The functions of many localized RNAs are indeed consistent with their
localization patterns, with late pathway RNAs acting in overall germ layer
induction and pattering (e.g., vegt ; Zhang et al., 1998 ) and early pathway
RNAs acting in the germ line (e.g., dazl ; Houston and King, 2000a ). How-
ever, it is becoming increasingly apparent that many of the so-called inter-
mediate RNAs may have multiple roles in the embryo. Several recent
studies point to new roles for localized RNAs in aspects of oogenesis and
oocyte maturation as well as potential structural roles. Localized RNAs
should not therefore be considered only as developmental determinants.
Some of the key localized mRNAs analyzed in maternal depletion studies
are described in the following sections.