Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 156

of the same trans -acting proteins are likely involved in RNA localization in
zebrafish. A broad deletion mapping study of the dazl 3 0 -UTR identified
several cis -elements mediating localization to the mitochondrial cloud, veg-
etal cortex, and cleavage furrow germ plasm ( Kosaka et al., 2007 ). The
required regions lack clusters of UGCAC motifs found in Xenopus some
mitochondrial cloud transcripts. Interestingly, the localization element from
Xenopus nos1 localized to the zebrafish mitochondrial cloud in a manner that
required those repeats ( Kosaka et al., 2007 ). These data support the idea that
while such motifs are not universally required for MCLS, transcripts with
these motifs are dependent on them for localization.
Analysis of sybu localization to the mitochondrial cloud in zebrafish rev-
ealed an intriguing requirement for RNA splicing ( Nojima et al., 2010 ).
Rescue of the cognate mutant, tokkaebi (tkk) , was accomplished by supplying
full-length sybu cDNA maternally through transgenesis, although the exog-
enous transcripts failed to localize to the vegetal pole. However, when a
transgene using the genomic intron-exon structure of sybu was used, the
exogenous RNAs localized vegetally and both the degree and efficiency
of rescue was increased ( Nojima et al., 2010 ). These data show that for sybu ,
at least, transcript splicing in the nucleus is necessary for localization, possibly
by influencing transcript selection or RNP assembly. Proper splicing is nec-
essary for oskar localization in Drosophila ( Hachet and Ephrussi, 2004 ), but
this mechanism is not well understood. Outside these examples, a general
requirement for splicing has not been demonstrated for vertebrate
mRNA localization.
3.7. mRNA localization in mammalian oocytes and embryos
In contrast to many invertebrates and vertebrates such as zebrafish and
Xenopus , mammalian embryos do undergo any obvious asymmetric inher-
itance of maternal determinants. Early mammalian blastomeres have a high
degree of developmental plasticity in chimeras and in ablation studies,
suggesting that regulative interactions are sufficient for embryonic develop-
ment ( Rossant and Tam, 2009; Zernicka-Goetz et al., 2009 ). In the mouse,
major transcription of zygotic genes and degradation of maternal transcripts
occurs at the 2-cell stage (zygotic genome activation, ZGA; Schier, 2007 ).
The three main tissue types of the blastocyst, the trophectoderm (TE), epi-
blast, and primitive endoderm (PE), are established through position and cell
polarity-dependent mechanisms ( Rossant and Tam, 2009; Zernicka-Goetz
et al., 2009 ). Briefly, cells that remain on the exterior of the blastocyst