Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 152

elements and mechanisms associated with other germ plasmRNAs have not
been studied in much detail. One interesting exception among the genes is
nif , in which the localization element is in the 5 0 -UTR instead of the 3 0
( Claussen et al., 2004 ). It is also an apparently universal but unexplained phe-
nomenon that early pathway mRNAs can also localize by the late pathway if
injected in stage III oocytes.
3.3. Intermediate/dual-pathway RNAs
There is a growing appreciation that many localized mRNAs do not fit
cleanly into either early or late pathways. Typically, these RNAs have some
MCLS but localize more broadly in the vegetal cortex than nos1 or xpat .
Additionally, these RNAs are retained in the germ plasm following fertili-
zation. RNAs in this so-called intermediate pathway include wnt11b , plin2
( fatvg/adrp ), dnd1 ( dead end homolog 1 ), rbpms , grip2 , and trim36 ( Table 4.2 ;
references therein). The localization elements of plin2 , dnd1 , and grip2 have
begun to be characterized. The plin2 RNA contains a single 25 nt element
necessary for localization, which differs from the repeated nature of other
elements ( Chan et al., 1999 ). Localization elements for dnd1 and grip2
through the late pathway have been defined ( Claussen et al., 2011;
Weidinger et al., 2003 ) and are largely similar in nature to typical late path-
way mRNAs. wnt11b and trim36 localize more strongly to the mitochon-
drial cloud of stage I oocytes and occupy a less expansive territory in the
vegtal cortex of stage VI oocytes than gdf1 or vegt ( Cuykendall and
Houston, 2009; Kloc and Etkin, 1998; Ku and Melton, 1993 ). It is unclear
whether these mRNAs represent a true “intermediate pathway” or whether
there is an unknown quantitative aspect to mRNA localization that varies
among transcripts. In support of this idea, a recent microarray analysis of
localized mRNAs ( Cuykendall and Houston, 2010 ) showed that there
was a wide range in the degree of vegetal cortex enrichment of different
RNAs. However, most of these transcripts have not been analyzed for local-
ization mechanisms either functionally or bioinformatically. It will be
important to analyze these and other RNAs in order to gain a more com-
plete picture of possible RNA localization mechanisms.
3.4. RNA localization in other amphibians
The vast majority of work on localized RNAs in amphibians has been done
in Xenopus , but the extent that the expression patterns and embryonic func-
tions of these RNAs are generally representative of anurans, or of all