Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 143

Figure 4.4 Patterns of mRNA localization in Xenopus. (A) In situ hybridization against a
late pathway mRNA, vegt, in stage IV and stage VI oocytes (vegetal views). vegt is
expressed broadly throughout the cortex of the vegetal hemisphere. Inset shows non-
localized expression of vegt in a stage I oocyte. (B) Expression of a germ plasm-localized
mRNA, dazl, during oogenesis. dazl is localized to the mitochondrial cloud in stage
I oocytes. Expression in later stages (stages III-VI) is localized to a compact domain at
the vegetal apex. (C) Expression of dazl in 4-cell embryos (vegetal views), showing char-
acteristic localization to germ plasm islands associating with the cleavage furrows.
Molecular analyses of mRNA localization mechanisms in Xenopus
oocytes have identified cis -acting localization elements, trans -acting proteins,
and different cellular organelles and cytoskeletal systems involved in local-
izing early- and late pathway RNAs. One surprising outcome from these
lines of investigation is that there is no unique consensus sequence that
determines the localization pattern of particular mRNAs, in contrast to
well-characterized cis -elements for mRNA polyadenylation ( Zhang et al.,
2010 ) or mRNA decay ( Alonso, 2012 ). Instead, clusters of short nucleotide
repeats mediate binding to a conserved complex of RNA-binding proteins.
Identifying the ultimate molecular determinants of localization specificity is
an important but unresolved problem in cell and developmental biology.
A number of localized RNAs that have characterized in some depth are
listed in Table 4.2 .
3.1. Late pathway RNAs
3.1.1 Localization of gdf1 RNA
gdf1 was the first and most prominent localized mRNA described in Xenopus
oocytes and has served as the primary model for localization mechanisms.
During stage I oocytes, gdf1 is unlocalized and uniform throughout the cyto-
plasm ( Forristall et al., 1995; Kloc and Etkin, 1995; Melton, 1987 ). By late
stage II/stage III, gdf1 associates with a wedge-shaped domain extending
from the GV to the vegetal cortex ( Deshler et al., 1997; Kloc and Etkin,
1995 ). Gdf1 RNA colocalizes with endoplasmic reticulum in this region,
thought to be specialized subdomain associated with machinery involved