Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 135

Cell junctions are absent from early blastomeres, and features of polarized
cells such as zona occludens, desmosomes, apical microvilli, and symmetric
division do not appear until the late 32- to 64-cell stage. This polarization
occurs cell autonomously in blastomeres that inherit egg membrane,
suggesting that the components necessary for definitive apicobasal polarity
are maintained in the apical cortical domain ( M¨ ller and Hausen, 1995 ).
The mechanisms for separating apical and basal domains in the absence of
definitive tight junctions are not known, but differences in membrane lipid
motility have been implicated ( Dictus et al., 1984 ).
2.2.2 Molecular regulation of egg cell polarity
Recent studies have begun to elucidate the molecular basis for the regulation
of cell polarity in the oocyte and egg. Atypical protein kinase C (Prkci/
aPKC/protein kinase C, iota) is localized uniformly at the membrane in
oocytes, and becomes enriched in the animal hemisphere during oocyte
maturation, along with Pard3 (Par3/par-3 partitioning defective 3 homolog;
Nakaya et al., 2000 ). Similar results were seen with Pard6 ( Choi et al., 2000 ),
another component of the apical PKC/Par complex, suggesting that acqui-
sition of apical polarity in the egg membrane begins at oocyte maturation.
More recently, animal hemisphere localization of Prkci in eggs was shown to
be dependent on Vangl2 (Vang-like 2; Cha et al., 2011 ), a component of
the planar cell polarity pathway. Like Prkci, the localization of exogenous
Lethal giant larvae homolog 2 (Llgl2)-GFP, a basolateral scaffolding protein,
is uniform around the oocyte cortex ( Cha et al., 2011 ). During oocyte
maturation, Llgl2-GFP becomes restricted to the vegetal cortex, likely as
a result of its exclusion from the animal region by Prkci ( Cha et al.,
2011 ). Atypical PKC signaling also regulates stable microtubules in Xenopus
oocytes ( Cha et al., 2011 ), similar to the case in Drosophila ( Tian and Deng,
2008 ), and may additionally be involved in the anchoring of some vegetally
localized mRNAs ( Cha et al., 2011 ). These data suggest integration at some
level between planar cell polarity signaling and epithelial cell polarity signal-
ing in the oocyte.
Thus, oocytes are nonpolarized, and the plasma membrane and cortex
express components of both apical and basolateral domains ( Fig. 4.2 ).
Oocyte maturation establishes a presumptive apicobasal polarity sup-
erimposed on the animal-vegetal axis, with the animal hemisphere
corresponding to apical and the vegetal hemisphere to basolateral. The
mechanisms establishing this polarization are unknown and may occur
largely independently of overall cytoskeletal changes, since animal-vegetal