Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 162

Grip2 is multi-PDZ domain protein of the glutamate receptor-interacting
protein family, also encoded by a germ plasm-restricted RNA ( Kaneshiro
et al., 2007; Tarbashevich et al., 2007 ). Inhibition of Grip2 translation using
MOs also affects PGC numbers and migration ( Tarbashevich et al., 2007 ).
The spread of PGCs along the anteroposterior axis is also aberrant in
Grip2-depleted embryos, a phenotype that has not been seen in other germ
plasm RNA loss-of-function experiments ( Tarbashevich et al., 2007 ). Grip2
could thus be involved in interpreting cell migration signals within PGCs.
4.2.4 buckyball
In zebrafish oogenesis, Buc protein and RNA are vegetally localized in associ-
ationwith the germplasm. Buckyball RNA is lost fromgermplasm in embryos,
but Buc protein remains germ plasm localized ( Bontems et al., 2009 ). Over-
expressionof Buc proteinduring embryogenesis induces the formationof germ
plasm and PGCs ( Bontems et al., 2009 ). The Buc protein sequence has homol-
ogy in the N-terminus to other vertebrate proteins, including a protein
encoded by a localized mRNA in Xenopus , velo1 ( Claussen and Pieler,
2004 ). Interestingly, although velo1 is a localized RNA, it does not localize
to the Balbiani body like buckyball ( Claussen and Pieler, 2004 ). The protein
is also thought to be restricted to some vertebrates, but has lost protein-coding
capacity in mammals. Buc is thought to be rapidly evolving ( Bontems et al.,
2009 ) and since germ plasm is present throughout Metazoa, invertebrates
may have cryptic homologues. Buc contains no known conserved motifs
and its biochemical function is unknown. A similar germ plasm-assembling
activity has been described for Xenopus Xpat in oocytes, another protein with-
out obvious functional domains ( HudsonandWoodland, 1998;Machadoet al.,
2005 ). However, it was not determined inwhether functional germ plasmwas
induced in Xenopus embryos. Itwouldbe interesting to compare the activities of
the two proteins in cross-species overexpression studies.
4.3. Axis formation
Several recent experiments have suggested that a subset of localized mRNAs
may play critical roles in forming the dorsal axis of the embryo. In Xenopus ,
axis specification requires the microtubule-based transport of the cortex
toward the future dorsal side. This movement is thought to enrich activators
of the Wnt/beta-catenin signaling pathway on the dorsal side of the blastula,
resulting in stabilization and nuclear translocation of beta-catenin and tran-
scriptional stimulation of dorsal-specific genes ( Houston, 2012; Weaver and
Kimelman, 2004 ).