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4.3.1 wnt11
Although some early work indicated that wnt11b was localized to the vegetal
cortex and could play a role in axis formation ( Ku and Melton, 1993 ), other
data suggested that beta-catenin regulation on the dorsal side was Wnt ligand
independent. However, maternal depletion of wnt11b mRNA suggested
that Wnt11b was indeed required for axis formation, in a secretion-
dependent manner ( Tao et al., 2005 ). The exact mechanism of Wnt11b
in dorsoventral patterning remains unclear, however. Both the dorsal
enrichment of wnt11b transcripts, in association with the germ plasm
( Tao et al., 2005 ), and dorsal-specific polyadenylation are suggested to occur
( Schroeder et al., 1999 ). Wnt11b signaling is also active in the full-grown
oocyte ( Kofron et al., 2007 ), possibly regulating the abundance of Axin-
containing beta-catenin degradation complexes. It will thus be important
to determine when Wnt11 is required for axis formation.
4.3.2 plin2
Two additional localized mRNAs, plin2 and trim36 , have roles in axis for-
mation and both likely act at the level of microtubule assembly and cortical
rotation ( Chan et al., 2007; Cuykendall and Houston, 2009 ). plin2 mRNA
localizes through an intermediate pathway during oogenesis, both to the
mitochondrial cloud and by the late pathway ( Chan et al., 2001 ). Maternal
mRNA depletion of plin2 caused a loss of dorsal axis formation ( Chan et al.,
2007 ). Interestingly, these embryos failed to undergo cortical rotation and
showed precocious aggregation of the germ plasm in eggs, although the
extent to which these two effects are correlated is not known ( Chan
et al., 2007 ). It is also uncertain how Plin2 might control cortical rotation.
Plin2 is a conserved member of the Perilipin family of proteins enriched on
lipid droplets, and Plin2 itself is detected on lipid vesicles in the cortex of
Xenopus oocytes ( Chan et al., 2007 ). These little understood organelles
are major sites of neutral lipid storage but also regulate lipid metabolism,
signaling, and trafficking ( Walther and Farese, 2009 ). Lipid droplets could
regulate metabolism in the cortex, supplying energy for microtubule-
dependent motor activity during cortical rotation, or they could serve as
cross-linkers or nucleation sites for higher-level microtubule organization.
Unfortunately, the functions of lipid droplets and their associated proteins
have not been well studied. Evidence also suggests that plin2 could function
as a structural RNA to control cytokeratin organization ( Kloc, 2009 ; see
Section 4.5 ).
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