71has also been implicated as a regulator of Wnt signaling (Kraus et al. 2012 ;
Maisonneuve et al. 2009 ), and therefore it is interesting that some Bic-C target
mRNAs, such as the AES/GRG5 transcription factor (Costa et al. 2013 ) and the Ddx5
RNA helicase (Guturi et al. 2014 ), encode proteins known to affect Wnt signaling.
These observations lead to a model in which Bic-C controls a network of mRNA
and many mRNAs in the network encode embryonic cell-fate regulators (Fig. 2.6).
This suggests that Bic-C acts as the control point of a posttranscriptional regulatory
network that establishes the proper balance of maternal proteins in the embryo
essential for normal development. Precedence for this kind of control by RNA-
binding proteins has been documented in other systems (Hogan et al. 2008 ; Gerber
et al. 2006 ; Ule and Darnell 2006 ; Licatalosi and Darnell 2010 ; Kershner and
Kimble 2010 ). Future experiments will require a systematic examination of the
functions of the Bic-C targets in the embryo with the goal of understanding how
different amounts of repression establish the proper balance of cell-fate determi-
nants throughout the embryo to drive normal development.
2.8 Summary: Maternal mRNA Regulation Establishes
and Elaborates Molecular Asymmetries in the Embryo
As discussed earlier, the primary animal-vegetal asymmetry in Xenopus is estab-
lished early in embryogenesis by localizing mRNAs and proteins in oogenesis,
oocyte maturation, and the early cleavage stages of embryos (Medioni et al. 2012 ;
King et al. 2005 ; Houston 2012 ) (Fig. 2.7). The localization of these molecules may
establish immediate molecular asymmetries, for example, if the molecule is an
active protein or an mRNA that is translated immediately after localization. In addi-
tion, this early localization can serve as preparation for a molecular asymmetry that
will not become evident until later in development, if, for instance, it involves an
Fig. 2.6 The Bic-C posttranscriptional network (Zhang et al. 2013 ). The vegetal cells of develop-
ing Xenopus embryos contain a high concentration of Bic-C that represses the translation of spe-
cific targets such as the AES, Cripto-1, and Ddx5 mRNAs. The protein products of Bic-C target
mRNAs are potentially concentrated in animal cells due to their repression by Bic-C in vegetal
cells
2 Controlling the Messenger...