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Recent experiments in Xenopus have shown that this self-organization ability
relies on the feedback regulation of Chrd flux in the embryo (De Robertis 2009 )
(Fig. 6.11b). The stability and activity of Chrd is regulated in the extracellular space
by proteolytic degradation, mediated by Bmp1, a member of the Tolloid family of
metallopeptidases. The activity of Bmp1 can be inhibited by the Sizzled (Szl) pro-
tein (Lee et al. 2006 ), a secreted Frizzled related protein. Szl is expressed ventrally
in Xenopus as well as zebrafish, and becomes restricted to an extreme ventral
domain in the ventral mesoderm (Salic et al. 1997 ; Yabe et al. 2003 ; Collavin and
Kirschner 2003 ; Martyn and Schulte-Merker 2003 ). Szl therefore promotes Chrd
activity, creating a relatively shallow gradient of dorsalizing activity, which can
explain the apparent paradox that szl loss-of-function embryos are ventralized
despite its expression ventrally (Yabe et al. 2003 ; Collavin and Kirschner 2003 ;
Martyn and Schulte-Merker 2003 ; Lee et al. 2006 ). Chrd and BMPs proteins can
form long-range gradients within Brachet’s cleft, the narrow intraembryonic space
in the gastrula separating the ectoderm from the mesendoderm, possibly using the
fibronectin-rich extracellular matrix to facilitate distribution (Plouhinec et al. 2013 ).
Self-organization becomes possible because key dorsal and ventral genes encode
proteins with opposing molecular activities, but are under differential transcrip-
tional control. Although the organizer expresses many growth factor antagonists,
this region also contains several BMP receptor agonists, admp (anti-dorsalizing
morphogenetic protein) and bmp2 (Moos et al. 1995 ; Joubin and Stern 1999 ; Lele
et al. 2001 ; Inomata et al. 2008 ). A subset of BMP antagonists are expressed in a
ventral domain, including bmper/crossveinless2 (Ambrosio et al. 2008 ) and bambi
(Onichtchouk et al. 1999 ). A central element of this regulatory network is the posi-
tive feedback control of the dorsal BMP agonists (De Robertis 2009 ): The promi-
nent ventrally expressed BMPs (bmp4/7) are positively regulated by BMP signaling
itself, whereas admp/bmp2 agonists are inhibited by BMP activity. Similarly, the
ventrally expressed BMP antagonists and szl are induced by BMPs, whereas chrd is
repressed. Additionally, all BMPs are inhibited by binding to Chrd and can be
released for action by Bmp1-mediated cleavage of Chrd.
Thus, in a dorsal half explant, loss of BMPs and Szl would lead to enhanced
Chrd cleavage and inhibition resulting in higher admp/bmp2 expression. These
ligands would then accumulate in the area of highest Bmp1/lowest Chrd activity,
activating bmp4/7 expression. Increasing levels of these “new” ventral BMPs would
limit the extent of chrd and admp expression as well as establish a new szl domain,
resetting the Chrd gradient to the size of the new “embryo” (Reversade and De
Robertis 2005 ). Analogous regulatory networks have been uncovered in zebrafish
(Lele et al. 2001 ; Xue et al. 2014 ) and are likely to exist in chick, as orthologues of
the main genes are present. Mammals appear to lack Admp and Szl (although Bmp2
is present), making it unclear whether similar axial self-organization occurs or to
what extent mechanistically similar processes have evolved. As the appreciation of
network effects in biology is growing, it will be interesting to discover whether
these principles of embedded antagonistic proteins under complementary transcrip-
tional control are general features of self-organization in development.
D.W. Houston