Embryonic Patterning of Xenopus Mesoderm by Bmp-4
179
boundaries of Xvent-l and Xvent-2 expression are regulated by the local
activity of BMP-4 in the marginal zone (Fig. 3). Xvent-2 requires a
lower BMP-4 dose to be induced than Xvent-l, and hence Xvent-2 is
expressed closer to the organizer, where BMP signalling is attenuated,
compared to Xvent-l (Dosch et al. 1997).
Consistent with Xvent genes acting in mediating BMP signalling,
overexpression on the dorsal side of both Xvent genes leads to ventralization of embryos (Fig. 4E) (Gawantka et al. 1995; Ladher et al. 1996;
Onichtchouk et al. 1996; Schmidt et al. 1996; Tidman-Ault et al. 1996).
Xvent proteins function as transcriptional repressors and dominantnegative forms have been generated by fusing their homeodomains to
the transcriptional activation domain of VP16 (Onichtchouk et al. 1998).
Microinjection of synthetic mRNA encoding dominant-negative Xvent
mRNAs leads to dorsalization of ventral mesoderm and induction of
secondary embryonic axes (Fig. 4F). The organizer gene XFD-l' is
repressed by, and is a direct target gene of, Xvent~ 1 (Friedle et al. 1998).
The DNA target consensus sequence recognized by Xvent-l is
CTATTTG, and thus is distinct from typical homeodomain consensus
sequences, probably due to the presence of threonine instead of the more
common valine or isoleucine residues in the DNA recognition helix
which is conserved in the Xvent protein family.
Fig. 4A-G. Xvent homeobox genes mediate BMP signalling. Whole-mount in
situ hybridizations showing expression of Bmp-4 (A), Xvent-l (B), and Xvent2 (C) in Xenopus gastrulae, vegetal view dorsal side up (arrowhead indicates
the dorsal blastopore lip). Note the different dorsal expression boundaries of
Xvent-l and Xvent-2 (dashed lines) indicative of different threshold responses
for BMP signalling. D Control embryo. E Embryo microinjected with Xvent-2
mRNA showing loss of head and axial structures, indicative of ventralization.
F Embryo microinjected with dominant negative (dn) Xvent-2 mRNA showing
a secondary embryonic axis, indicative of dorsalization. G Model of Xvent
gene action in response to different BMP-4 concentrations. Left, high BMP-4
activity in lateroventral marginal zone drives the expression of both Xvent
genes. Coexpression of both Xvent genes specifies cells as the most ventral
type of mesoderm: blood and mesenchyme, repressing muscle and notochord.
Right, low BMP-4 activity in dorsolateral marginal zone is not sufficient for
Xvent-l but for Xvent-2 expression. Xvent-2 expression promotes muscle differentiation, but represses notochord. (From Onichtchouk et al. 1996;
Onichtchouk et al. 1998)
179
boundaries of Xvent-l and Xvent-2 expression are regulated by the local
activity of BMP-4 in the marginal zone (Fig. 3). Xvent-2 requires a
lower BMP-4 dose to be induced than Xvent-l, and hence Xvent-2 is
expressed closer to the organizer, where BMP signalling is attenuated,
compared to Xvent-l (Dosch et al. 1997).
Consistent with Xvent genes acting in mediating BMP signalling,
overexpression on the dorsal side of both Xvent genes leads to ventralization of embryos (Fig. 4E) (Gawantka et al. 1995; Ladher et al. 1996;
Onichtchouk et al. 1996; Schmidt et al. 1996; Tidman-Ault et al. 1996).
Xvent proteins function as transcriptional repressors and dominantnegative forms have been generated by fusing their homeodomains to
the transcriptional activation domain of VP16 (Onichtchouk et al. 1998).
Microinjection of synthetic mRNA encoding dominant-negative Xvent
mRNAs leads to dorsalization of ventral mesoderm and induction of
secondary embryonic axes (Fig. 4F). The organizer gene XFD-l' is
repressed by, and is a direct target gene of, Xvent~ 1 (Friedle et al. 1998).
The DNA target consensus sequence recognized by Xvent-l is
CTATTTG, and thus is distinct from typical homeodomain consensus
sequences, probably due to the presence of threonine instead of the more
common valine or isoleucine residues in the DNA recognition helix
which is conserved in the Xvent protein family.
Fig. 4A-G. Xvent homeobox genes mediate BMP signalling. Whole-mount in
situ hybridizations showing expression of Bmp-4 (A), Xvent-l (B), and Xvent2 (C) in Xenopus gastrulae, vegetal view dorsal side up (arrowhead indicates
the dorsal blastopore lip). Note the different dorsal expression boundaries of
Xvent-l and Xvent-2 (dashed lines) indicative of different threshold responses
for BMP signalling. D Control embryo. E Embryo microinjected with Xvent-2
mRNA showing loss of head and axial structures, indicative of ventralization.
F Embryo microinjected with dominant negative (dn) Xvent-2 mRNA showing
a secondary embryonic axis, indicative of dorsalization. G Model of Xvent
gene action in response to different BMP-4 concentrations. Left, high BMP-4
activity in lateroventral marginal zone drives the expression of both Xvent
genes. Coexpression of both Xvent genes specifies cells as the most ventral
type of mesoderm: blood and mesenchyme, repressing muscle and notochord.
Right, low BMP-4 activity in dorsolateral marginal zone is not sufficient for
Xvent-l but for Xvent-2 expression. Xvent-2 expression promotes muscle differentiation, but represses notochord. (From Onichtchouk et al. 1996;
Onichtchouk et al. 1998)