172
C. Niehrs et al.
chant et al. 1998), as would be expected if the ratio between BMP-4 and
BMP antagonists determines cell fates.
While dose-dependent patterning by BMP-4 would be expected if the
protein acts as a morphogen, the possibility remains that BMP-4 only
induces different doses of a yet unknown morphogen. This raises the
question of whether BMP-4 protein is able to signal directly over a long
range as dpp is in Drosophila (Lecuit et al. 1996; Nellen et al. 1996). To
address this question, Bmp-4 mRNA was microinjected into blastomeres of 32-cell stage embryos together with ~-galactosidase containing a nuclear localization signal as lineage tracer. Embryos were subsequently dorsalized by incubation in Liel to inhibit ventral gene
expression in order to visualize an induced marker gene, Xvent-l. Figure 2B shows that Xvent-l is induced by Bmp-4 and that the expression
extends approximately five to ten cell diameters beyond the injected
cells containing light-labeled nuclei (lacZ), indicating that the induction
of Xvent-l by Bmp-4 is non-cell-autonomous and long-range.
However, the possibility remains that Bmp-4 acts by inducing some
other factor that is ultimately responsible for long-range signalling. To
address this possibility, we have made use of XsmadJ, an intracellular
downstream target of BMP-4 signalling, that mediates its effects, presumably by directly interacting with Bmp-4 target genes. Like Bmp-4,
XsmadJ induces ventral mesoderm and ventralizes dorsal mesoderm. It
rescues the effects of dominant-negative BMP receptor on mesoderm as
well as ectoderm (Sekelsky et al. 1995; Graff et al. 1996; Hoodless et al.
1996; Liu et al. 1996; Meersman et al. 1997; Niehrs 1996). If Bmp-4 acts
indirectly by inducing another signalling factor, then XsmadJ, faithfully
reproducing all effects of Bmp-4, should also function at long range,
despite being a transcription factor. If, however, Bmp-4 acts directly on
mesodermal cells, only expression of the cytokine, but not that of its
intracellular transducer, should yield long-range effects. To distinguish
between these possibilities XsmadJ mRNA was coinjected with ~galactosidase as lineage tracer and embryos were dorsalized by incubation in LiCl. Figure 2B shows that Xvent-J is induced by XsmadJ.
Unlike in the case of Bmp-4 injection, expression of Xvent- J is restricted
to the ~-galactosidase positive cells, indicating that the induction of
Xvent-l by XsmadJ is cell-autonomous. We cannot exclude the possibility, however, that BMP-4 signalling involves a relay that is independent
of Xsmadl function, e.g., by some novel receptor. Given that BMP-4 is
C. Niehrs et al.
chant et al. 1998), as would be expected if the ratio between BMP-4 and
BMP antagonists determines cell fates.
While dose-dependent patterning by BMP-4 would be expected if the
protein acts as a morphogen, the possibility remains that BMP-4 only
induces different doses of a yet unknown morphogen. This raises the
question of whether BMP-4 protein is able to signal directly over a long
range as dpp is in Drosophila (Lecuit et al. 1996; Nellen et al. 1996). To
address this question, Bmp-4 mRNA was microinjected into blastomeres of 32-cell stage embryos together with ~-galactosidase containing a nuclear localization signal as lineage tracer. Embryos were subsequently dorsalized by incubation in Liel to inhibit ventral gene
expression in order to visualize an induced marker gene, Xvent-l. Figure 2B shows that Xvent-l is induced by Bmp-4 and that the expression
extends approximately five to ten cell diameters beyond the injected
cells containing light-labeled nuclei (lacZ), indicating that the induction
of Xvent-l by Bmp-4 is non-cell-autonomous and long-range.
However, the possibility remains that Bmp-4 acts by inducing some
other factor that is ultimately responsible for long-range signalling. To
address this possibility, we have made use of XsmadJ, an intracellular
downstream target of BMP-4 signalling, that mediates its effects, presumably by directly interacting with Bmp-4 target genes. Like Bmp-4,
XsmadJ induces ventral mesoderm and ventralizes dorsal mesoderm. It
rescues the effects of dominant-negative BMP receptor on mesoderm as
well as ectoderm (Sekelsky et al. 1995; Graff et al. 1996; Hoodless et al.
1996; Liu et al. 1996; Meersman et al. 1997; Niehrs 1996). If Bmp-4 acts
indirectly by inducing another signalling factor, then XsmadJ, faithfully
reproducing all effects of Bmp-4, should also function at long range,
despite being a transcription factor. If, however, Bmp-4 acts directly on
mesodermal cells, only expression of the cytokine, but not that of its
intracellular transducer, should yield long-range effects. To distinguish
between these possibilities XsmadJ mRNA was coinjected with ~galactosidase as lineage tracer and embryos were dorsalized by incubation in LiCl. Figure 2B shows that Xvent-J is induced by XsmadJ.
Unlike in the case of Bmp-4 injection, expression of Xvent- J is restricted
to the ~-galactosidase positive cells, indicating that the induction of
Xvent-l by XsmadJ is cell-autonomous. We cannot exclude the possibility, however, that BMP-4 signalling involves a relay that is independent
of Xsmadl function, e.g., by some novel receptor. Given that BMP-4 is