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Expression of a truncated activin type II receptor, which blocks signaling by activin, neuralizes explants of embryonic cells that would otherwise become epidermal cells. This neuralization is direct and does not require the presence of mesoderm. The induced neural tissue expresses general molecular markers of the central nervous system as well as an array of neural markers along the anteroposterior axis. In the context of the whole embryo, expression of this truncated activin receptor diverts prospective ectoderm and endoderm to a neural fate. We propose that inhibition of the activin type II receptor signaling causes the cells of Xenopus embryos to adopt a neural fate. These results, along with previous experiments performed in Drosophila, suggest that the formation of the nervous system in vertebrates and invertebrates occurs by a common strategy.
Figure 1. Expression of a Truncated Activin Receptor in Animal Cap
Explants Leads to Induction of Neural and Cement Gland Markers
Northern blot analysis of molecular markers in animal caps from embryos
injected with A7XARI. Both N-CAM and 84ubulin isotype II are
expressed in caps previously injected with AlXAR7, but not in caps
injected with the control globin RNA. The neural-specific 8-tubulin isotype
is the middle band (Richter et al., 1988; arrow). XAG 1 is a cement
gland marker. Muscle-specific actin is a marker for axial mesoderm
and is the bottom of three bands (arrow) detected by a probe that also
hybridizes to two cytoskeletal actins transcripts ubiquitously expressed.
Fibronectin is a control for RNA recovery. The amount of
synthetic mRNA injected per embryo is shown at the top. RNA was
extracted from ten animal cap explants for each lane. The lane labeled
Tailbud is RNA from five uninjected tailbud embryos used as control.
Figure 2. The Neural Tissue induced by
d 7XARl Is Patterned
(Top) Neural tissue in the tailbudembryo was
detected by staining with MAb X3, which
stains the CNS. dlXAR7 or globin RNA was
injected into a fertilized embryo at the 2-cell
stage (1 ngkgg), and animal caps were explanted
at the early blastula stage. An animal
cap (right) from a globin RNA-injected egg
shows no staining, whereas expression of
AlXARl (left) induces neural cells in a cluster
that stains with MAb 3C3.
(Bottom) Staining of the sensory neurons by
MAb Tor 25.4. As for MAb 3C3, injection of
AlXAR7 RNA (left), but not globin (right), induces the expression of the sensory neural antigen
recognized by MAb Tor 25.4.
Figure 3. The Neural Tissue Induced by LI lXAR1 Expresses a Range
of Anterior-Posterior Neural Markers
RT-PCR analysis was performed for various markers on animal caps
explanted from fertilizedeggsthatwere injected with 1 ngof AlXARl at
the 2-cell stage. Controls include uninjected and globin RNA-injected
embryos. The lane marked control contains all the ingredients of the
RT-PCR reaction except for reverse transcriptase. The lane marked
embryos is a positive control for the RT-PCR signal and contains RNA
from the tailbud stage. Opsin is an eye marker that derives from the
forebrain. En-2 demarcates the midbrain-hindbrain junction. Tanabin
is a marker of rhombomeres 2, 4, 6, and 8, the trigeminal ganglia,
and a few cells in the eye and spinal cord. Krox-20 marks rhombomeres
3 and 5 of the hindbrain. Xlhbox-6 is a posterior spinal cord marker.
Muscle actin is a marker for somites, and N-CAM is a general neural
marker. The EF-la lane demonstrates that comparable amounts of
total RNA were present in all reactions.
Figure 4. Injection of a Dominant Negative Activin Receptor into the
Animal Pole of the 2-Cell Xenopus Embryo Leads to Exaggerated
Neuralization
(A) Control embryo injected with 2 ng of globin RNA displays normal
phenotype.
(6, C, and D) Embryos injected with 2 ng of AlXfWl display ectopic
neural structures such as an enlarged brain (B, C) and ectopic eyes
(B, C, and D). Arrows point to the eyes.
(E-H) Histological sections of embryos injected with AlXAR7. (E) and
(F) are transverse sections through the head of the control embryo.
(G) and (H) are transverse sections through the head of the embryo
injected with AlXARl shown in (C). (E) and (G) are transverse sections
at the level of the eye. The AIXAM-injected embryos (G) show an
expansion of the brain, ectopic neural tissue (EN), and a ventral positioning
of retinal pigments (RP). (F) and (H) are sections at the level
of theoticvesicle(OV)and heart(H). Theembryosexpressing AlXARl
(H) show ectopic and hypertrophic neural tissue.
Figure 5. Injection of d 7XARl into Vegetal Blastomeres of E-Cell Embryos
(A) Embryos 1 and 2 have been injected with 0.5 ng of B-gal and 0.5
ng of globin (control) RNA in a single vegetal blastomere. Embryos 3
and 4 have been injected with 0.5 ng of P-gal and 0.5 ng of d lXARl
in a single vegetal blastomere. While most of the B-gal-positive cells
in the control embryos populate endodermal derivatives, the cells that
have received the d7XAR7 RNA are excluded from endoderm and
have migrated to the dorsal-anterior region of the embryo. (B) Embryo
@cell stage) coinjected with P-gal and AlXAR7 in a single vegetal
blastomere and double stained with a neural-specific antibody 3C3.
A significant number of the B-gal-labeled cells have contributed to the
nervous system (see left eye), and the neural tissue has expanded
beyond the lateral edge of the neural tube (arrow).
