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In Xenopus development, dorsal mesoderm is thought to play a key role in both induction and patterning of the nervous system. Previously, we identified a secreted factor, noggin, which is expressed in dorsal mesoderm and which can mimic that tissue's neural-inducing activity, without inducing mesoderm. Here the neural tissue induced in ectodermal explants by noggin is further characterized using four neural-specific genes: two putative RNA-binding proteins, nrp-1 and etr-1; the synaptobrevin sybII; and the lipocalin cpl-1. First we determine the expression domain of each gene during embryogenesis. Then we analyze expression of these genes in noggin-treated explants. All markers, including the differentiated marker sybII, are expressed in noggin-induced neural tissue. Furthermore, cpl-1, a marker of dorsal brain, and etr-1, a marker absent in much of the dorsal forebrain, are expressed in non-overlapping territories within these explants. We conclude that the despite the absence of mesoderm, noggin-induced neural tissue shows considerable differentiation and organization, which may represent dorsal-ventral patterning of the forebrain.
Fig. 1. Sequence of the etr-1 protein encoded by cDNA 17-5. The diagram above shows the domains of the protein as described in the text. The
identification of the RRMs is indicated at the left; each RRM is presented so that the conserved RNP1 and RNP2 (boxed sequences) are
aligned. The consensus sequence at the bottom is from Burd and Dreyfuss (1994) and Birney et al. (1993). The nucleotide sequence of the etr-1
cDNA is deposited in GenBank under accession number U16800.
Fig. 2. Alignment of the Xenopus sybII predicted protein, encoded by the 17-30 cDNA, with the rat SYBI and SYBII sequences, with dots
representing identity. The first Q of the sequence corresponds to the start of the cDNA, while the asterisk in the rat sequences represents amino
terminal sequences omitted here. The nucleotide sequence for Xenopus sybII is deposited in GenBank under accession number U16801.
Fig. 3. Whole-mount in situ hybridizations, showing expression patterns of nrp-1 (A-C), sybII (D-F), etr-1 (G-I) and cpl-1 (J-L) at various
stages of Xenopus development. All embryos are oriented with anterior to the left; sections are shown with dorsal pointing up. (A) Dorsal
view of stage 13 (early neurula) embryo, showing nrp-1 expression throughout the neural plate, except for the prospective floorplate.
(B) Stage 18 (late neurula), dorsal view, showing nrp-1 expression in neural folds, around the blastopore, and in lateral stripes (indicated
by arrowhead) surrounding the brain. (C) Side view of nrp-1 expression at stage 26 (tailbud), in the CNS, otic vesicle (arrowhead) and
facial nerve (arrow). (D) Side view of stage 24 embryo, stained for sybII expression in the spinal cord, hindbrain and trigeminal ganglion
but not the anteriorbrain. (E) sybII is expressed throughout the CNS, including eyes, epiphysis (future pineal gland, indicated by open
arrow) and cranial ganglia and nerves (vagus nerve indicated by solid arrow), by stage 37 (swimming tadpole). (F) Transverse section of
sybII staining in the ventral, outer spinal cord of a stage 24 embryo. (G) Dorsal view of etr-1’s punctate staining in the neural folds and
trigeminal placodes (arrowhead) of a stage 18 embryo. (H) Side view of stage 24 embryo, showing strong etr-1 expression in spinal cord,
hindbrain, ventral mid- and forebrain, and epiphysis (open arrow). The line drawn through the head indicates the level of the section
shown in I. (I) Expression of etr-1 in a transverse section through the ventralforebrain of a stage 24 embryo. (J) Dorsal view of cpl-1
expression in the anterior neural ridge of a stage 13 embryo. (K) cpl-1 expression (blue stain) in the dorsal brain at stage 23 (side view);
XAG-1 expression (brown stain, indicated by bracket) marks the cement gland. The line represents the level of the section shown in L.
(L) Transverse section through the ventralforebrain and eyes of a stage 24 embryo, showing cpl-1 expression only at the dorsal midline of
the forebrain. A-E, G, H, J and K are the same magnification; scale bar in A represents 0.5 mm. Scale bar in F, for F, I and L, represents
0.1 mm.
Fig. 4. Expression of nrp-1 (A,B), sybII (C, D), etr-1 (E, G, H) and cpl-1 (F-H) in noggin-treated
animal caps (except control caps in B and activin caps in D). Explants were cultured until initial
tailbud stages (23-26), except for sybII-stained explants (C,D), which were cultured until stage 35.
(A) Strong and uniform nrp-1 expression in noggin-treated explants; uninduced caps (B) show no
expression. (C) At stage 35, sybII is expressed diffusely throughout most noggin-treated explants and
in extensive patches in activin-treated explants (D). (E) etr-1 expression in noggin-treated animal
caps is restricted to regions of the explants. (F) cpl-1 is expressed in noggin-treated animal caps in
distinct patches, which are separate from cement glands (marked by XAG-1 expression, stained
brown and indicated by arrowheads). (G) Double staining, showing non-overlapping expression of
etr-1 (purple) and cpl-1 (turquoise) in noggin-treated animal caps. (H) Double staining of u.v.-
ventralized animal caps treated with noggin, showing the same pattern of etr-1 and cpl-1 expression
as shown in G. Scale bar represents 0.5 mm.
