The emergence of nerve cells is one of the key novelties in animal evolution1. How
cells of an embryo are committed to a neuronal fate is therefore a central question
in biology. A related issue is the formation of a central nervous system (CNS), which
is coordinating the action of sensory cells and neurons in higher metazoan animals.
The general view holds that the bilaterian CNS can be traced back through evolution
to a nerve net in a cnidarian-like ancestor2. Cnidarians are simple diploblastic animals
with a gastrula-like body plan, and they constitute the sister group of all bilaterians3
4
5
6
7
8
9. The cnidarian nervous system is organized as a nerve net with an increased density
of neurons at the oral and aboral end of the body axis5
6
8. It also exhibits a remarkable sophistication of sensory cells and organs such as
rhopalia8. Although the picture of the cnidarian nervous system is not complete, their
neurons are known to be rich in neuropeptides, and comparative genomics in distant
cnidarian species has revealed an almost complete set of homologous neurogenic transcription
factors (TFs) and signalling molecules patterning the nervous system in bilaterians5
8. Cnidarian neurogenic TFs and neuropeptide-positive neurons exhibit a clear position
dependency along the oral–aboral body axis5
6
8
9
10
11
12
13
14. However, the genetic mechanisms controlling the formation of the cnidarian nervous
system are largely unknown.
In bilaterians, several secreted factors have been demonstrated to be crucial for
the early segregation of the embryonic ectoderm into a neurogenic and a non-neurogenic
tissue during neural induction. Bmp signalling is the critical factor for the early
development of the CNS and for its anterior–posterior (AP) and dorsal–ventral (DV)
patterning. Bmp signalling is suppressed by antagonists such as Chordin and Noggin
on the anterior–dorsal side in chordates15 and on the ventral side in arthropods,
flatworms and other gastroneuralians16
17
18. On the basis of experiments in Xenopus and Drosophila, which showed that an inhibition
of Bmp signalling is sufficient to induce the neuroectoderm, it was proposed that
the primary function of Bmp signalling in bilaterians is the inhibition of the neural
fate, and accordingly, the default state of the ectoderm would be the neural fate15
16
19
20. This hypothesis was questioned for vertebrates because mutants lacking Bmp antagonists
still develop neurons21. Also in other deuterostomes and in annelids, perturbation
of Bmp signalling did not change the expression of neural makers22
23
24. Therefore, Bmp/Chordin signalling is not necessarily linked to neural induction
in bilaterians21.
Wnt/β-catenin signalling has multiple and even opposite functions in neural development
of deuterostomes, depending on the developmental stage21. Canonical Wnt signalling
is required for neurogenesis at the dorsal side of early chordate embryos and for
the AP and DV patterning of the CNS21. At later stages of chordate development, the
pathway has a function in the posteriorization of the neural ectoderm26, which is
similar to hemichordates and echinoderms27
28. The neurogenic function of Wnt signalling for the transition of proliferating
neuronal precursor cells to differentiating neurons in deuterostomes25 has been recently
described in lophotrochozoans29. Thus, a neurogenic function of Wnt/β-catenin signalling
might be an ancestral bilaterian trait29. To understand which signalling pathway(s)
controls neurogenesis in cnidarians, we focused our analysis on the signalling pathways
and TFs that control the formation of the oral nervous system in N. vectensis. Many
cnidarian polyps develop a conspicuous ring-like nerve plexus around their mouth8
10
30. This cnidarian nerve ring was repeatedly depicted as a beginning of the bilaterian
CNS10
31
32.
The first sign for neurogenesis in N. vectensis is the salt-and-pepper-like expression
of neural marker messenger RNAs (mRNAs) such as NvAchaete-scute homolog A (NvAshA),
NvElav1 and Rfamide in blastula epithelium6
13
14. Recent studies provided evidence for their involvement in neural development of
N. vectensis embryos. A whole-genome microarray analysis on NvAshA-overexpressing
embryos has identified NvElav1, NvRfamide and genes involved neurotransmission as
NvAshA targets13. It was demonstrated that NvElav1 is required at least ectodermal
development of neuropeptides-expressing neurons16. The pervasive expression of NvAshA
suggests that the early embryonic epithelium of N. vectensis has the potential to
generate various neuronal cell types that form the larval nerve net13. In addition
to this pervasive expression of neural genes, the presumptive blastopore region of
the blastula expresses NvAshB, which is also involved in NvRfamide neuropeptide gene
expression13. This suggests that the blastoporal side of the early embryos has distinct
neurogenic feature(s). The blastopore of the gastrula in fact develops into a prominent
neurogenic domain at the planula stage, which is characterized by some neural markers,
NvAshC, NvSoxC, NvMusashi and NvRfamide that are expressed dominantly or exclusively
around the blastopore6
12
13
14
33. This oral neurogenic domain of the embryo develops into an elaborate nerve plexus
at the oral side of the planula larva and primary polyp that comprises a number of
subsystems with separate physiological properties8.
Our study revealed that β-catenin signalling is essential for early neurogenesis during
the development of the oral nervous system that starts at the blastula/gastrula transition.
Wnt/β-catenin signalling is known to be active at the blastoporal side and defines
the primary oral–aboral axis in N. vectensis blastulae34
35. By comparison, Bmp2/4 is also expressed at the blastoporal side, however, with
a strong bias towards a secondary ‘directive’ body axis at later development36
37
38. We also show that in a subsequent developmental phase Bmp signalling has crucial
influences on the regionalized development of the nervous system along both the primary
and secondary (directive) body axes. Our data indicate that the sequential action
of β-catenin and Bmp signalling in the cnidarian N. vectensis reflects the evolutionary
emergence of these major signalling axes in the evolution of the nervous system.
Results
Early development of neuropeptide-positive neurons
The cnidarian nervous system is rich in neuropeptides8
11
39. Among these, the short amidated neuropeptides RFamide and GLWamide, belonging
to R[F/Y]amide and [G/V/L]Wamide groups, respectively, are known to have deep evolutionary
roots in the common Cnidaria/Bilateria ancestor40. These neuropeptides serve as specific
markers for mature neurons in cnidarians8, and in bilaterians they are expressed in
neuronal subpopulations of the CNS41
42.
We studied various developmental stages of N. vectensis that belongs to anthozoans,
the most basal class in cnidarians (Fig. 1a)7
43
44, and is known to show ‘bilateral organization of the endoderm45. Analyses using
antibodies specific for the mature form of RFamide and GLWamide neuropeptides demonstrated
that functional and peptidergic neurons are already present in early planulae (Fig.
1b,c)6
14. It has also recently been shown that NvElav1 is expressed in a substantial part
of neurons during embryogenesis6
14. An NvElav1+ neuron-specific transgenic reporter line, where the mOrange fluorescent
protein is expressed under the NvElav1 regulatory elements, demonstrated the development
of the NvElav1::mOrange+ neurons at gastrula and planula stages14. A quantitative
analysis revealed that the RFamidergic (RFa+) and GLWamidergic (GLWa+) neuronal subpopulations
correspond to 10% of all neurons at the late planula stage (Fig. 1b). The RFa+ neurons
develop in the entire ectoderm and form an elaborated nerve plexus at the oral side,
but they did not form an aboral sensory cluster (Fig. 1c)6
14, which is often observed in planula larvae of hydrozoans46. In late planulae and
after metamorphosis into a primary polyp, a domain rich in RFa+ perikaria was formed
in the blastoporal and the hypostomal/tentacle region (Fig. 1c; Supplementary Fig.
1)6. GLWa+ neurons differentiate in the lateral ectoderm and in the oral endoderm
of early planulae (Fig. 1d; Supplementary Fig. 2). The endodermal GLWa+ neurons formed
a neuronal cluster in an asymmetric manner on one side at the oral region of the planula
larvae (Fig. 1c,d), whereas the ectodermal neurons are distributed symmetrically and
mainly in the midst body region (Supplementary Fig. 2). The asymmetry of the endodermal
cluster of GLWa+ neurons completely vanished in primary polyps (Supplementary Fig.
1). The spatial arrangement of neurons at the oral region is not unique to the peptidergic
neurons because Elav+ neurons form a ring-like sensory cell cluster around the blastopore
at late planula stage (Fig. 1d). The formation of RFa+ and GLWa+ neurons at the oral
side of early planula larvae suggests that those precursors were generated directly
at the blastoporal region in early developmental stages, for example, gastrula. We
therefore analysed the TFs expressed at the blastopore region and signalling pathways
establishing the oral nervous system.
Identification and expression of oral neurogenic TFs
We screened the EST and genomic database of N. vectensis for TFs that have conserved
neurogenic functions among bilaterians. We determined the temporal and spatial expression
patterns of the TFs during N. vectensis embryogenesis and analysed their neurogenic
activity. Our reverse transcriptase (RT)-PCR and whole-mount in situ hybridization
(WISH) data demonstrate that several neurogenic genes were strongly upregulated during
early embryonic development (Fig. 2a,b; Supplementary Fig. 3). We found that NvAtonal-related
protein 3 (NvArp3) formed a synexpression group at the prospective and established
blastopore site together with the previously described NvSoxB2a and NvAshB
12
13 during the blastula and gastrula stage (Fig. 2a,b). NvSoxB2a was considered to
be an unclassified NvSox1 (ref. 12), but has recently been identified as the orthologous
gene to the bilaterian SoxB2 genes and was reclassified as NvSoxB2 (ref. 47). Since
the C-terminal region of NvSoxB2a is similar to bilaterian SoxB2 (Supplementary Fig.
4), NvSoxB2a might have a conserved neurogenic function47. At the planula larva stage,
NvSoxB2a and NvAshB were restricted to the oral side of the pharynx, while NvArp3
was expressed in cells distributed broadly at the endodermal layer. The concurrent
expression of NvSoxB2a with bHLH genes in N. vectensis is in accord with results from
bilaterians, where SoxB2 genes are expressed in neural precursors together with bHLH
genes48. We identified additional bHLH TFs showing also distinct blastoporal expression
patterns at the oral region, that is, NvArp4 in endoderm, NvArp7 in pharynx and NvAshC
in ectoderm13. Furthermore, we were able to find NvArp6 as the first neurogenic TF
being asymmetrically expressed along the secondary (directive) axis (Fig. 2a,b).
We next analysed the genetic relationships between these early oral neurogenic genes
(that is, NvSoxB2a, NvAshB and NvArp3) and other early neurogenic and neural marker
genes (NvAshA, NvSoxB2c (former SoxB2), NvElav1 and NvRfamide) as well as the late
oral neurogenic genes (NvAshC, D, NvArp2, 4, 6, 7, NvSoxB2d, C, NvMusashi, NvRfamide
and NvGlwamide) using morpholino (MO) antisense oligonucleotides against the early
blastoporal neurogenic genes (Supplementary Fig. 5). Figure 2c,d show that in morphants
of NvSoxB2a, NvAshB and NvArp3 an upregulation of all early genes at the blastula
stage, except for NvAshA and NvRfamide, indicating that NvSoxB2a, NvAshB and NvArp3
are not required for the initial steps of most of these genes, but rather form a negative
gene regulatory circuit. At the planula stage, however, a suppression of NvSoxB2a
resulted in the downregulation of NvAshB and NvArp3 (Fig. 2e), indicating that NvSoxB2a
maintains the expression of these early oral bHLH genes. At the planula stage, expression
of the lateral genes (Fig. 2f) and the late oral genes (that is, neurogenic TFs and
neuropeptides) (Fig. 2g) were affected in these morphants. This clearly demonstrated
the crucial function of early oral neurogenic genes for the development of the oral
nervous system. The MO experiments indicated also that these early oral neurogenic
TFs are involved at least partially in the transcriptional control of lateral neural
markers from the blastula to planula stage (Fig. 2d,f). The function of the early
oral genes is specific to neurogenesis because expression of NvTwist, a marker for
endoderm was not suppressed in these morphants (Fig. 2f).
We further analysed the neurogenic activity of the early oral TFs by monitoring the
mature RFa+ and GLWa+ neurons. For all morphants, we confirmed that these genes are
required for ectodermal and endodermal development of RFa+ and GLWa+ neurons (Fig.
3a,b; Supplementary Figs 6 and 7). Interestingly, development of NvElav1::mOrnage+
neurons were inhibited only in NvArp3 MO-injected planula larvae (Fig. 3c,d), which
is consistent with the specific involvement of NvArp3 on NvElav1 expression at the
planula stage (Fig. 2f). These results clearly indicate that NvSoxB2a, NvAshB and
NvArp3 have neurogenic function and that the presumptive blastopore region of the
blastula represents an early neurogenic field, which gives rise to neuronal populations
of the oral (RFa+/GLWa+) and endodermal (NvElav1
+) nervous systems. As NvSoxB2a and NvAshB are required for the expression of many
early and late oral neuronal genes, they might mainly be involved in the commitment
of embryonic cells that gives rise to neural precursor cells, whereas NvArp3 might
have a selective function to specify certain neuronal cell types such as Elav1+ neurons.
β-Catenin signalling in early neurogenesis
To identify signals required for expression of the early oral neurogenic genes, we
searched for factors turned on in the blastula at the site of presumptive blastopore
region. RT-PCR and WISH analyses indicated that NvBrachyury (NvBra), a target of canonical
Wnt/β-catenin signalling in cnidarians49, is strongly upregulated at the blastula
stage (Supplementary Fig. 8). Although β-catenin protein seems to be expressed in
all blastomeres at the early-cleavage stages, it has been described to become stabilized
and translocated to the nuclei at the embryonic region where gastrulation will occur34
and NvTcf expression is maintained (Supplementary Fig. 8)50. Wnt/β-catenin activity
is maintained in later developmental stages (planula and polyp) at the blastopore/mouth35
50 and was postulated to pattern gene expression (including NvRfamide) along the oral–aboral
(primary) body axis51
52. The β-catenin function on the early phase of neurogenesis, however, has not yet
been clarified. The concurrent temporal and spatial expression pattern of early neurogenic
genes with downstream target genes of the β-catenin pathway at the blastula stage
prompted us to investigate the involvement of β-catenin signalling in the early development
of the oral nervous system.
To examine the function of the β-catenin pathway in the neural induction, we stimulated
β-catenin signalling by injecting mRNA for a dominant-negative GSK3-β (DN-GSK3β) into
fertilized eggs. The augmentation of β-catenin signalling induced increased expression
of the neurogenic genes NvSoxB2a, NvAshB and NvArp3, as well as a β-catenin target
gene NvBra at the blastula stage (Fig. 4a). A more prominent and ectopic upregulation
of neurogenic gene expression could be observed when early embryos were treated with
the GSK3-β inhibitors Alsterpaullone (ALP), BIO or 1-Azakenpaullone (1-AZA) (Fig.
4a,b; Supplementary Figs 8b and 9). To analyse β-catenin regulation of the neurogenic
gene expression at early embryonic stages, we treated embryos only for a short time
(that is, from the egg to the blastula stage). The temporal activation of β-catenin
signalling with GSK3β inhibitors was also needed to circumvent potential toxic effects
caused by a long-term treatment with these inhibitors51, so that the mortality rate
of planulae remained unchanged (Supplementary Fig. 10). Under these conditions, we
observed an ‘oralized’ phenotype with an ectopic expression of early neurogenic TFs
(Fig. 4; Supplementary Fig. 8b). In blastula embryos, the injection of NvTcf MO or
treatment of Tcf/β-catenin inhibitor iCRT14 (ref. 53) suppressed the expression of
NvSoxB2a, NvAshB and NvArp3 (Fig. 4b,c). These data therefore indicate that the β-catenin
pathway positively regulates expression of the oral neurogenic TFs at the blastula
stage.
Next, we analysed the β-catenin regulation of early genes, NvAshA, NvSoxB2c, NvElav1
and NvRfamide that show lateral expression patterns at blastula. Most of these genes
except for NvElav1 were not upregulated under augmented β-catenin signalling (Fig.
4d), whereas NvAshA, NvSoxB2c and NvElav1 were strongly suppressed by the β-catenin
inhibitor (Fig. 4e). These data indicate that β-catenin signalling is globally required
at an early developmental stage for the expression of oral and lateral neural genes
and neurogenic TFs.
The wide spectrum of β-catenin dependency of neurogenic TFs suggests that rather for
specifying certain neuronal cell types, β-catenin signalling is required for the early
fate decision of orally and laterally localized neural progenitors giving rise to
multiple neuronal cell types in the planula nervous system. Consistent with this idea,
the activation of β-catenin signalling at early embryogenesis (egg-blastula) also
induced strong upregulation of other neural genes that show a later oral expression
at the gastrula stage (Fig. 5a). All neurogenic and neural marker genes analysed were
suppressed at planula stage after the iCRT14 inhibitor treatment (Fig. 5b–d). The
early stage-specific activation of the β-catenin pathway resulted in an ectopic development
and increase in the number of both, RFa+ and GLWa+ neurons at the planula stage (Fig.
5e,f,i,j; Supplementary Fig. 11). The iCRT14 treatment confirmed the β-catenin requirement
for the development of RFa+, GLWa+ and Elav::mOrange+ neurons (Fig. 5g–i,k,l). Thus,
global and localized actions of β-catenin signalling are essential for the neural
commitment of early embryonic cells and for the formation of a region with a high
neurogenic potential that then develops into the oral nervous system.
Bmp signalling in late neural patterning at the oral side
We next analysed the effect of Bmp signalling on the development of the oral nervous
system in N. vectensis. The asymmetric expression of NvBmp2/4, NvBmp5–8 and their
antagonist NvChordin became visible around the blastoporal lip only after the gastrula
stage (Fig. 6a)37
38
54. Although an early expression of NvBmp2/4 and NvChordin was reported at the blastula
stage by RT-PCR and microarray analyses37
55, no regionalized expression was detectable at this stage. The oral expression of
NvBmp2/4, NvBmp5–8 and NvChordin was depending on the β-catenin signalling, as the
expression was suppressed by iCRT14 (Supplementary Fig. 12). An augmentation of β-catenin
signalling by 1-AZA induced upregulation of NvBmp2/4 and NvBmp5–8 expression, whereas
NvChordin expression was inhibited, suggesting that the dual role of β-catenin signalling
on the NvChordin expression. When we analysed the level of activated Bmp signalling
by determining phosphorylation levels of the NvSmad1 C terminus, we found a strong
signal at the gastrula stage (Fig. 6b), suggesting that the local expression of NvBmp2/4
and NvBmp5–8 in the gastrula is required for an activation of Bmp signalling. To test
whether Bmp signalling functions in the delimited expression of early neurogenic TFs
at the prospective blastoporal side of the blastula, we exposed early embryos to increasing
concentrations of exogenous human Bmp2 protein (hBmp2) to globally elevate the Bmp
pathway. hBmp2 treatment induced the phosphorylation of NvSmad1 in a dose-dependent
manner (Fig. 6c). The expression of NvChordin was strongly suppressed by the augmented
Bmp signalling (Fig. 6d)—as has been demonstrated in bilaterians and N. vectensis
36—whereas the polarized expression of early oral neurogenic TFs at the future oral
side of the blastula and gastrula was not altered (Fig. 6d). Consistently, development
of the RFa+ and GLWa+ neurons was not affected by the early hBmp2 treatment (Supplementary
Fig. 13). The hBmp2 suppression of NvChordin in the early embryos was confirmed by
quantitative PCR (qPCR), whereas no significant effect was observed on the expression
level of the early oral TFs (Fig. 6e). Elimination of Bmp signalling by co-injection
of MOs for NvBmp2/4 and NvBmp5–8 (ref. 36) confirmed that the expression of the early
neurogenic TFs is independent of Bmp signalling at the blastula stage (Fig. 6f).
Finally, we extended the hBmp2 treatment up to the planula stage to test the effect
of Bmp signalling on TF genes that are also expressed in late neurogenesis. Surprisingly,
under these conditions, the expression of early oral NvArp3 and most of the late oral
neurogenic TFs as well as NvGlwamide were largely reduced by exogenous hBmp2 in a
concentration-dependent manner (Figs 6e and 7a,b). When we further analysed the expression
of the same set of genes in Bmp morphants, we also observed a significant suppression
of these early and late oral neurogenic genes and of NvRfamide and NvGlwamide (Figs
6f and 7c,d), which resulted in a decreased number of RFa+ and GLWa+ neurons at planula
stage (Fig. 7e; Supplementary Fig. 14). These data indicate that Bmp signalling is
positively and negatively involved in the development of the oral nervous system.
This dual function of Bmp signalling is reminiscent to the Bmp-dependent transcriptional
regulation of other oral genes36.
NvArp6 induces the asymmetric pattern of NvGLWa+ neurons
Among the TFs with a blastoporal expression pattern, NvArp6 was exceptional in that
it exhibits a distinct asymmetric pattern along both body axes, that is, the oral–aboral
and the directive axis (Fig. 8a). Double WISH analyses indicated that NvArp6 is expressed
at the same side of the directive axis as NvChordin (Fig. 8b), and NvBmp2/4 and NvBmp5–8
(ref. 37). Different from NvChordin, which was restricted to the oral ectoderm, the
gradient of NvBmp2/4, NvBmp5–8 and NvArp6 expression along the primary axis was reaching
the aboral side of endoderm. It was proposed that the overlapping expression of the
NvBmp ligands and its antagonist NvChordin on the same side of the embryo creates
a complex gradient of Bmp activity along the primary and directive axes36. Accordingly,
the overlapping expression domains of NvBmp2/4 and NvArp6 indicate a Bmp-mediated
suppression of NvArp6 transcription. To test this hypothesis, we either activated
or suppressed Bmp signalling and determined NvArp6 expression levels at the planula
stage. NvArp6 expression was significantly suppressed by hBmp2 treatment and activated
in NvBmp2/4/5–8 double morphants (Fig. 8a,c–e). Note that the NvSoxB2a was left unchanged
in the Bmp-depleted planula larvae (Fig. 8d). These data clearly indicate that Bmp
signalling has a suppressive function in inducing NvArp6 expression along the primary
and directive axes.
We next analysed whether the asymmetric development of the GLWa+ neuronal subset along
the directive axis was NvArp6 dependent. In NvArp6 morphants, we found an almost complete
inhibition of GLWa+ neurons (Fig. 8f), whereas RFa+ neurons were not affected. A quantitative
analysis using NvArp6 morphants confirmed this principal finding (Fig. 8g,h). These
data clearly show that NvArp6 activity is specifically required for differentiation
of GLWa+ neurons. Our data also indicate that the localized induction of NvArp6 by
Bmp suppression demarcates a specific neurogenic region in the oral nervous system
along directive axis to give rise to the GLWa+ neuronal subdomain.
Discussion
Here, we analysed neural development in embryos of the cnidarian sea anemone N. vectensis.
We focused our study on the genetic mechanisms neuralizing embryonic cells and regulating
the development of the oral nervous system, which forms a conspicuous nerve plexus
around the mouth of many polyps8
10
30 and was repeatedly depicted as the precursor of the bilaterian CNS10
31
32. We used neuronal markers that are abundantly or exclusively expressed during the
early development of the oral nervous system at the side of the blastopore of the
embryo and planula larva8
12
13
14
33. By deciphering the consecutive actions of β-catenin and Bmp signalling in the
development of the oral nervous system in N. vectensis embryos, our findings replicate
the evolutionary emergence of these major signalling axes in animal evolution.
β-Catenin signalling is essential for gastrulation and oral development in N. vectensis
34
35. In this study, we demonstrate that β-catenin is the early inducer of the oral
nervous system of N. vectensis. β-Catenin signalling is highly activated at the future
oral side at the blastula stage and even before (Supplementary Fig. 8)34, which causes
an upregulation of early and late neurogenic and neural marker genes at the oral side,
while inhibition of β-catenin showed opposite effects. Thus, β-catenin signalling
is essential for early cell fate decisions of progenitor cells giving rise to multiple
neural cell types. Since the target genes of β-catenin signalling, for example, NvBra
and NvAshB, start being upregulated before canonical Wnt ligands NvWnt3 and NvWnt4
become detectable55, the higher activity of β-catenin signalling at the future oral
side of early embryos might include NvDishevelled-mediated stabilization of β-catenin
protein at the oral region56.
Another intriguing finding revealed in this study is the β-catenin dependency of expression
of the early lateral neural genes NvAshA, NvSoxB2c and NvElav1 at the blastula stage.
Although it is yet unknown how the locally activated β-catenin signalling contributes
to the early expression of these genes at the blastula lateral epithelium (for example,
we cannot rule out that the effects of β-catenin in neuronal cell formation are indirect),
our findings may suggest that β-catenin protein that is expressed uniformly in early
blastomeres has a role to predispose the blastula epithelium to become neurogenic,
as it has been suggested by previous studies on vertebrates57. Our data support observations
made in distinct bilaterians showing a function of β-catenin signalling in early neurodevelopment29
57
58
59
60.
When we tested the function of Bmp signalling on the early induction of oral neuronal
genes, we could not detect any inhibitory Bmp effect, which was unexpected since Bmp/Chordin
signalling is frequently considered as the primary and general CNS inducer15
20. Our findings are, however, in accord with data on early neural development in
several slow-evolving bilaterians. In annelids22 and in hemichordates24, Bmp treatments
did not inhibit early neurogenic gene expression. We therefore conclude that the ancestral
mode of neural induction is probably not the Bmp-mediated event, which was described
for Xenopus and Drosophila
21. Instead, the β-catenin signalling dependency of neural development at the blastoporal
site appears to represent the evolutionary conserved and ancestral mode of neural
development with a deep root in the common ancestor of cnidarians and bilaterians.
Treatment of gastrulae and planula larvae with inhibitors of GSK3 (for example, ALP
or 1-AZA) triggered β-catenin activation and the expansion of oral markers including
RFa+ neurons6, or even the formation of multiple polyp heads52. This clearly suggests
an additional function of Wnt/β-catenin signalling for maintaining and/or patterning
the cnidarian nervous system. Since the oral end of the cnidarian body plan is considered
to represent the posterior end of the bilaterian AP body axis61
62
63, Wnt/β-catenin signalling might also be involved in the oral–aboral patterning
of the pervasive cnidarian nerve net. Bmp signalling was also postulated, however,
to have a role in maintaining RFa+ neurons at the planula stage36. How can these observations
fit with our principal finding that Bmp signalling is indispensable for development
of the oral nervous system in N. vectensis? When we analysed Bmp signalling at the
planula stage, we discovered that RFa+ and GLWa+ neurons as well as early and late
oral neurogenic TFs (NvArp2, -3, -4, -5, -7, NvAshB, -C, -D) exhibited indeed a distinct
Bmp sensitivity. Only for NvAshB this Bmp dependency was weak, and it was absent for
NvSoxB2a. Therefore, Bmp signalling is an important factor in the development of the
oral nervous system. However, this Bmp regulation is complex. When Bmp signalling
was either suppressed or over-activated at gastrula and planula stages, neurogenic
TFs and GLWamide neuropeptide became downregulated under both conditions. This is
reminiscent to the blastoporal regulation of NvSoxB1 and NvNk2.1 that are likewise
downregulated in NvChordin and NvBmp2/4 morphants36. Thus, a balanced level of Bmp
signalling seems to be crucial for the development of the oral nervous system. Because
the expression of NvBmp2/4, NvBmp5–8 and NvChordin genes is dependent on β-catenin
signalling, the Wnt/β-catenin regulation of neural patterning might be mediated, at
least in part, by the Bmp activity. This point is of interest for future studies.
A striking function of Bmp signalling at the planula stage is the asymmetric induction
of neurons along the (secondary) directive axis. NvArp6-expressing cells and GLWa+
neurons form a distinct asymmetric pattern along the directive axis, which is Bmp
dependent. This asymmetric suppression of Bmp is reminiscent to an evolutionarily
conserved Bmp function in the mediolateral patterning of the trunk nervous system
in bilaterians22. It should be pointed out that Bmp inhibition (which activates NvArp6
but suppresses other oral neuronal TFs) did not result in more and symmetric development
of GLWa+ neurons. Therefore, GLWa+ neurons depend on additional neurogenic TFs, which
are similarly restricted to the oral site.
Our data provide a new view on the evolution of the nervous system, and we propose
that the cnidarian oral nervous system exhibits some important features that are essential
for the evolution of the bilaterian CNS (Fig. 9). (i) The oral nervous system is demarcated
from other parts of the diffused nervous system and exhibits a distinct condensation
of neuronal perikaria with neuronal processes extending to the periphery (tentacles
and aboral side). The condensations of neuronal perikaria in the oral nervous system
is an anatomical feature related to the formation of a CNS in bilaterians with distinct
and functionally specialized neuronal domains (ganglia)64. (ii) Development of the
cnidarian oral nervous system is dependent on early β-catenin- and late Bmp signalling
activities. The function of these signalling pathways in the oral nervous system development
is highly reminiscent to their function in the specification of neural cell types
in the CNS along bilaterian body axes. (iii) There is an evidence for a common set
of orthologous neuronal genes expressed in the cnidarian oral nervous system and the
bilaterian CNS. The neuropeptides belonging to R[F/Y]amide and [G/V/L]Wamide groups
have been shown to form clusters in bilaterian CNS41
42. In addition to neurogenic TFs used in this study, some of marker genes for bilaterians
CNS (that is, SoxB1 and Nk2.1) are exclusively expressed in the oral nervous system
in N. vectensis
12
36. These findings suggest that the genetic signature of the cnidarian oral nervous
system is shared, at least in part, by that of the bilaterian CNS.
Most recently the genome of comb jellies (ctenophores), another pre-bilaterian clade
with a primitive nervous system, was published65
66. Many neuronal genes are missing in ctenophores, for example, those for the synthesis
and transmission of the classical neurotransmitters GABA and acetylcholine. However,
several of the ‘neurogenic’ TFs (for example, SoxB and bHLH) and RNA-binding proteins
(Elav and Musashi) are present in ctenophore genomes65
66. In the ctenophore, SoxB and RFamide are strongly expressed at neuron-enriched
body regions, that is, the aboral sensory organ and the polar fields67
68. In addition, members of Bmp and β-catenin signalling are also highly expressed
at this region during development69
70. The fact that these markers are expressed during the neural development in ctenophores
is questioning, whether the nervous system of ctenophores had really a different evolutionary
origin from the cnidarian-bilaterian clade as recently proposed66.
To understand the emergence of the bilaterian CNS from simple neural clusters, deeper
insights into the anatomical and molecular features of the cnidarian nervous systems
are very helpful. Although this knowledge is still far from being complete, our data
on the development of the oral nervous system in N. vectensis give an insight how
a first step in the evolution of nervous system centralization may have been accomplished.
The cnidarian nervous system is also an intriguing example, how basic positional information,
which is organized in bilaterians in a Cartesian coordinate system26, can be translated
into the development of the nervous system.
Methods
Culture of N. vectensis
N. vectensis were cultured at a salinity one-third of normal seawater (that is, brackish
water, 108.32 g l−1 Tropic Marine sea salt, pH 7.6). Adult males and females were
separately cultured at 18 °C in the dark and fed two to three times per week. On 2
days after the feeding, the culture boxes were washed. For spawning the release of
eggs and sperms, the culture boxes were incubated at 26 °C under the light for at
least 14 h. After fertilization, embryos were cultured at 20 °C.
Gene isolation from N. vectensis and plasmid construction
Gene-specific primers were then designed to recover both 3′ and 5′ rapid amplification
of complementary DNA (cDNA) ends (RACE) fragments using the GeneRacer kit (Invitrogen)
with annealing temperatures between 55 and 65 °C for 1st and nested PCR. Gene-specific
primer sequences are listed in Supplementary Table 1. RACE products were cloned using
pGEM-T (Promega) and sequenced. Overlapping 5′and 3′ RACE fragments were aligned to
obtain mRNA sequences. The DN form of GSK3β (K83R) was made by two-step PCR-based
mutagenesis. The PCR product was inserted into the pCS2 plasmid and sequenced. For
the TnT assays, NvSoxB2a, NvAshB, NvArp3 and NvArp6 genes were subcloned with partial
5′ untranslated region (UTR) and the complete open-reading frame into pGEM-T vectors
and sequenced. The insert was cut using EcoRI and inserted into the pCS2 plasmid.
Quantitative and semi-quantitative RT-PCR
For semi-quantitative RT-PCR, the level of amplified product was compared in the exponential
range of amplification. qRT-PCR was performed on the Bio-Rad DNAEngine equipped with
Chromo4 real-time PCR detector (Bio-Rad) using Absolute QPCR SYBR Green mix (Thermo
Scientific). The primer sequences are listed in Supplementary Table 1.
Embryo treatment with Bmp protein or GSK3β inhibitors
Embryos were treated with recombinant hBmp2 (a gift from Dr Wölfl) at concentrations
from 0.1 to 1 μg ml−1. As a control, 1 μg ml−1 bovine serum albumin (BSA) was applied.
hBmp2 protein was added at one-two-cell-stage embryos and cDNAs were prepared at the
blastula (12–14 h.p.f.), gastrula (30 h.p.f.) or planula (74 h.p.f.) stages. For WISH
experiments, hBmp-treated blastulae and gastrulae (12 h.p.f. (blastula) and 24 h.p.f.
(gastrula) were fixed. For experiments using the GSK3β inhibitors to enhance β-catenin
signalling, embryos were treated with ALP, BIO or 1-AZA (Sigma) at the concentrations
indicated. cDNAs were prepared at the blastula and early gastrula stages (12–18 h.p.f.)
for the qPCR analyses. For immunostaining of neuropeptides and mOrange protein, embryos
were treated with GSK3β inhibitors from one- and two-cell stage to blastula stage.
After being washed, the embryos were cultured in Nematostella medium until early (72 h.p.f.)
or late (98 h.p.f.) planula stages. To pharmacologically inhibit β-catenin signalling,
embryos were treated with 10 μM or 50 μM iCRT14 (Sigma), a Tcf/β-catenin inhibitor,
for 14 h (blastula), 3 days (early planula) or 4 days (late planula).
Microinjection of mRNA and MO antisense oligonucleotides
pCS2 constructs, pCS2-MT-β-Eng (DN-β-catenin) (a gift from Dr McCrea), pCS2-NvBmp2/4
and DN-GSK3β, were linearized using appropriate restriction enzymes. Capped mRNAs
were synthesized using the SP6 mMessage mMachine mRNA synthesis kit (Ambion). Capped
mRNAs or MO antisense oligonucleotide for control (5′- CCTCTTACCTCAGTTACAATTTATA ),
neurogenic TFs NvSoxB2a (MO1: 5′- TCTAAATCCCGTAGAAGTTCTAGGT -3′, MO2: 5′- CGTCAAGTTTACTCGTTCCGGCACT
-3′), NvAshB (MO1: 5′- AGGAAGCCTCCATCGGAATCTCCAT -3′, MO2: 5′- TTGTAAGCGAGAAGCACTCACATGC
-3′), NvArp3 (MO1: 5′- TTTCTGTACCTTCGCCTTTTGTCAT -3′, MO2: 5′- CTGAGAGCCGGTGAGTTTTCTTGAT
-3′), NvArp6 (MO1: 5′- TCATTGAGTGAGTGCATCCGGCTTC -3′, MO2: 5′- GCGCACTCTCTGTTTGATCGTAAGT
-3′), NvBmp2/4 (5′- GTAAGAAACAGCGTAAGAGAAGCAT -3′)36, NvBmp5–8 (5′- GTAACAGGTCTCGTATTCTCCGCAT
-3′)36 and NvTcf (5′- CTGAGGCATACTACCGTCTCATGTG -3′)55 were injected at 250 μM concentration
into fertilized eggs with 0.2 μg μl−1 rhodamine- or Alxa488-dextran (Invitrogen).
Embryos were then rinsed with N. vectensis medium and incubated until desired stages.
TnT assay
The translation-blocking MOs were tested using the TnT SP6 quick coupled transcription/translation
system (Promega). pCS2 plasmids (250–500 ng) containing MO-target sites (5′ UTR and
first ATG) were incubated with control or gene-specific MOs (50 μM) at 30 °C for 90 min.
The biotinylated proteins synthesized were detected by western blotting using horseradish
peroxidase-conjugated streptavidin (1:10,000 dilution).
Whole-mount in situ hybridization
In situ hybridization was performed as described previously37 with following modifications:
specimens were fixed with 4% paraformaldehyde/PBST (PBS, 0.1% Tween 20) for 1 h, then
washed with methanol for 3 times and stored at −20 °C. Hybridization of 0.6- to 1.4-kb
digoxygenin-labelled antisense RNA probes were carried out using a hybridization solution
containing 1% SDS at 50–65 °C for at least 22 h. For post-hybridization washes, specimens
were washed by serial dilutions (75, 50 and 25%) of hybridization solution with 2
× SSC at 55 °C. After processing of the digoxygenin-labelled probe with BM purple
(Roche), specimens were washed with PBST.
Western blotting
N. vectensis embryos were solubilized with 2 × SDS–polyacrylamide gel electrophoresis
sample buffer and immediately boiled at 90 °C for 5 min. After being sonicated, proteins
separated by SDS–polyacrylamide gel electrophoresis were transferred to a PVDF (polyvinylidene
difluoride) membrane (Millipore). The membranes were blocked with 5% BSA in buffer
consisting of 50 mM Tris-HCl, pH 8.0, 2 mM CaCl2, 80 mM NaCl and 0.2% Nonidet P-40,
incubated with the blocking solution containing anti-phospho-Smad1/5/8 antibody (Cell
Signaling Technology) (1:200 dilution), anti-Smad1/5/8 (N-18) antibody (Santa Cruz
Biotechnology) (1:300 dilution) or anti-β-tubulin (2-28-33) antibody (Sigma) (1:1,000
dilution) followed by incubation with horseradish peroxidase-conjugated secondary
antibodies (Jackson ImmunoResearch) (1:5,000 dilution) in blocking solution. Immunoreactive
proteins were detected with an ECL immunoblotting detection reagent (Amersham Pharmacia
Biotech). Full images of western blots are shown in Supplementary Fig. 15.
Immunostaining and fluorescent microscopy
The NvElav1::mOrange transgenic embryos were fixed with 4% paraformaldehyde/PBT for
1 h at room temperature and stained using anti-DsRed antibody (1:300 dilution)(Clontech),
as was previously reported14. For neuropeptide staining, embryos and primary polyps
were fixed by Zamboni’s fixative o/n at 4 °C. After permeabilization in PBS+0.1% Triton
X-100, specimens were blocked with blocking solution (10 mg ml−1 BSA+5% normal goat
serum in PBS), incubated with anti-FMRFamide antibody (1:600 dilution) (Sigma) or
anti-GLWamide antibody (1:400 dilution) (a gift from Dr Koizumi) in the blocking solution
containing 0.1% Triton X-100, and then with Alexa488- or Alexa568-conjugated anti-Rabbit
antibody (1:800 dilution) (Invitrogen). Specimens were imaged using a Nikon ECLIPSE
80i fluorescent microscope equipped with DIGITAL SIGH DSU1 (Nikon). For confocal microscopy,
a fluorescent microscope ECLIPSE Ti (Nikon) equipped with an A1R MP confocal scanner
(Nikon) was used.
Author contributions
H.W. performed most of the experiments. A.K. and M.F. performed some WISH analyses.
H.W., K.A., S.Ö., T.F. and T.W.H. designed the project. H.W. and T.W.H. wrote the
manuscript.
Additional information
How to cite this article: Watanabe, H. et al. Sequential actions of β-catenin and
Bmp pattern the oral nerve net in Nematostella vectensis. Nat. Commun. 5:5536 doi:
10.1038/ncomms6536 (2014).
Supplementary Material
Supplementary Information
Supplementary Figures 1-15 and Supplementary Table 1