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Formation of the embryonic organizer is restricted by the competitive influences of Fgf signaling and the SoxB1 transcription factors.

Kuo CL, Lam CM, Hewitt JE, Scotting PJ - PLoS ONE (2013)

Bottom Line: We show that ectopic expression of organizer genes induced solely by the inhibition of SoxB1 function is dependent upon the activation of fgf expression.These data allow us to describe a comprehensive signaling network in which the SoxB1 factors restrict organizer formation by inhibiting Fgf, Nodal and Wnt signaling, as well as independently repressing the targets of that signaling.The organizer therefore forms only where Nodal-induced Fgf signaling overlaps with Wnt signaling and the SoxB1 proteins are absent.

View Article: PubMed Central - PubMed

Affiliation: Centre for Genetics and Genomics, School of Biology, University of Nottingham, QMC, Nottingham, United Kingdom.

ABSTRACT
The organizer is one of the earliest structures to be established during vertebrate development and is crucial to subsequent patterning of the embryo. We have previously shown that the SoxB1 transcription factor, Sox3, plays a central role as a transcriptional repressor of zebrafish organizer gene expression. Recent data suggest that Fgf signaling has a positive influence on organizer formation, but its role remains to be fully elucidated. In order to better understand how Fgf signaling fits into the complex regulatory network that determines when and where the organizer forms, the relationship between the positive effects of Fgf signaling and the repressive effects of the SoxB1 factors must be resolved. This study demonstrates that both fgf3 and fgf8 are required for expression of the organizer genes, gsc and chd, and that SoxB1 factors (Sox3, and the zebrafish specific factors, Sox19a and Sox19b) can repress the expression of both fgf3 and fgf8. However, we also find that these SoxB1 factors inhibit the expression of gsc and chd independently of their repression of fgf expression. We show that ectopic expression of organizer genes induced solely by the inhibition of SoxB1 function is dependent upon the activation of fgf expression. These data allow us to describe a comprehensive signaling network in which the SoxB1 factors restrict organizer formation by inhibiting Fgf, Nodal and Wnt signaling, as well as independently repressing the targets of that signaling. The organizer therefore forms only where Nodal-induced Fgf signaling overlaps with Wnt signaling and the SoxB1 proteins are absent.

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Sox3 represses expression of chd and gsc independently of repressing fgf expression.(A) Model of the signalling network that controls organizer formation. Sox3 plays a central role in this model to repress Fgf signalling in addition to independently repressing other genes needed for organizer formation. Injection of fgf3 RNA at the 1–2 cell stage dramatically expanded both chd and gsc expression in the animal hemisphere (Ba,b,e,f). Injection of wild-type sox3 RNA not only inhibited the endogenous expression of chd and gsc (Bc,g), but was also able to partially inhibit the expansion of chd and gsc expression that was induced by injection of fgf3 (Bd,h). (C,D) Graphical representation of the numbers of embryos affected in these experiments. Lateral view and dorsal is to the right in upper panels, viewed from animal pole in lower panels. The proportion of embryos exhibiting these phenotypes is shown at the bottom right of each panel.
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pone-0057698-g004: Sox3 represses expression of chd and gsc independently of repressing fgf expression.(A) Model of the signalling network that controls organizer formation. Sox3 plays a central role in this model to repress Fgf signalling in addition to independently repressing other genes needed for organizer formation. Injection of fgf3 RNA at the 1–2 cell stage dramatically expanded both chd and gsc expression in the animal hemisphere (Ba,b,e,f). Injection of wild-type sox3 RNA not only inhibited the endogenous expression of chd and gsc (Bc,g), but was also able to partially inhibit the expansion of chd and gsc expression that was induced by injection of fgf3 (Bd,h). (C,D) Graphical representation of the numbers of embryos affected in these experiments. Lateral view and dorsal is to the right in upper panels, viewed from animal pole in lower panels. The proportion of embryos exhibiting these phenotypes is shown at the bottom right of each panel.

Mentions: Based on the above observations, a network of signals leading to organizer formation can be proposed as shown in Fig. 4A. We have shown previously that repression of gsc and chd expression by SoxB1 factors appears to be independent of the repression of sqt and boz which are upstream activators of gsc and chd[9]. However, whether the inhibition of gsc and chd by SoxB1 factors is direct or entirely due to its inhibition of Fgf signaling required testing. In order to do this, we combined overexpression of Sox3 and Fgf3 and analysed the effect upon the expression of gsc and chd. Fgf3 overexpression alone caused a dramatic expansion of both gsc and chd expression throughout the epiblast (Fig. 4Ba,b,e,f,C,D). This is consistent with the data of Maegawa etal. (2006) who showed that Fgf3 or Fgf8 could rescue the loss of chd expression in mutant fish that lacked functional ß-catenin2 [3]. However, when Fgf3 and Sox3 were overexpressed together, we found that, although Fgf3 still induced gsc and chd expression over broad parts of the embryo, in the regions where Sox3 was expressed (as indicated by staining for the overexpressed HA-Sox3 protein, Fig. S6 in the supplementary material), gsc and chd expression was absent (Fig. 4Bc,d,g,h,C,D). Since the direct effects of Sox3 are cell autonomous, this is consistent with Sox3 repressing gsc and chd in the patches where it is expressed, while Fgf3 (which has a broader effect due to its extracellular diffusion) can only expand expression in those regions where cells are not over expressing Sox3. It seems, therefore, that in addition to repressing expression of the fgfs, Sox3 also independently represses the expression of gsc and chd downstream of Fgf signaling.


Formation of the embryonic organizer is restricted by the competitive influences of Fgf signaling and the SoxB1 transcription factors.

Kuo CL, Lam CM, Hewitt JE, Scotting PJ - PLoS ONE (2013)

Sox3 represses expression of chd and gsc independently of repressing fgf expression.(A) Model of the signalling network that controls organizer formation. Sox3 plays a central role in this model to repress Fgf signalling in addition to independently repressing other genes needed for organizer formation. Injection of fgf3 RNA at the 1–2 cell stage dramatically expanded both chd and gsc expression in the animal hemisphere (Ba,b,e,f). Injection of wild-type sox3 RNA not only inhibited the endogenous expression of chd and gsc (Bc,g), but was also able to partially inhibit the expansion of chd and gsc expression that was induced by injection of fgf3 (Bd,h). (C,D) Graphical representation of the numbers of embryos affected in these experiments. Lateral view and dorsal is to the right in upper panels, viewed from animal pole in lower panels. The proportion of embryos exhibiting these phenotypes is shown at the bottom right of each panel.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3585176&req=5

pone-0057698-g004: Sox3 represses expression of chd and gsc independently of repressing fgf expression.(A) Model of the signalling network that controls organizer formation. Sox3 plays a central role in this model to repress Fgf signalling in addition to independently repressing other genes needed for organizer formation. Injection of fgf3 RNA at the 1–2 cell stage dramatically expanded both chd and gsc expression in the animal hemisphere (Ba,b,e,f). Injection of wild-type sox3 RNA not only inhibited the endogenous expression of chd and gsc (Bc,g), but was also able to partially inhibit the expansion of chd and gsc expression that was induced by injection of fgf3 (Bd,h). (C,D) Graphical representation of the numbers of embryos affected in these experiments. Lateral view and dorsal is to the right in upper panels, viewed from animal pole in lower panels. The proportion of embryos exhibiting these phenotypes is shown at the bottom right of each panel.
Mentions: Based on the above observations, a network of signals leading to organizer formation can be proposed as shown in Fig. 4A. We have shown previously that repression of gsc and chd expression by SoxB1 factors appears to be independent of the repression of sqt and boz which are upstream activators of gsc and chd[9]. However, whether the inhibition of gsc and chd by SoxB1 factors is direct or entirely due to its inhibition of Fgf signaling required testing. In order to do this, we combined overexpression of Sox3 and Fgf3 and analysed the effect upon the expression of gsc and chd. Fgf3 overexpression alone caused a dramatic expansion of both gsc and chd expression throughout the epiblast (Fig. 4Ba,b,e,f,C,D). This is consistent with the data of Maegawa etal. (2006) who showed that Fgf3 or Fgf8 could rescue the loss of chd expression in mutant fish that lacked functional ß-catenin2 [3]. However, when Fgf3 and Sox3 were overexpressed together, we found that, although Fgf3 still induced gsc and chd expression over broad parts of the embryo, in the regions where Sox3 was expressed (as indicated by staining for the overexpressed HA-Sox3 protein, Fig. S6 in the supplementary material), gsc and chd expression was absent (Fig. 4Bc,d,g,h,C,D). Since the direct effects of Sox3 are cell autonomous, this is consistent with Sox3 repressing gsc and chd in the patches where it is expressed, while Fgf3 (which has a broader effect due to its extracellular diffusion) can only expand expression in those regions where cells are not over expressing Sox3. It seems, therefore, that in addition to repressing expression of the fgfs, Sox3 also independently represses the expression of gsc and chd downstream of Fgf signaling.

Bottom Line: We show that ectopic expression of organizer genes induced solely by the inhibition of SoxB1 function is dependent upon the activation of fgf expression.These data allow us to describe a comprehensive signaling network in which the SoxB1 factors restrict organizer formation by inhibiting Fgf, Nodal and Wnt signaling, as well as independently repressing the targets of that signaling.The organizer therefore forms only where Nodal-induced Fgf signaling overlaps with Wnt signaling and the SoxB1 proteins are absent.

View Article: PubMed Central - PubMed

Affiliation: Centre for Genetics and Genomics, School of Biology, University of Nottingham, QMC, Nottingham, United Kingdom.

ABSTRACT
The organizer is one of the earliest structures to be established during vertebrate development and is crucial to subsequent patterning of the embryo. We have previously shown that the SoxB1 transcription factor, Sox3, plays a central role as a transcriptional repressor of zebrafish organizer gene expression. Recent data suggest that Fgf signaling has a positive influence on organizer formation, but its role remains to be fully elucidated. In order to better understand how Fgf signaling fits into the complex regulatory network that determines when and where the organizer forms, the relationship between the positive effects of Fgf signaling and the repressive effects of the SoxB1 factors must be resolved. This study demonstrates that both fgf3 and fgf8 are required for expression of the organizer genes, gsc and chd, and that SoxB1 factors (Sox3, and the zebrafish specific factors, Sox19a and Sox19b) can repress the expression of both fgf3 and fgf8. However, we also find that these SoxB1 factors inhibit the expression of gsc and chd independently of their repression of fgf expression. We show that ectopic expression of organizer genes induced solely by the inhibition of SoxB1 function is dependent upon the activation of fgf expression. These data allow us to describe a comprehensive signaling network in which the SoxB1 factors restrict organizer formation by inhibiting Fgf, Nodal and Wnt signaling, as well as independently repressing the targets of that signaling. The organizer therefore forms only where Nodal-induced Fgf signaling overlaps with Wnt signaling and the SoxB1 proteins are absent.

Show MeSH
Related in: MedlinePlus