SoxB1 (SpSoxB1) is the maternally supplied SoxB-group (Sox1/2/3-like) HMG-box transcription factor of the purple sea urchin. Its mRNA is uniform in the egg and the protein is nuclear in every blastomere of 4- and 8-cell embryos, but from the fourth cleavage it is cleared post-translationally from micromere nuclei and then progressively from the whole vegetal (endomesodermal) region, a turnover that depends on nuclear beta-catenin and can be triggered by Pmar1; after blastula stage nuclear SoxB1 is confined to the ectoderm, making it the earliest known spatially restricted transcriptional regulator along the animal-vegetal axis. SoxB1 binds Sox cis-regulatory elements in a sequence-specific manner (for example the Sox site of the SpAN promoter, which it bends strongly and which is essential for promoter activity) and it acts as a transcriptional activator of nodal and of most oral-ectoderm, aboral-ectoderm and ciliated-band regulatory genes, while repressing its own gene and otx beta1/2. Independently of DNA binding it antagonises the transcriptional cofactor function of nuclear beta-catenin/TCF, so that elevated SoxB1 animalises the embryo and reduced SoxB1 raises TCF/beta-catenin reporter output; the mutual antagonism between SoxB1 and nuclear beta-catenin sets the boundary between ectoderm and the endomesoderm gene regulatory network. Later, SoxB1 persists in uncommitted neural precursors of the ectoderm and foregut endoderm and is lost from neurons as they differentiate.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000122 negative regulation of transcription by RNA polymerase II | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT inference from the SoxB family (Dichaete, mouse Sox1/2/3/14, human SOX3/SOX14, zebrafish). In the sea urchin SoxB1 does repress some Pol II genes: it limits its own transcript level by negative feedback in presumptive ectoderm (soxb1 morpholino raises soxb1 mRNA) and it represses otx beta1/2 in the ectoderm. Direct binding to those genes has not been shown, and most of its targets are activated rather than repressed, so this is a real but secondary output. Reason: Negative autoregulation and otx beta1/2 repression are documented by knockdown, but activation is the predominant mode of SoxB1 action; keep as a non-core process. Supporting Evidence: PMID:25655703 As these analyses show, SoxB1 controls its own level of expression through negative feedback on the soxb1 gene, so that soxb1 MASO produces a large increase in the prevalence of its own mRNA. PMID:25655703 Furthermore, SoxB1 evidently represses otxΞ²1/2 transcription. PMID:15689377 However, in mesomeres, SoxB1 appears to be subject to negative autoregulation that helps to maintain tight regulation of SoxB1 mRNA levels in presumptive ectoderm. |
| GO:0000978 RNA polymerase II cis-regulatory region sequence-specific DNA binding | IBA GO_REF:0000033 | ACCEPT | Summary: Deep Sox/HMG-node inference. Directly corroborated in this species: in vitro translated SpSoxB1 forms a specific complex with the Sox cis element of the SpAN promoter, the same complex present in embryo nuclear extracts is supershifted by anti-SpSoxB1 antiserum, and mutation of the site reduces SpAN promoter activity in vivo 20-fold. Sox sites in the nodal 5' module are likewise essential. Reason: EMSA plus antiserum supershift on a defined Pol II cis-regulatory element and in vivo site mutagenesis demonstrate sequence-specific cis-regulatory DNA binding for the sea urchin protein itself. Supporting Evidence: PMID:10556071 In vitro translated SpSoxB1 forms a specific complex with this cis element whose mobility is identical to that formed by a protein in nuclear extracts. An anti-SpSoxB1 rabbit polyclonal antiserum specifically supershifts this DNA-protein complex PMID:11763999 Regulatory sites that bind SpSoxB1 and CBF (CCAAT binding factor) are essential for strong transcriptional activity because mutations of these elements reduce promoter activity in vivo 20- and 10-fold, respectively. |
| GO:0001228 DNA-binding transcription activator activity, RNA polymerase II-specific | IBA GO_REF:0000033 | ACCEPT | Summary: Inherited activator activity of the Sox family. In the sea urchin SoxB1 is required for zygotic nodal expression through Sox sites in the nodal cis-regulatory module, and soxb1 morpholino downregulates almost all regulatory genes of the ciliated band and oral/aboral ectoderm, which Barsi et al. interpret as common activation by SoxB1. One caveat: at the SpAN promoter multimerised Sox sites do not activate a basal promoter, and SoxB1 acts architecturally by bending DNA rather than by classical transactivation, so the activator function is target-context dependent. Reason: Cis-regulatory dependence of nodal on Sox sites and broad loss of ectodermal regulatory-gene expression on knockdown support activator activity for this protein; the SpAN architectural case is recorded as a caveat, not a contradiction. Supporting Evidence: PMID:25655703 However, most of its target genes utilize SoxB1 as a transcriptional activator. PMID:17855430 This work shows that Tcf, SoxB1 and Univin play essential roles in the regulation of nodal expression in the sea urchin and suggests that some of the regulatory interactions controlling nodal expression predate the chordates. PMID:11763999 Here we show that multimerized SpSoxB1 elements cannot activate transcription from the SpAN basal promoter in vivo. |
| GO:0003677 DNA binding | IEA GO_REF:0000104 | ACCEPT | Summary: UniRule inference from the HMG box (residues 61-129). Correct but uninformative next to the experimentally corroborated sequence-specific cis-regulatory binding term. Reason: True as a generic parent; the specific activity is captured by the GO:0000978 row. Supporting Evidence: PMID:10556071 In vitro translated SpSoxB1 forms a specific complex with this cis element whose mobility is identical to that formed by a protein in nuclear extracts. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Immunostaining with anti-SpSoxB1 antiserum shows nuclear protein in all blastomeres of 4- and 8-cell embryos, lost from micromere nuclei at fourth cleavage and later confined to ectoderm nuclei; its nuclear presence is the regulated variable in the animal-vegetal patterning system. Reason: Nuclear localisation is directly observed in this species and is where the protein acts. Supporting Evidence: PMID:10556071 SpSoxB1 maternal transcripts are uniformly distributed in the unfertilized egg and the protein accumulates to similar, high concentrations in all nuclei of 4- and 8-cell embryos. PMID:10556071 The vegetal region lacking nuclear SpSoxB1 gradually expands so that, after blastula stage, only cells in differentiating ectoderm accumulate this protein in their nuclei. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic subcellular-location mapping; concordant with the antiserum staining of nuclear SpSoxB1 in cleavage-stage and ectoderm nuclei. Reason: Redundant with the IBA nucleus row but correct. Supporting Evidence: PMID:10556071 the protein accumulates to similar, high concentrations in all nuclei of 4- and 8-cell embryos. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the SOX family signature. Correct but general; the specific positive and negative Pol II regulation rows carry the informative content. Reason: Generic parent of the experimentally corroborated GO:0045944 and GO:0000122 rows. Supporting Evidence: PMID:10556071 This factor is the earliest known spatially restricted regulator of transcription along the animal-vegetal axis of the sea urchin embryo. |
| GO:0007420 brain development | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Propagated from vertebrate Sox1/2/3 and Drosophila Dichaete brain phenotypes. The sea urchin embryo and pluteus larva have no brain; the nervous system consists of an apical organ, ciliated-band neurons and foregut neurons. SoxB1 does mark uncommitted neural precursors in these domains, but the organ-level term does not apply to this taxon. Reason: Brain development is an anatomical term with no counterpart in echinoderm embryos; the neural role is better captured by the neuron differentiation row. Propagation Review Root cause: PROPAGATION BAD Failure modes: LINEAGE OR TAXON MISMATCH CONTEXT OR TISSUE MISMATCH Sources checked: PANTHER:PTN008496689 SUPPORTS SOURCE BUT NOT TARGET SoxB-group node whose brain-development inference is seeded by vertebrate Sox1/2/3 and Drosophila Dichaete; echinoderm embryos and larvae have no brain, so the organ-level term does not transfer even though the neural-precursor role is conserved. Supporting Evidence: PMID:21576476 Just as these factors disappear when neurons differentiate in mouse embryos, so also is SoxB1 eliminated from differentiating neurons in ectoderm and foregut endoderm in sea urchin embryos. |
| GO:0030182 neuron differentiation | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: The neural-precursor role of SoxB-group factors is conserved in the sea urchin. SoxB1 is expressed in all three neurogenic territories, is lost from neurons as they differentiate in ectoderm and foregut, and soxb1 morpholino alters expression of proneural regulators in a territory-specific way. The early gastrulation block of soxb1 knockdown prevents a direct test of its requirement for foregut neurons, and the neural function is one of several roles of this pleiotropic factor. Reason: Supported by expression and knockdown data in this species, but it is a later, non-core role relative to ectoderm specification and beta-catenin antagonism. Supporting Evidence: PMID:21576476 Just as these factors disappear when neurons differentiate in mouse embryos, so also is SoxB1 eliminated from differentiating neurons in ectoderm and foregut endoderm in sea urchin embryos. PMID:30413529 Expression of the six proneural transcription factors shows that the response to knockdown of soxC , sip1 or soxb1 differs when the three territories are compared |
| GO:0045944 positive regulation of transcription by RNA polymerase II | IBA GO_REF:0000033 | ACCEPT | Summary: SoxB1 positively regulates transcription of Pol II genes in this species through identified Sox sites: the SpAN promoter (site mutation reduces activity 20-fold in vivo, via DNA bending), the nodal 5' cis-regulatory module, and, by morpholino, nearly all regulatory genes of the ciliated band and oral/aboral ectoderm. This is the core output of the factor in the ectoderm GRN. Reason: SoxB1 is the gene product performing the activation step at Sox sites of Pol II promoters and modules; direct cis-regulatory evidence exists for SpAN and nodal. Supporting Evidence: PMID:11763999 Regulatory sites that bind SpSoxB1 and CBF (CCAAT binding factor) are essential for strong transcriptional activity because mutations of these elements reduce promoter activity in vivo 20- and 10-fold, respectively. PMID:25655703 Remarkably, almost all of the regulatory genes specifically expressed within these domains are downregulated by interference with SoxB1 expression, implying their common activation by this factor. |
| GO:0008301 DNA binding, bending | IDA PMID:11763999 SpSoxB1 serves an essential architectural function in the pr... | NEW | Summary: Circular-permutation bandshift assays with in vitro translated SpSoxB1 show that it strongly bends the SpAN Sox element, and multimerised Sox sites cannot activate the SpAN basal promoter; the authors conclude that SpSoxB1 works at this promoter as an architectural factor whose bending brings upstream activators into contact with the basal promoter. Reason: Direct biochemical demonstration of DNA bending by the sea urchin protein, and the mechanism proposed for its essential role at SpAN; not captured by the sequence-specific binding row. Supporting Evidence: PMID:11763999 However, like other factors containing HMG-class DNA binding domains, SpSoxB1 does induce strong bending of DNA. PMID:11763999 The results described above suggest a model in which SpSoxB1 promotes transcription of the SpAN gene primarily through DNA bending, rather than through transcriptional activation. |
| GO:0090090 negative regulation of canonical Wnt signaling pathway | IMP PMID:14499650 Tight regulation of SpSoxB factors is required for patternin... | NEW | Summary: Morpholino or dominant-negative reduction of SpSoxB1 elevates TCF/Lef-beta-catenin-dependent luciferase reporter expression in vivo, and SpSoxB1 overexpression suppresses all vegetal fates, phenocopying cadherin-mediated sequestration of beta-catenin. The activity does not require DNA binding (a non-binding mutant retains it), pointing to protein-level antagonism of the beta-catenin/TCF transcriptional complex. Reciprocally, nuclear beta-catenin drives SoxB1 protein turnover in vegetal cells, and the GRN literature treats SoxB1 as the antagonist of beta-catenin cofactor function that keeps ectoderm out of the endomesoderm program. Reason: SoxB1 itself dampens canonical Wnt transcriptional output (reporter readout rises on knockdown, falls on overexpression), so it participates in the negative regulation rather than merely being required upstream; this is its defining role in the endomesoderm GRN. Supporting Evidence: PMID:14499650 Reduction in SpSoxB1 level results in elevated TCF/Lef-beta-catenin-dependent expression of a luciferase reporter gene in vivo, indicating that in the normal embryo this protein suppresses the primary vegetal signaling mechanism that is required for specification of mesenchyme and endoderm. PMID:14499650 Suppression of vegetal fates involves interference at the protein-protein level because a mutation of SpSoxB1 that prevents its binding to DNA does not significantly reduce this activity. PMID:15689377 Patterning of cell fates along the sea urchin animal-vegetal embryonic axis requires the opposing functions of nuclear beta-catenin/TCF-Lef, which activates the endomesoderm gene regulatory network, and SoxB1, which antagonizes beta-catenin and limits its range of function. PMID:18413610 SoxB1 is believed to act as an antagonist of the transcriptional cofactor function of Ξ²-catenin. |
| GO:0001715 ectodermal cell fate specification | IMP PMID:25655703 Geometric control of ciliated band regulatory states in the ... | NEW | Summary: SoxB1 is the pan-ectodermal transcription factor of the embryo. Nuclear SoxB1 becomes confined to ectoderm, soxb1 morpholino downregulates almost all regulatory genes of the ciliated band and oral/aboral ectoderm GRNs, Sox sites are essential for zygotic nodal expression, and ectopic SpSoxB1 converts the embryo to an animalised (ectoderm-only) phenotype. Later authors describe soxb1 as involved in early specification of ectoderm. Reason: SoxB1 performs the regulatory step (activation of the ectodermal regulatory-state genes) that specifies ectodermal fate; comparator Otx carries the same term for aboral ectoderm in this species. Endoderm and gastrulation requirements are treated as indirect and not proposed. Supporting Evidence: PMID:25655703 Remarkably, almost all of the regulatory genes specifically expressed within these domains are downregulated by interference with SoxB1 expression, implying their common activation by this factor. PMID:14499650 Here we show that elevated and ectopic expression of this factor suppresses differentiation of all vegetal cell types, a phenotype that is very similar to that caused by the suppression of beta-catenin nuclear function by cadherin overexpression. PMID:30413529 soxb1 was a special case in that it is involved in early specification of ectoderm ( Barsi et al., 2015 ) and is eliminated post-translationally from the EM ( Angerer et al., 2005 ). |
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Download this section (compressed HTML)Q: Does SoxB1 bind beta-catenin or TCF directly in sea urchin nuclei, or does it compete for a shared co-factor, and which SoxB1 domain mediates the antagonism?
Q: Which of the ciliated-band and oral/aboral ectoderm regulatory genes downregulated by soxb1 morpholino carry functional Sox sites, i.e. which are direct SoxB1 targets?
Experiment: Co-immunoprecipitation or proximity-labelling of SpSoxB1 with beta-catenin and Tcf in blastula nuclear extracts, combined with the DNA-binding-defective SoxB1 mutant, to identify the physical basis of Wnt antagonism.
Hypothesis: SoxB1 sequesters beta-catenin or Tcf at the protein level, independently of its HMG-box DNA binding.
Experiment: ChIP-seq of SpSoxB1 at blastula stage compared with the soxb1 morpholino transcriptome, to separate direct from indirect ectoderm GRN targets.
Hypothesis: Most ciliated-band and ectoderm regulatory genes downregulated by soxb1 MASO are bound directly by SoxB1 at Sox sites.
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular mechanism by which SoxB1 antagonises beta-catenin/TCF is unresolved; the activity survives a DNA-binding mutation, implying a protein-level interaction, but a direct SoxB1-beta-catenin or SoxB1-TCF interaction has not been demonstrated in the sea urchin.
OPEN BIOLOGY
Provenance (the field's own admissions):
Gap: Whether the requirement of SoxB1 for gastrulation and endoderm differentiation reflects direct SoxB1 targets in the vegetal plate or an indirect consequence of disturbed ectoderm/endomesoderm boundary setting is not established; no endoderm process term is proposed here.
OPEN BIOLOGY
Provenance (the field's own admissions):
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