Tcf (SpTcf/Lef) is the single Tcf/Lef-family HMG-box transcription factor of the purple sea urchin and the nuclear effector of the maternal beta-catenin system that polarizes the embryo along its animal-vegetal axis. The protein carries an N-terminal beta-catenin-binding region and a C-terminal HMG box that recognizes Tcf sites in the cis-regulatory modules of the early endomesoderm regulatory genes (pmar1, wnt8, blimp1, otx, foxa and others). In animal-half blastomeres, where beta-catenin is degraded, Tcf bound by the co-repressor Groucho keeps these genes silent; in the vegetal micromeres and macromere descendants, where Dishevelled stabilizes beta-catenin and it enters the nucleus, beta-catenin displaces Groucho and converts Tcf into a transcriptional activator. Tcf is thus an obligatory input to nearly every node at the top of the endomesoderm gene regulatory network. It drives pmar1 in the micromeres, opening the double-negative gate of the skeletogenic lineage, sustains the wnt8/Blimp1 community-effect loop that spreads the vegetal regulatory state outward, and activates the veg2 endoderm regulatory genes; the same Tcf sites later extinguish the endoderm programme in mesoderm precursors once Notch signalling reduces nuclear beta-catenin. A dominant-negative Tcf animalizes the embryo, abolishing skeletogenic and non-skeletogenic mesenchyme, endoderm and aboral ectoderm, while a constitutively active Tcf vegetalizes it.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003677 DNA binding | IEA GO_REF:0000104 | ACCEPT | Summary: Electronic inference from the HMG box (InterPro IPR009071, residues 316-384) via the Tcf/Lef UniRule. Tcf acts in vivo through Tcf target sites that were demonstrated by mutation in the wnt8 cis-regulatory modules and that control most of the early endoderm regulatory genes, so DNA binding is correct although generic. No in vitro binding assay with the S. purpuratus protein is cached, so the more specific cis-regulatory-region binding term is not asserted; the informative activities are added as NEW activator and repressor rows. Reason: The HMG-box domain inference agrees with the cis-regulatory evidence that Tcf sites mediate the factor's input; the term is true but general. Supporting Evidence: PMID:16289024 The genomic target sites for these factors were demonstrated by mutation in one of the cis-regulatory modules. PMID:21623371 the same Tcf sites that are used to initiate the endoderm GRN in the veg2 lineage are used again to extinguish it in mesoderm precursors |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic subcellular-location inference. Tcf is the nuclear partner of beta-catenin that reads Tcf sites in cis-regulatory DNA; its function is exercised in the nucleus, and the GRN literature describes beta-catenin transiting to the nucleus to form an active complex with Tcf. No immunolocalization of SpTcf/Lef itself is cached, but the inference is sound and this is the core location. Reason: A sequence-specific transcription factor that activates and represses target genes through cis-regulatory sites is nuclear; the electronic inference matches the functional evidence. Supporting Evidence: PMID:18413610 Dsh regionally prevents the degradation of cytoplasmic Ξ²-catenin ( 27 ), which transits to the nucleus, forming an active complex with the Tcf transcription factor. PMID:10664150 Expression of SpTcf/Lef was maximal when beta-catenin became localized to nuclei of vegetal blastomeres, consistent with its acting in combination with beta-catenin to specify vegetal cell fates. |
| GO:0016055 Wnt signaling pathway | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the TCF/LEF family signature. The biology is right but the term is the generic parent. Tcf is specifically the transcriptional endpoint of the beta-catenin-dependent (canonical) branch. Dominant-negative Tcf phenocopies blockade of beta-catenin nuclearization, activated Tcf vegetalizes, and LiCl (GSK3 inhibition) potentiates endogenous Tcf transactivation, so the canonical Wnt term is the accurate one. Reason: Replace the generic Wnt signaling term with canonical Wnt signaling pathway, the beta-catenin/Tcf branch that Tcf executes at the level of transcription (see NEW row for the experimental evidence). Proposed replacements: canonical Wnt signaling pathway Supporting Evidence: PMID:10664150 Expression of a dominant-negative SpTcf/Lef inhibited primary and secondary mesenchyma, endoderm, and aboral ectoderm formation in a manner similar to that observed when nuclear accumulation of beta-catenin was prevented. PMID:10625549 the transactivational activity of endogenous sea urchin TCF is potentiated by LiCl treatment, which vegetalizes embryos by inhibiting GSK3, consistent with an in vivo interaction between endogenous beta-catenin and TCF |
| GO:0001228 DNA-binding transcription activator activity, RNA polymerase II-specific | IMP PMID:10664150 Involvement of Tcf/Lef in establishing cell types along the ... | NEW | Summary: In the vegetal blastomeres, where beta-catenin is nuclear, Tcf activates the endomesoderm regulatory genes. A dominant-negative SpTcf/Lef blocks formation of the beta-catenin-dependent vegetal cell types; Tcf sites are required for endomesodermal expression of the wnt8 cis-regulatory module; and the GRN perturbation analysis places beta-catenin/Tcf as a direct activating input into pmar1, wnt8 and the veg2 endoderm genes. Activated Tcf (constitutive activator) vegetalizes the embryo and LiCl potentiates endogenous Tcf transactivation (Vonica et al.; species not stated in the cached abstract). Reason: Both the loss-of-function (dominant-negative) phenotype in S. purpuratus and the requirement for Tcf sites in an endomesodermal cis-regulatory module show that Tcf, with beta-catenin, is a sequence-specific transcriptional activator of the endomesoderm genes. Supporting Evidence: PMID:10664150 Expression of a dominant-negative SpTcf/Lef inhibited primary and secondary mesenchyma, endoderm, and aboral ectoderm formation in a manner similar to that observed when nuclear accumulation of beta-catenin was prevented. PMID:16289024 The Tcf1/beta-catenin and Blimp1/Krox inputs are both necessary for normal endomesodermal expression mediated by this cis-regulatory module PMID:18413610 The pmar1 genes are activated by the Ξ²-catenin/Tcf transcription complex plus Otx, i.e., by two of the micromere-specific inputs just enumerated ( 25 , 30 ). PMID:10625549 In contrast, microinjected RNA encoding an activated TCF produces a highly "vegetalized" embryo. |
| GO:0001227 DNA-binding transcription repressor activity, RNA polymerase II-specific | IDA PMID:16289024 cis-Regulatory inputs of the wnt8 gene in the sea urchin end... | NEW | Summary: Where beta-catenin is not nuclear, Tcf represses the same target genes. In the wnt8 cis-regulatory system of S. purpuratus, Tcf1 sites in the early module act to repress ectopic transcription in prospective ectoderm cells, and the Tcf sites of the veg2 endoderm genes are reused to extinguish the endoderm GRN in mesoderm precursors when Notch signalling lowers nuclear beta-catenin. The repressive state depends on the co-repressor Groucho, which competes with beta-catenin for Tcf binding (shown for the Lytechinus variegatus orthologue). Reason: Tcf-site mutation in a S. purpuratus cis-regulatory module derepresses transcription in the animal half, demonstrating that the DNA-bound factor represses in the absence of nuclear beta-catenin; this is the other half of the Tcf switch and is the mechanism that restricts the endomesoderm GRN to the vegetal half. Supporting Evidence: PMID:16289024 In a second regulatory region, which initiates expression in micromere and macromere descendant cells early in cleavage, Tcf1 sites act to repress ectopic transcription in prospective ectoderm cells. PMID:21623371 A possible explanation is that in cells receiving Notch signalling, the availability of nuclear Ξ²-catenin is reduced, leading to Tcf/Groucho-mediated repression. PMID:15708573 LvGroucho interacts with Tcf to antagonize the expression of key endomesoderm regulatory genes. |
| GO:0060070 canonical Wnt signaling pathway | IMP PMID:10664150 Involvement of Tcf/Lef in establishing cell types along the ... | NEW | Summary: Tcf is the transcriptional effector of nuclear beta-catenin in the sea urchin embryo. Dominant-negative SpTcf/Lef reproduces the phenotype of blocking beta-catenin nuclearization; all of beta-catenin's axis-patterning activity is mediated by TCF; and beta-catenin/Tcf is itself an output of Wnt8 reception, closing the wnt8 community-effect loop. Tcf performs the terminal step of the pathway (transcriptional response), so it participates in the pathway rather than merely being required for it. Reason: The dominant-negative phenotype and its identity with the beta-catenin-blockade phenotype in S. purpuratus establish Tcf as the nuclear component of the canonical (beta-catenin-dependent) Wnt pathway. Supporting Evidence: PMID:10664150 Our results suggest that SpTcf/Lef functions by interacting with beta-catenin to specify cell fates along the sea urchin animal-vegetal axis. PMID:10625549 We also provide evidence indicating that all of beta-catenin's activity in patterning the sea urchin AV axis is mediated by TCF. PMID:19104065 the wnt8 gene requires inputs from both Ξ²-catenin/Tcf (i.e., from the same signal transduction system that it activates) and from Blimp1 factor for expression |
| GO:0001714 endodermal cell fate specification | IMP PMID:10664150 Involvement of Tcf/Lef in establishing cell types along the ... | NEW | Summary: Dominant-negative SpTcf/Lef abolishes endoderm formation, and the endoderm GRN of the veg2 lineage is launched by cis-regulatory Tcf sites in most of its eight early regulatory genes. Tcf itself performs the transcriptional step that initiates the endoderm regulatory state, so this is direct participation, not a downstream consequence. As comparators, the vertebrate Tcf/Lef orthologues carry germ-layer specification terms for the equivalent beta-catenin-dependent role. Reason: Loss of Tcf function removes endoderm, and the mechanism is the Tcf-site-dependent activation of the endoderm regulatory genes; Tcf is the factor that executes the specification step. Supporting Evidence: PMID:10664150 Expression of a dominant-negative SpTcf/Lef inhibited primary and secondary mesenchyma, endoderm, and aboral ectoderm formation in a manner similar to that observed when nuclear accumulation of beta-catenin was prevented. PMID:21623371 the cis-regulatory Tcf responsiveness of early endodermal genes results first in the activation of an endodermal GRN and then, together with cleavage geometry, in the spatial separation of endodermal and mesodermal regulatory states and hence of biological fates PMID:17038519 while TCF activity is required early for launching the endomesoderm gene regulatory network, it has to be downregulated at blastula stage in the mesodermal lineage |
| GO:0007501 mesodermal cell fate specification | IMP PMID:10664150 Involvement of Tcf/Lef in establishing cell types along the ... | NEW | Summary: Dominant-negative SpTcf/Lef abolishes both primary (skeletogenic) and secondary mesenchyme. In the micromeres beta-catenin/Tcf together with Otx directly activates pmar1, the gene that opens the double-negative gate and is necessary and sufficient for skeletogenic specification; beta-catenin/Tcf is also the input to wnt8, whose signal drives non-skeletogenic mesoderm and endoderm specification in the adjacent tiers. Tcf executes the first transcriptional step of mesoderm specification in the micromere lineage. Reason: The dominant-negative phenotype and the direct beta-catenin/Tcf input into pmar1 show that Tcf performs the regulatory step that specifies the skeletogenic mesoderm lineage. Supporting Evidence: PMID:10664150 Expression of a dominant-negative SpTcf/Lef inhibited primary and secondary mesenchyma, endoderm, and aboral ectoderm formation in a manner similar to that observed when nuclear accumulation of beta-catenin was prevented. PMID:18413610 The pmar1 genes are activated by the Ξ²-catenin/Tcf transcription complex plus Otx, i.e., by two of the micromere-specific inputs just enumerated ( 25 , 30 ). PMID:9847248 this localization is required for the establishment of all vegetal cell fates and the production of micromere-derived signals |
| GO:0000122 negative regulation of transcription by RNA polymerase II | IDA PMID:16289024 cis-Regulatory inputs of the wnt8 gene in the sea urchin end... | NEW | Summary: The process counterpart of the repressor-activity row. Mutation of the Tcf1 sites in the early wnt8 cis-regulatory module of S. purpuratus derepresses transcription in prospective ectoderm, and Tcf sites in the veg2 endoderm genes are required to extinguish their expression in mesoderm precursors, so Tcf directly reduces RNA polymerase II transcription of its targets where beta-catenin is absent from the nucleus. Reason: Site-mutation evidence shows Tcf-dependent repression of a defined target gene in the territory lacking nuclear beta-catenin. Supporting Evidence: PMID:16289024 Tcf1 sites act to repress ectopic transcription in prospective ectoderm cells. PMID:21623371 the same Tcf sites that are used to initiate the endoderm GRN in the veg2 lineage are used again to extinguish it in mesoderm precursors |
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Download this section (compressed HTML)Q: Does the sea urchin genome encode a single Tcf/Lef gene, and does SpTcf/Lef produce alternative isoforms (as vertebrate TCF7L2 does) with different activator or repressor potential?
Q: Is the C-terminal region of SpTcf/Lef a target of the Notch-induced NLK kinase in S. purpuratus, as shown for TCF in Paracentrotus lividus, and does phosphorylation account for the clearance of the endoderm GRN from mesoderm precursors?
Experiment: ChIP or DamID for tagged SpTcf/Lef at 16- to 60-cell stages, with separated animal and vegetal halves, to test whether Tcf occupies the wnt8, pmar1 and foxa Tcf sites in both territories while co-occupancy with beta-catenin is vegetal-only and with Groucho is animal-only.
Hypothesis: Tcf is constitutively DNA-bound and the activator/repressor switch is determined solely by which cofactor it recruits.
Experiment: Co-immunoprecipitation or in vitro pull-down of S. purpuratus beta-catenin and Groucho with the N-terminal (IPR013558) and HMG-box regions of SpTcf/Lef, and rescue of the dominant-negative phenotype with binding-site mutants.
Hypothesis: The N-terminal beta-catenin-binding region of SpTcf/Lef is necessary for activation of the endomesoderm GRN and dispensable for Groucho-mediated repression.
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Direct physical binding of the S. purpuratus Tcf protein to beta-catenin and to Groucho has not been assayed; the interactions are inferred from LiCl and dominant-negative behaviour in S. purpuratus and from interaction assays with the Lytechinus variegatus orthologue.
OPEN BIOLOGY
Provenance (the field's own admissions):
Gap: Whether Tcf occupies its target cis-regulatory sites in vivo in both the animal (repressed) and vegetal (activated) territories, as the switch model requires, has not been shown by chromatin binding assays; the evidence is from site mutation in reporter constructs.
OPEN BIOLOGY
Provenance (the field's own admissions):
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