Lim1 is the sea urchin ortholog of the vertebrate Lhx1/Lhx5 LIM-homeodomain transcription factors, a 480-residue protein with two N-terminal LIM zinc-finger domains followed by a LIM-class homeodomain. It is a zygotically expressed regulatory gene of the embryonic ectoderm: transcripts appear around hatching (about 8 h) in a ring of ectoderm cells immediately adjacent to the vegetal endomesoderm, extending across both the oral and the aboral face of the embryo (the "veg1" or "border" ectoderm), and decline by gastrulation. Its expression is independent of Nodal signalling and of the Nodal-dependent homeodomain repressor Not, and in Lytechinus it is the earliest gene activated in the border ectoderm by a short-range Wnt5 signal from the adjacent endoderm, with Nodal and BMP2/4 later restricting the border-ectoderm genes to subregions. In Hemicentrotus, ectopic HpLim1 mRNA suppresses gut, mesoderm and aboral ectoderm differentiation and drives embryo-wide expression of an oral ectoderm marker, indicating that the protein acts as a sequence-specific transcriptional regulator of ectodermal regulatory state.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000977 RNA polymerase II transcription regulatory region sequence-specific DNA binding | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference from the LIM-homeobox node (fly Lim1, mouse Lhx1/Lhx5/Lhx2/Lmx1b, worm lin-11/lim-6/mec-3/ttx-3). Lim1 carries an intact LIM-class homeodomain and behaves as a regulatory gene in every sea urchin GRN study; sequence-specific binding to RNA polymerase II regulatory regions is the expected activity of the protein even though no S. purpuratus DNA-binding assay has been reported. Reason: Sound phylogenetic inference for a LIM-homeodomain transcription factor; consistent with the gene's role as an ectodermal regulatory gene. No target-specific evidence of loss or divergence. Supporting Evidence: PMID:10400389 HpLim1 contains two LIM domains and a LIM-class homeodomain, and amino acid sequences of these three domains are highly homologous to corresponding domains of Lim1 of other animals. |
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IBA GO_REF:0000033 | ACCEPT | Summary: Core molecular function. Lim1 is a LIM-homeodomain transcription factor and is treated as a regulatory gene (transcription factor) in the oral/aboral ectoderm GRN and in the border ectoderm subcircuit; ectopic HpLim1 reprograms embryo-wide differentiation, as expected for a transcriptional regulator. Reason: Well-placed PAINT inference for the LIM-homeobox family, congruent with the gene's behaviour in sea urchin GRN studies. The activator/repressor sign has not been determined for the sea urchin protein, so the parent term is the right level. Supporting Evidence: PMID:24227654 This signal activates a unique subcircuit of the ectoderm gene regulatory network, including the transcription factors IrxA, Nk1, Pax2/5/8 and Lim1, which are ultimately restricted to subregions of the border ectoderm (BE). PMID:10400389 HpLim1 mRNA-injected embryos became spherical with markedly reduced gut formation, failed to express marker proteins for aboral ectoderm and mesoderm, and mainly expressed an oral ectoderm marker. |
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the homeobox conserved-site signature (IPR017970). Correct for Lim1, which has a canonical LIM-class homeodomain; redundant with the IBA row for the same term. Reason: The homeodomain signature mapping gives the correct core activity; it duplicates the phylogenetically grounded IBA row above. Supporting Evidence: PMID:10400389 HpLim1 contains two LIM domains and a LIM-class homeodomain |
| GO:0003677 DNA binding | IEA GO_REF:0000120 | MODIFY | Summary: Generic parent derived from the homeodomain Pfam/InterPro entry (IPR001356) via a UniRule. Not wrong, but uninformative; the specific sequence-specific regulatory region binding term is already asserted by PAINT. Reason: Too general for a sequence-specific homeodomain transcription factor; GO:0000977 (already present as an IBA row) is the informative descendant. Proposed replacements: RNA polymerase II transcription regulatory region sequence-specific DNA binding Supporting Evidence: PMID:10400389 HpLim1 contains two LIM domains and a LIM-class homeodomain |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Nuclear localisation inferred for the LIM-homeobox family. A homeodomain transcription factor acts in the nucleus; no sea urchin immunolocalisation exists, but the inference is uncontroversial. Reason: Correct cellular component for a homeodomain transcription factor. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: UniRule/subcellular-location mapping to nucleus; duplicates the IBA row with a weaker qualifier. Reason: Correct, redundant with the PAINT-derived is_active_in nucleus row. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000002 | MODIFY | Summary: Generic InterPro2GO process term from the homeobox signature. Correct but less specific than the RNA polymerase II-level term that PAINT already assigns. Reason: Lim1 is a eukaryotic homeodomain transcription factor acting on RNA polymerase II genes; GO:0006357 (already present as an IBA row) is the informative descendant. Proposed replacements: regulation of transcription by RNA polymerase II |
| GO:0006357 regulation of transcription by RNA polymerase II | IBA GO_REF:0000033 | ACCEPT | Summary: Core biological process for a transcription factor that is a node of the ectoderm gene regulatory network. The sign of regulation (activation vs repression) and the direct targets of Lim1 in the border ectoderm have not been identified in any sea urchin, so the neutral regulation term is appropriate. Reason: Sound PAINT inference; lim1 is treated as a regulatory gene of the veg1/border ectoderm state in the S. purpuratus and Lytechinus GRN papers. Supporting Evidence: PMID:22771578 no effects were seen on expression of the other veg1 oral ectoderm regulatory genes nk2.2 and lim1 PMID:24227654 Cells in the ectoderm at the boundary, hereafter called border ectoderm (BE), express a group of basic helix-loop-helix (bHLH) and homeodomain transcription factors including IrxA, Nk1, Pax2/5/8 and Lim1. |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO mapping from the two Lhx1/5-type LIM domains (IPR049618, IPR049619). LIM domains are zinc-coordinating protein-interaction modules; the zinc ions are structural and do not describe what the protein does. Reason: Chemically true (each LIM domain chelates two zinc ions) but not a function of the gene product; retained as a non-core structural descriptor rather than a core activity. Supporting Evidence: PMID:10400389 HpLim1 contains two LIM domains and a LIM-class homeodomain |
| GO:0030182 neuron differentiation | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Deep PAINT node (PTN000654232) placed on the shared neuronal function of LIM-homeobox genes across fly, worm, fish and mouse. In the sea urchin embryo lim1 is expressed from hatching to gastrulation in the border ectoderm, a region that later contributes to the neurogenic ciliary band, but no study has examined a neuronal role of sea urchin Lim1, and its documented expression window precedes neurogenesis. Reason: The phylogenetic placement is reasonable and there is no target-specific evidence of loss, but the sea urchin literature documents only an early ectodermal patterning role; neuron differentiation is at most a later, untested function and is not core for this gene. Supporting Evidence: PMID:24227654 The earliest marker expressed in the BE was Lim1. It was expressed throughout the BE beginning at 8 hpf and was diminished by gastrulation. |
| GO:0007398 ectoderm development | IEP PMID:19268450 A perturbation model of the gene regulatory network for oral... | NEW | Summary: Expression-pattern inference. In S. purpuratus lim1 is transiently and specifically expressed in the ring of ectoderm abutting the vegetal endomesoderm, spanning oral and aboral ectoderm, from hatching until gastrulation, and in Lytechinus it is the first gene activated in this border ectoderm by the endodermal Wnt5 signal. Ectopic HpLim1 in Hemicentrotus converts embryos to an oral-ectoderm marker state. Reason: The only S. purpuratus-specific evidence is expression plus upstream (Wnt5, Nodal, BMP2/4, Not) perturbations in which lim1 is a readout, so the annotation is kept at the IEP level and at the general ectoderm-development term rather than a cell-fate specification term, which would require a lim1 loss-of-function result. No ancestor or descendant of this term is already on the gene. Supporting Evidence: PMID:19268450 there is a separate domain along the interface with the vegetal endomesoderm that extends all across both oral and aboral ectoderm, as indicated by expression of the lim1 gene (Kawasaki et al. 1999) PMID:24227654 The earliest marker expressed in the BE was Lim1. It was expressed throughout the BE beginning at 8 hpf and was diminished by gastrulation. PMID:10400389 HpLim1 mRNA-injected embryos became spherical with markedly reduced gut formation, failed to express marker proteins for aboral ectoderm and mesoderm, and mainly expressed an oral ectoderm marker. |
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Download this section (compressed HTML)Q: Is Lim1 a direct activator or repressor in the border ectoderm, and which of the other border ectoderm regulatory genes (nk1, irxA, pax2/5/8, vegf3) or the endoderm/ectoderm boundary repressors are its direct targets?
Q: Does S. purpuratus lim1 show the same Wnt5 dependence that was demonstrated for Lim1 in Lytechinus variegatus, and is the response direct (Tcf sites in a lim1 cis-regulatory module)?
Q: Does Lim1 contribute to the later neurogenic ciliary band, as the PAINT neuron-differentiation inference would predict, or is its role confined to the pregastrular patterning of the ectoderm/endoderm border?
Experiment: lim1 morpholino or Cas9 knockdown in S. purpuratus with Nanostring/QPCR and in situ readout of border ectoderm genes (nk1, irxA, pax2/5/8, vegf3, wnt5) and of adjacent endoderm genes at 12-24 h, to define the regulatory outputs of Lim1 and the sign of its action.
Experiment: BAC-reporter dissection of the lim1 locus to identify the border-ectoderm cis-regulatory module and test Tcf (Wnt5), Smad (Nodal/BMP2/4) and Not sites by site mutagenesis.
Experiment: Double in situ hybridisation of lim1 with synaptotagmin B and other neural markers at pluteus stage to test whether lim1-expressing border ectoderm cells contribute to ciliary band neurons.
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