ets1-a (P18755, Xenopus laevis) — review notes

Project context: projects/NEURAL_CREST_ORIGINS.md, Tier 1 (neural crest specifiers,
candidate vertebrate innovations). Task: place Ets1 in the correct NC GRN layer and
check whether its GO annotations reflect that.

Identity

• UniProt P18755 "Protein c-ets-1-A", 438 aa; PNT (SAM/pointed) domain 49-134, ETS
  DNA-binding domain 332-412, autoinhibitory helices HI-1/HI-2/H4/H5 flanking the ETS
  domain (by similarity to mouse P27577). Xenbase cross-reference: ets1.S
  (XB-GENE-6254436). The second homeolog (ets1b / ets1.L) exists; most frog
  loss-of-function reagents knock down both.
• All 21 existing GOA rows are electronic or ISS/IBA; there is no Xenopus experimental
  annotation on P18755.

Molecular activity

• Classic ETS-family sequence-specific transcription factor; activator in most contexts.
  In chick cranial NC, Ets1 directly binds and activates the Sox10E2 enhancer
  PMID:20139305
• In Xenopus, overexpressed Ets1a binds the id3 promoter and recruits HDAC1 to repress
  it (context-dependent repression)
  PMID:26198637
  The construct was explicitly the ets1a ORF [file:XENLA/ets1-a/ets1-a-deep-research-falcon.md "FLAG-tagged frog Ets1a occupied its predicted binding region in embryo chromatin-immunoprecipitation assays."]
  Note: this is a gain-of-function result; excess Ets1 represses NC markers, so it is
  a dosage effect rather than evidence that Ets1 normally inhibits NC formation.
• Cofactor: Vgll3 PMID:28870996

Expression

• Xenopus: maternal, then NC and derivatives, plus hemangioblast/endothelial lineages
  PMID:9303349
  PMID:9303349
• Pre-migratory and migratory crest PMID:32713114
• Chick: onset at HH7-8 in cranial neural folds, AFTER the border genes, maintained in
  migrating crest PMID:27339986

Network position (layer)

• Chick cranial NC circuit: Brn3c/Lhx5/Dmbx1 (anterior border) -> Tfap2b/Sox8 -> Ets1
  PMID:27339986
• Ets1 (with Sox9, cMyb) is a direct input to the cranial Sox10 enhancer
  PMID:20139305
• Gain of function/sufficiency: Sox8 + Tfap2b + Ets1 reprogram trunk NC to cranial
  identity and chondrogenic potential; Ets1 alone insufficient
  PMID:27339986
  PMID:27339986
• Ets1 required for Alx1 in facial mesenchyme PMID:27339986
• Chick delamination: Ets1 required and sufficient for cranial-type cell recruitment
  PMID:17987123
  PMID:17987123

Conclusion on layer: Ets1 is NOT a neural plate border specifier (onset is after
border genes, downstream of Tfap2b) and not a competence/pluripotency factor. It is a
late neural crest specifier / cranial-identity factor whose principal Xenopus
loss-of-function output is delamination and migration rather than initial induction.

Xenopus loss of function

• Ets1 MO (both homeologs) — initial foxd3/snai2 not prevented, migration/streams
  disrupted [file:XENLA/ets1-a/ets1-a-deep-research-falcon.md "Thus, impaired migration and delamination are better-established physiological outcomes than an absolute requirement for initial whole-embryo crest induction."]
• NC-targeted vs heart-mesoderm-targeted MO
  PMID:25691536
  PMID:25691536
• Downstream msmb3 (X. tropicalis mutant RNA-seq; X. laevis follow-up)
  PMID:32713114

Evolution (outgroup)

• Lamprey: Ets1 absent from premigratory/migratory NC; deployed later than in amniotes
  PMID:31645763
• Little skate has it: gnathostome acquisition
  PMID:31645763
• So Ets1's NC role is a gnathostome-level recruitment (not even pan-vertebrate),
  consistent with the project's Tier 1 framing, and verifies the PMID:27339986 claim
  (it is the chick cranial circuit Sox8/Tfap2b/Ets1).

Annotation decisions (summary)

• MF: ACCEPT RNAPII-specific DNA-binding TF activity; MODIFY generic DNA binding to
  RNAPII cis-regulatory region sequence-specific DNA binding; NEW activator activity
  (GO:0001228), based on direct Sox10E2 activation by the ortholog.
• BP: NEW neural crest cell migration (GO:0001755) and neural crest cell delamination
  (GO:0036032); NEW endocardium development (GO:0003157). Comparator check: Xenopus
  TFs hic1.L, kmt2d.L, sox10.S, snai2.S carry GO:0001755; chick SOX9 carries
  GO:0036032. Not proposing neural crest cell fate specification: in Xenopus initial
  specifier expression is not Ets1-dependent; the chick Sox10E2 data are a question
  for experts.
• Vascular/erythroid ISS terms (from human ETS1 cell-culture work) kept as non-core
  or flagged; endothelial expression in frog supports the vascular ones.
• Cytoplasm: non-core.