lin28a (Xenopus laevis, Q8JHC4) — curation notes

Project: projects/NEURAL_CREST_ORIGINS.md, Tier 3 (blastula pluripotency programme retained in the crest).
Reviewed 2026-10-07 (claude-code).

Identity

Molecular activity (mostly mammalian, by homology)

Developmental roles in Xenopus

  1. Early embryo / germ layers (X. tropicalis, lin28a+lin28b compound morphants). "Both lin28a and lin28b
    are expressed in pluripotent cells of the Xenopus embryo and are enriched in cells that respond to
    mesoderm-inducing signals"; "The development of axial and paraxial mesoderm is severely abnormal in lin28
    knockdown (morphant) embryos"; "the ability of pluripotent cells from the embryo to respond to the FGF and
    activin/nodal-like mesoderm-inducing pathways is compromised"; and "this early function is probably
    independent of the recognised role of Lin28 proteins in negatively regulating let-7 miRNA biogenesis"
    PMID:23344711. lin28 is an FGF target in X. laevis PMID:19333377. Interpretation: Lin28 sets the
    competence/timing of pluripotent cells' response to inducers, not maintenance of a stem-cell pool.
  2. Metamorphic timing (X. laevis, heat-shock overexpression). "Lin28 overexpression before activation of
    TH signaling delays metamorphosis and inhibits the expression of TH target genes" PMID:28359807; a
    Lin28aΔC (no RHA interaction) still delays metamorphosis, "indicating that the translational regulation
    domain is not required to inhibit the HPT axis" PMID:31397023. Gain-of-function only, but it fits the
    conserved heterochronic role.

Neural crest: what is the evidence?

Layer decision

Lin28a is, on frog evidence, a general pluripotency / developmental-timing RNA-binding factor (early germ
layer competence; metamorphic timing). On chick evidence it is a neural crest competence/multipotency
factor acting through the Lin28/let-7 axis — the same layer as Myc/Id3, and mechanistically linked (let-7
targets Myc). It is not a specifier and not a border specifier. Frog crest evidence is absent, so no NC-branch
term is added to Q8JHC4. The chick paper is uncurated in GO; chick LIN28A (Q45KJ5) would be the right
place for an IMP GO:0014029 (competence-factor convention) — raise with curators.

Module: the competence part could hold a Lin28a annoton only with the chick member (Q45KJ5) as the grounded
participant; the frog member is not yet supported. Recommend recording it as a knowledge gap rather than an
annoton grounded on Q8JHC4.

Comparator check (QuickGO, 2026-10-07)

Query: annotation/downloadSearch?geneProductId=<all lin28 gene products in taxa 9606,10090,10116,7955,9031, 8355,8364 from geneproduct/search?query=lin28>&goId=GO:0014029,GO:0014032,GO:0014033,GO:0001755,GO:0014034, GO:0014036,GO:0019827,GO:0048863,GO:0048864&goUsage=descendants&goUsageRelationships=is_a,part_of,occurs_in.
Gene products included: human LIN28A Q9H9Z2, LIN28B Q6ZN17; mouse Lin28a Q8K3Y3, Lin28b Q45KJ6; rat; zebrafish
lin28a Q803L0, lin28ab, lin28b; chicken LIN28A Q45KJ5, LIN28B Q45KJ4; X. laevis lin28a Q8JHC4, lin28b Q8AVK2 +
TrEMBL; X. tropicalis Q5EB47 + TrEMBL.

Result: no Lin28 family member in any of these species carries any neural crest branch term
(GO:0014029/14032/14033/14034/14036/1755 and descendants). GO:0019827 rows: human LIN28A IMP (PMID:19703396);
mouse ISO/IEA from human; ISS from human on chicken Q45KJ5, X. tropicalis Q5EB47, zebrafish Q803L0, X. laevis
Q8JHC4. Mouse Lin28a has GO:0048863 stem cell differentiation IMP (PMID:18292307, PMID:18604195).
Interpretation: the absence of crest terms is not a convention against annotating Lin28 to crest — the one
crest paper (chick, PMID:30520734) is simply uncurated. But the frog gene has no crest data of its own, so the
gap to fill is on chick, not frog.

GO:0019827 stem cell population maintenance — judged against frog evidence

Source: ISS from human LIN28A IMP (PMID:19703396, "Knockdown of TUT4 and Lin28 reduces the level of stem cell
markers, suggesting that they are required for stem cell maintenance" — cultured cells). Frog evidence: the
Xenopus role is responsiveness of transient pluripotent blastula cells to mesoderm inducers (PMID:23344711),
and metamorphic timing (PMID:28359807). No frog evidence of maintaining a stem cell population. The ISS
transfers an ES-cell culture phenotype to an embryo that has no self-renewing pluripotent population. Action:
MARK_AS_OVER_ANNOTATED (not wrong for mammals; over-reach for frog). It is also not the right home for the
crest competence role: the chick evidence is about holding multipotency within a migrating population over time,
which GO:0019827 would describe only loosely.

Other decisions

Evolution / outgroups