APLP1 (human) review notes

UniProt P51693 (APLP1_HUMAN). Taxon NCBITaxon:9606.

These notes were added as a 2026 update to an already-complete review. They cover one
topic only: APLP1's proposed role as a neuronal receptor for pathological alpha-synuclein
fibrils, which the existing review did not mention (the file contained zero occurrences of
"synuclein" or "fibril" before this update). Nothing else in the review was changed.

Why this was added

APLP1 is now routinely named as one of three candidate neuronal mediators of alpha-synuclein
preformed-fibril (PFF) uptake and spread, alongside FAM171A2 and LAG3:
PMID:41270468.

What is actually established for APLP1

Direct biophysics (strongest evidence). The isolated APLP1 E1 domain binds the acidic
C-terminus of alpha-synuclein by an electrostatic mechanism shared with LAG3 D1, with strong
preference for the fibrillar over the monomeric state:
PMID:34172566 and PMID:34172566. Cached abstract-only.

Cellular/in vivo (partner-dependent). APLP1 is presented as acting through a complex
with LAG3: PMID:38821932. The genetic rescue is from a double knockout, not an Aplp1 single knockout:
PMID:38821932. So the APLP1-only contribution has not been isolated in vivo.

That paper carries an Author Correction, which is a statistics/figure-legend fix to Fig. 4
and not a retraction: PMID:39080298.

Why the field is unsettled

• LAG3, the partner the APLP1 mechanism depends on, is disputed.
  PMID:34309222 and
  PMID:34309222. Important caveat: this paper does not
  mention APLP1 anywhere (checked: zero occurrences in the cached full text). It is
  evidence against LAG3, and touches APLP1 only indirectly.
• A third candidate receptor was reported in 2025.
  PMID:39977508. This paper likewise does not mention APLP1 (zero occurrences).
• Provenance is concentrated. PMID:27708076 (LAG3, Science 2016), PMID:34172566 (PNAS
  2021) and PMID:38821932 (Nat Commun 2024) share senior authorship (Dawson/Mao, with Liu
  for the biophysics). The 2021 and 2024 papers are therefore not independent replications
  of the 2016 proposal.
• The one paper that puts all three side by side is computational. AlphaFold3 modelling,
  not experiment: PMID:41270468, and it could not reproduce the LAG3 interaction at all:
  PMID:41270468.

Curation position taken

No annotation was added or changed. Checked the whole of APLP1-goa.tsv: no existing
GO annotation on APLP1 concerns alpha-synuclein, fibril binding, or endocytic uptake of
aggregates, so nothing in existing_annotations is affected and no action changed.

A receptor molecular function was deliberately not proposed, for three reasons:
1. The in vivo evidence is double-knockout only, so APLP1 cannot be separated from LAG3.
2. The LAG3 half of the mechanism is actively contested.
3. The only source that frames APLP1 as one of three receptors is a review with AlphaFold3
predictions; structure predictions cannot support a GO molecular function.

What was recorded instead: the six references above (with findings and reference_review),
a knowledge_gaps entry stating precisely what is unknown and what would resolve it, four
suggested_questions, and two suggested_experiments.

Ontology note

GO has GO:0001540 amyloid-beta binding but no alpha-synuclein binding term (checked
via QuickGO term search). The nearest available parent is GO:0051787 misfolded protein binding, which loses the ligand identity. A proposed term is recorded under the knowledge
gap's proposed_terms; it is an ontology-coverage request and is not an assertion that
APLP1 should carry it.

Open question for experts

Does APLP1 bind and internalize alpha-synuclein fibrils in LAG3-null neurons? An Aplp1
single-knockout uptake/spreading experiment, replicated outside the originating
laboratories, would settle whether APLP1 is a receptor in its own right or only a
co-factor of a partner that may not be expressed in neurons at all.