AMIGO3

AMIGO3 is a type I transmembrane protein composed of six leucine-rich repeats flanked by cysteine-rich LRR-N and LRR-C caps and a single immunoglobulin domain, and it acts as a cell adhesion molecule capable of homophilic and heterophilic binding with other AMIGO family members (PMID:12629050). It dimerizes through an LRR-LRR interface, a contact required for proper cell-surface expression and stable folding (PMID:21983541). Functionally, AMIGO3 serves as a co-receptor within the NgR1/p75-TROY inhibitory signaling complex, where it substitutes for LINGO-1, binds NgR1 and p75/TROY, and transduces CNS myelin-derived inhibitory signals through RhoA/ROCK activation to suppress axon growth (PMID:23613963, PMID:34650403). Loss of AMIGO3 disinhibits axon regeneration: shRNA knockdown in primary DRG and retinal neurons promotes neurite outgrowth under myelin-inhibitory conditions (PMID:23613963), and in vivo knockdown in dorsal root ganglion neurons restores NT3-stimulated dorsal column axon regeneration, compound action potential conduction, and sensory-locomotor function (PMID:30013050). AMIGO3 is also upregulated after status convulsion and contributes to myelin sheath damage, with its downregulation relieving myelin ultrastructural impairment via ROCK/RhoA suppression (PMID:34650403).

1. 2003 Medium

   Establishing AMIGO3's domain architecture and adhesive behavior defined it as an LRR/Ig cell adhesion molecule capable of homophilic and heterophilic interactions, framing it as a candidate neuronal surface receptor.

   Evidence Molecular cloning, domain analysis, recombinant ectodomain binding and neurite outgrowth assays in hippocampal neurons

   PMID:12629050

   Open questions at the time

   • Did not identify the in vivo binding partners or downstream signaling
   • Adhesion shown for ectodomains; physiological neuronal role unresolved
2. 2011 Medium

   Structural work resolved how AMIGO proteins assemble, showing AMIGO3 forms LRR-LRR dimers required for surface expression and folding, explaining a prerequisite for its receptor function.

   Evidence Crystal structure of AMIGO-1 at 2.0 Å with SAXS, light-scattering and interface mutagenesis applied to AMIGO-3

   PMID:21983541

   Open questions at the time

   • AMIGO-3 dimerization inferred from SAXS/modeling, not a direct AMIGO3 crystal structure
   • Did not address heteromeric assembly with NgR1/p75/TROY
3. 2013 High

   Identifying AMIGO3 as a LINGO-1-substituting co-receptor in the NgR1/p75-TROY complex answered how myelin inhibitory signals are transduced, placing AMIGO3 at the receptor level upstream of RhoA.

   Evidence Reciprocal co-IP in brain lysates and non-neuronal cells, RhoA activation assay, shRNA knockdown with neurite outgrowth in primary DRG and retinal neurons, in vivo lesion expression profiling

   PMID:23613963

   Open questions at the time

   • Stoichiometry and direct binding interface within the complex not defined
   • Whether AMIGO3 binds myelin ligands directly was not established
4. 2018 High

   In vivo knockdown demonstrated that removing AMIGO3 disinhibits axon regeneration with functional recovery, confirming it is a causally required mediator of growth inhibition rather than a correlate.

   Evidence Non-viral shRNA knockdown in DRG neurons, dorsal column injury model, compound action potential electrophysiology, behavioral testing, Western blot/immunofluorescence

   PMID:30013050

   Open questions at the time

   • Did not dissect the RhoA/ROCK step at molecular resolution in vivo
   • Durability and clinical translatability of the regenerative effect not addressed
5. 2021 Medium

   Extending AMIGO3 to a seizure context showed it drives myelin sheath damage via ROCK/RhoA, generalizing its inhibitory signaling role beyond traumatic axon injury to demyelinating pathology.

   Evidence Kainic acid status convulsion model in immature mice, shRNA knockdown, Western blot for MBP and ROCK/RhoA components, TEM of myelin ultrastructure

   PMID:34650403

   Open questions at the time

   • Single lab, single publication
   • Cell type driving the effect (neuron vs oligodendrocyte) not resolved

• The direct molecular ligand sensed by the AMIGO3-containing receptor complex and the structural basis of AMIGO3 incorporation into the NgR1/p75-TROY assembly remain undefined.
• No direct AMIGO3-myelin ligand interaction characterized
• No structure of the AMIGO3-containing receptor complex
• Whether AMIGO3 substitution for LINGO-1 is regulated transcriptionally or post-translationally is unknown