LRRC4B

LRRC4B (NGL-3) is a postsynaptic leucine-rich-repeat adhesion molecule that drives bidirectional excitatory synapse formation by trans-synaptically engaging presynaptic LAR-family receptor tyrosine phosphatases (PMID:19252495, PMID:20139422). Its LRR domain binds the first two FNIII domains of LAR, PTPσ, and PTPδ, with Gln-96 in the first LRR critical for LAR binding and presynaptic induction; LAR and PTPσ promote bidirectional synapse formation whereas PTPδ drives only presynaptic differentiation (PMID:20139422). Intracellularly, NGL-3 localizes to the postsynaptic density and anchors to PSD-95 through its PDZ-binding motif (PMID:19252495, PMID:24298159). The synaptogenic output of the NGL-3/LAR system requires the scaffolding protein afadin, which associates with the LAR-binding catenins, while a separate interaction with presynaptic L1cam promotes formation of excitatory presynaptic puncta (PMID:28695613, PMID:38871241). During long-term depression, NGL-3 is sequentially cleaved by MMPs and γ-secretase downstream of NMDAR activation (PMID:24298159), and loss of NGL-3 abolishes hippocampal LTD by aberrantly hyperactivating Akt/GSK3β signaling, indicating that NGL-3 normally suppresses Akt to permit LTD (PMID:31166939).

1. 2009 High

   Established NGL-3 as a postsynaptic adhesion molecule capable of organizing excitatory synapses bidirectionally through a presynaptic phosphatase partner, answering whether NGL-3 has synaptogenic activity and a trans-synaptic ligand.

   Evidence Co-culture synaptogenesis assay, Co-IP, neuronal knockdown/overexpression, antibody clustering, and soluble LAR competition in rat hippocampal neurons

   PMID:19252495

   Open questions at the time

   • Did not map the binding interface or critical residues
   • In vivo requirement not tested
2. 2009 Medium

   Defined the intracellular coupling of NGL-3 to the postsynaptic scaffold, showing it binds PSD-95 via its PDZ-binding motif and resides at the postsynaptic density.

   Evidence Co-IP and immunofluorescence colocalization in hippocampal neurons

   PMID:19252495 PMID:24298159

   Open questions at the time

   • Functional consequence of disrupting the PSD-95 interaction not isolated
   • Other intracellular partners not characterized
3. 2010 High

   Resolved the molecular interface and broadened the receptor repertoire, identifying the LRR/FNIII contacts, the critical Gln-96 residue, and distinguishing bidirectional (LAR, PTPσ) from unidirectional (PTPδ) signaling.

   Evidence Domain-mapping binding assays, Q96 site-directed mutagenesis, co-culture synaptogenesis, and Co-IP

   PMID:20139422

   Open questions at the time

   • Structural basis of bidirectional vs unidirectional outcome unresolved
   • No co-crystal structure
4. 2013 Medium

   Showed that NGL-3 is a regulated proteolytic substrate, undergoing NMDAR-driven ectodomain shedding by MMPs followed by γ-secretase intramembrane cleavage during LTD-inducing activity.

   Evidence Pharmacological inhibition of NMDARs, MMPs, and γ-secretase with biochemical detection of cleavage products in cultured neurons and hippocampal slices

   PMID:24298159

   Open questions at the time

   • Specific MMP not identified
   • Fate and signaling role of cleavage fragments unknown
   • Single lab
5. 2017 Medium

   Identified afadin as a required scaffolding cofactor for NGL-3-induced presynaptic differentiation, linking the system to LAR-binding catenins.

   Evidence Co-culture assay with afadin-deficient neurons, Co-IP from brain lysate, immunofluorescence and immunoelectron microscopy

   PMID:28695613

   Open questions at the time

   • Direct vs indirect afadin interaction not resolved
   • Catenin contribution shown only by co-IP
   • Single lab
6. 2019 High

   Defined the in vivo physiological role, showing NGL-3 is required for hippocampal LTD by restraining Akt/GSK3β signaling rather than by maintaining synapse number.

   Evidence Constitutive knockout mice, electrophysiology, Akt-inhibitor pharmacological rescue, and Western blot for Akt/GSK3β phosphorylation

   PMID:31166939

   Open questions at the time

   • Mechanistic link between NGL-3 and Akt suppression not defined
   • How adhesion signaling couples to Akt unknown
7. 2024 Medium

   Extended the presynaptic ligand repertoire to L1cam and connected NGL-3 to a circuit-level therapeutic response, showing its requirement for electroacupuncture-induced synaptogenesis in post-stroke depression.

   Evidence Ectodomain binding assay with L1cam, VGluT1 puncta formation assay, cortical neuron-specific conditional knockout, and behavioral tests in a PSD mouse model

   PMID:38871241

   Open questions at the time

   • L1cam binding interface not mapped
   • Relationship between L1cam and LAR signaling unclear
   • Limited methodological detail

• How NGL-3 adhesion signaling is biochemically transduced to suppress Akt/GSK3β, and how its proteolytic processing integrates with this signaling during LTD, remain unresolved.
• No defined molecular pathway from NGL-3 to Akt
• Role of cleavage fragments in intracellular signaling unknown
• No structural model of receptor complexes