NFASC

Neurofascin (NFASC) is an alternatively spliced L1-family immunoglobulin cell adhesion molecule whose two principal isoforms organize functionally distinct axonal domains required for saltatory conduction and synaptic targeting. The neuronal isoform NF186 localizes to axon initial segments and nodes of Ranvier via ankyrinG binding through its cytoplasmic FIGQY motif—a binding regulated by tyrosine phosphorylation—where it recruits voltage-gated sodium channel β1/β3 subunits through its first Ig and second FNIII domains and directs GABAergic pinceau synapse formation onto Purkinje neurons (PMID:8947556, PMID:9804856, PMID:11470829, PMID:15479642). The glial isoform NF155, expressed by myelinating oligodendrocytes, engages the axonal Caspr–contactin complex through its Ig domains 5–6 to establish paranodal septate-like junctions; conditional loss of NF155 abolishes these junctions, causes potassium channel redistribution, severe conduction deficits, and lethality (PMID:11839274, PMID:19185024, PMID:20371806). Autoantibodies against neurofascin isoforms are pathogenic in multiple sclerosis, inhibiting axonal conduction in a complement-dependent manner and exacerbating disease upon passive transfer in an EAE model (PMID:17846150).

1. 1992 High

   Identification of neurofascin as an alternatively spliced L1-family Ig-superfamily member established the molecular framework for understanding how a single gene generates functionally distinct adhesion molecules in the nervous system.

   Evidence cDNA cloning, protein biochemistry, and antibody perturbation of axon growth in chick brain

   PMID:1377696

   Open questions at the time

   • Functional roles of individual splice isoforms were not defined
   • Intracellular binding partners were unknown
   • Relevance to myelinated axon organization was not yet explored
2. 1996 High

   Discovery that the NF186 isoform concentrates at nodes of Ranvier and axon initial segments together with ankyrinG and sodium channels resolved where neurofascin functions and implicated it as a scaffold organizer of excitable axonal domains.

   Evidence Isoform-specific antibody generation and immunofluorescence co-localization in myelinated nerves

   PMID:8947556

   Open questions at the time

   • Direct binding between neurofascin and ankyrinG had not been biochemically demonstrated
   • Whether neurofascin was required or merely co-localized was unresolved
3. 1998 High

   Mapping the ankyrin-binding site to the cytoplasmic FIGQY motif and showing that tyrosine phosphorylation abolishes this interaction provided a regulatory switch mechanism controlling neurofascin's anchorage to the spectrin–actin cytoskeleton.

   Evidence Phosphorylation-state manipulation and cell aggregation/sorting assays in neuroblastoma cells; deletion/point-mutant analysis with GFP-ankyrinG recruitment in HEK293 cells

   PMID:9371782 PMID:9804856

   Open questions at the time

   • The kinase and phosphatase controlling FIGQY phosphorylation in vivo were not identified
   • Whether phosphorylation-dependent switching operates at nodes versus AIS was unknown
4. 1998 High

   Demonstration that neurofascin binds tenascin-R, axonin-1, and F11 in an isoform-dependent manner expanded its extracellular interaction repertoire beyond homophilic adhesion, linking alternative splicing to ligand selectivity.

   Evidence Cellular binding assays, competition experiments, and neurite outgrowth assays on neurofascin-Fc substrates

   PMID:9722619

   Open questions at the time

   • In vivo relevance of tenascin-R modulation of neurofascin interactions was not tested
   • Whether these interactions occur at nodes or paranodes was unclear
5. 2001 High

   Genetic loss of ankyrinG in Purkinje neurons proved that ankyrinG is the upstream organizer that recruits neurofascin, NrCAM, βIV spectrin, and Nav1.6 to the AIS, establishing a hierarchical assembly model in which ankyrinG is the master scaffold.

   Evidence Cerebellar ankyrinG knockout mouse with immunofluorescence analysis of AIS protein composition

   PMID:11724816

   Open questions at the time

   • Whether neurofascin has any ankyrinG-independent roles at the AIS was not addressed
   • The order of recruitment among ankyrinG-dependent proteins was not resolved
6. 2001 High

   Identification of a direct interaction between neurofascin and sodium channel β1/β3 subunits—mediated by neurofascin's first Ig domain and second FNIII repeat—revealed the mechanism by which neurofascin recruits sodium channel complexes to early forming nodes.

   Evidence Co-immunoprecipitation, domain-mapped cell binding assays, immunofluorescence co-localization in sciatic nerve across a developmental time course

   PMID:11470829

   Open questions at the time

   • Whether this interaction is sufficient to cluster Nav channels in the absence of other nodal components was not tested
   • Binding affinities for β1 vs β3 were not quantified
7. 2001 High

   Discovery of the neurofascin–syntenin-1 intracellular interaction via the PDZ domain suggested an additional signaling or trafficking axis specific to neurofascin within the L1 family.

   Evidence Yeast two-hybrid, ligand-overlay, surface plasmon resonance, and co-localization in heterologous cells

   PMID:11152476

   Open questions at the time

   • Functional consequence of syntenin-1 binding for neurofascin trafficking or signaling was not determined
   • In vivo relevance was not tested
8. 2002 High

   Identification of the glial NF155 isoform as the receptor for the axonal Caspr–contactin complex established the molecular basis of paranodal axoglial junctions and explained how glial and neuronal membrane domains are coupled.

   Evidence Cell-binding assays with transfected cells, brain lysate pulldown, and myelinating co-culture inhibition with NF155 antibodies and recombinant ectodomain

   PMID:11839274

   Open questions at the time

   • Which NF155 domains mediate Caspr–contactin binding had not been mapped
   • Whether NF155 is required for junction maintenance in addition to formation was unknown
9. 2004 High

   Demonstration that AIS-localized NF186 directs GABAergic basket interneuron pinceau innervation onto Purkinje neurons expanded neurofascin's role from ion channel scaffolding to synapse targeting, showing that the ankyrinG–NF186 complex serves as a positional cue for presynaptic partners.

   Evidence BAC transgenic reporter mice, conditional neurofascin manipulation, analysis of basket axon innervation patterns

   PMID:15479642

   Open questions at the time

   • The extracellular ligand on basket axons recognizing NF186 was not identified
   • Whether NF186 similarly directs other synapse types was not explored
10. 2007 High

    Discovery that anti-neurofascin autoantibodies from MS patients are directly pathogenic—inhibiting conduction in a complement-dependent manner and exacerbating EAE upon passive transfer—established neurofascin as an autoimmune target and linked nodal/paranodal disruption to demyelinating disease mechanisms.

    Evidence Proteomics-based antigen identification, hippocampal slice electrophysiology, and passive transfer in a rat MOG-EAE model

    PMID:17846150

    Open questions at the time

    • Relative pathogenic contribution of anti-NF186 vs anti-NF155 antibodies was not resolved
    • Prevalence and clinical specificity of these antibodies across MS subtypes was not established
11. 2009 High

    Conditional and inducible ablation of NF155 in myelinating glia proved that NF155 is required both for initial formation and ongoing maintenance of paranodal junctions, and that its loss causes potassium channel redistribution, conduction failure, ataxia, and lethality.

    Evidence Developmental (PLP-Cre) and adult-inducible (PLP-CreER) conditional knockout mice with electrophysiology and immunofluorescence

    PMID:19185024

    Open questions at the time

    • Rate of paranodal disassembly after NF155 loss and its relationship to protein half-life were not quantified
    • Whether NF155 loss also affects nodal stability was not examined
12. 2010 High

    Pinpointing Ig domains 5–6 as both necessary and sufficient for NF155's interaction with contactin and for paranodal junction integrity resolved the molecular interface of the tripartite paranodal complex and showed these domains are dispensable for nodal NF186 function.

    Evidence In vivo knock-in mouse with Ig5–6 deletion plus in vitro binding assays with deletion constructs

    PMID:20371806

    Open questions at the time

    • Atomic-resolution structure of the NF155–contactin–Caspr complex has not been determined
    • Whether additional extracellular domains contribute to binding affinity or specificity in vivo is unresolved

• Key unresolved questions include the structural basis of the NF155–Caspr–contactin tripartite complex at atomic resolution, the identity of kinases and phosphatases regulating FIGQY phosphorylation in vivo, and the extracellular ligand on basket axons that recognizes NF186 for synapse targeting.
• No atomic-resolution structure of the paranodal complex
• FIGQY kinase/phosphatase identity unknown in vivo
• Presynaptic receptor for NF186-mediated synapse targeting not identified