Spatiotemporal ablation of myelinating glia-specific neurofascin (Nfasc NF155) in mice reveals gradual loss of paranodal axoglial junctions and concomitant disorganization of axonal domains.

The evolutionary demand for rapid nerve impulse conduction led to the process of myelination-dependent organization of axons into distinct molecular domains. These domains include the node of Ranvier flanked by highly specialized paranodal domains where myelin loops and axolemma orchestrate the axoglial septate junctions. These junctions are formed by interactions between a glial isoform of neurofascin (Nfasc(NF155)) and axonal Caspr and Cont. Here we report the generation of myelinating glia-specific Nfasc(NF155) null mouse mutants. These mice exhibit severe ataxia, motor paresis, and death before the third postnatal week. In the absence of glial Nfasc(NF155), paranodal axoglial junctions fail to form, axonal domains fail to segregate, and myelinated axons undergo degeneration. Electrophysiological measurements of peripheral nerves from Nfasc(NF155) mutants revealed dramatic reductions in nerve conduction velocities. By using inducible PLP-CreER recombinase to ablate Nfasc(NF155) in adult myelinating glia, we demonstrate that paranodal axoglial junctions disorganize gradually as the levels of Nfasc(NF155) protein at the paranodes begin to drop. This coincides with the loss of the paranodal region and concomitant disorganization of the axonal domains. Our results provide the first direct evidence that the maintenance of axonal domains requires the fence function of the paranodal axoglial junctions. Together, our studies establish a central role for paranodal axoglial junctions in both the organization and the maintenance of axonal domains in myelinated axons.

Pillai AM ，Thaxton C ，Pribisko AL ，Cheng JG ，Dupree JL ，Bhat MA ... -  《-》

In vivo deletion of immunoglobulin domains 5 and 6 in neurofascin (Nfasc) reveals domain-specific requirements in myelinated axons.

Thaxton C ，Pillai AM ，Pribisko AL ，Labasque M ，Dupree JL ，Faivre-Sarrailh C ，Bhat MA ... -  《-》

Biallelic mutations in neurofascin cause neurodevelopmental impairment and peripheral demyelination.

Axon pathfinding and synapse formation are essential processes for nervous system development and function. The assembly of myelinated fibres and nodes of Ranvier is mediated by a number of cell adhesion molecules of the immunoglobulin superfamily including neurofascin, encoded by the NFASC gene, and its alternative isoforms Nfasc186 and Nfasc140 (located in the axonal membrane at the node of Ranvier) and Nfasc155 (a glial component of the paranodal axoglial junction). We identified 10 individuals from six unrelated families, exhibiting a neurodevelopmental disorder characterized with a spectrum of central (intellectual disability, developmental delay, motor impairment, speech difficulties) and peripheral (early onset demyelinating neuropathy) neurological involvement, who were found by exome or genome sequencing to carry one frameshift and four different homozygous non-synonymous variants in NFASC. Expression studies using immunostaining-based techniques identified absent expression of the Nfasc155 isoform as a consequence of the frameshift variant and a significant reduction of expression was also observed in association with two non-synonymous variants affecting the fibronectin type III domain. Cell aggregation studies revealed a severely impaired Nfasc155-CNTN1/CASPR1 complex interaction as a result of the identified variants. Immunofluorescence staining of myelinated fibres from two affected individuals showed a severe loss of myelinated fibres and abnormalities in the paranodal junction morphology. Our results establish that recessive variants affecting the Nfasc155 isoform can affect the formation of paranodal axoglial junctions at the nodes of Ranvier. The genetic disease caused by biallelic NFASC variants includes neurodevelopmental impairment and a spectrum of central and peripheral demyelination as part of its core clinical phenotype. Our findings support possible overlapping molecular mechanisms of paranodal damage at peripheral nerves in both the immune-mediated and the genetic disease, but the observation of prominent central neurological involvement in NFASC biallelic variant carriers highlights the importance of this gene in human brain development and function.

Efthymiou S ，Salpietro V ，Malintan N ，Poncelet M ，Kriouile Y ，Fortuna S ，De Zorzi R ，Payne K ，Henderson LB ，Cortese A ，Maddirevula S ，Alhashmi N ，Wiethoff S ，Ryten M ，Botia JA ，Provitera V ，Schuelke M ，Vandrovcova J ，SYNAPS Study Group ，Walsh L ，Torti E ，Iodice V ，Najafi M ，Karimiani EG ，Maroofian R ，Siquier-Pernet K ，Boddaert N ，De Lonlay P ，Cantagrel V ，Aguennouz M ，El Khorassani M ，Schmidts M ，Alkuraya FS ，Edvardson S ，Nolano M ，Devaux J ，Houlden H ... -  《-》

Neurofascins are required to establish axonal domains for saltatory conduction.

Sherman DL ，Tait S ，Melrose S ，Johnson R ，Zonta B ，Court FA ，Macklin WB ，Meek S ，Smith AJ ，Cottrell DF ，Brophy PJ ... -  《NEURON》

Gangliosides contribute to stability of paranodal junctions and ion channel clusters in myelinated nerve fibers.

Paranodal axo-glial junctions are important for ion channel clustering and rapid action potential propagation in myelinated nerve fibers. Paranode formation depends on the cell adhesion molecules neurofascin (NF) 155 in glia, and a Caspr and contactin heterodimer in axons. We found that antibody to ganglioside GM1 labels paranodal regions. Autoantibodies to the gangliosides GM1 and GD1a are thought to disrupt nodes of Ranvier in peripheral motor nerves and cause Guillain-Barré syndrome, an autoimmune neuropathy characterized by acute limb weakness. To elucidate ganglioside function at and near nodes of Ranvier, we examined nodes in mice lacking gangliosides including GM1 and GD1a. In both peripheral and central nervous systems, some paranodal loops failed to attach to the axolemma, and immunostaining of Caspr and NF155 was attenuated. K(+) channels at juxtaparanodes were mislocalized to paranodes, and nodal Na(+) channel clusters were broadened. Abnormal immunostaining at paranodes became more prominent with age. Moreover, the defects were more prevalent in ventral than dorsal roots, and less frequent in mutant mice lacking the b-series gangliosides but with excess GM1 and GD1a. Electrophysiological studies revealed nerve conduction slowing and reduced nodal Na(+) current in mutant peripheral motor nerves. The amounts of Caspr and NF155 in low density, detergent insoluble membrane fractions were reduced in mutant brains. These results indicate that gangliosides are lipid raft components that contribute to stability and maintenance of neuron-glia interactions at paranodes.

Susuki K ，Baba H ，Tohyama K ，Kanai K ，Kuwabara S ，Hirata K ，Furukawa K ，Furukawa K ，Rasband MN ，Yuki N ... -  《GLIA》