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Nodes of Ranvier act as barriers to restrict invasion of flanking paranodal domains in myelinated axons.

ABSTRACT: Accumulation of voltage-gated sodium (Na(v)) channels at nodes of Ranvier is paramount for action potential propagation along myelinated fibers, yet the mechanisms governing nodal development, organization, and stabilization remain unresolved. Here, we report that genetic ablation of the neuron-specific isoform of Neurofascin (Nfasc(NF¹??)) in vivo results in nodal disorganization, including loss of Na(v) channel and ankyrin-G (AnkG) enrichment at nodes in the peripheral nervous system (PNS) and central nervous system (CNS). Interestingly, the presence of paranodal domains failed to rescue nodal organization in the PNS and the CNS. Most importantly, using ultrastructural analysis, we demonstrate that the paranodal domains invade the nodal space in Nfasc(NF¹??) mutant axons and occlude node formation. Our results suggest that Nfasc(NF¹??)-dependent assembly of the nodal complex acts as a molecular boundary to restrict the movement of flanking paranodal domains into the nodal area, thereby facilitating the stereotypic axonal domain organization and saltatory conduction along myelinated axons.

SUBMITTER: Thaxton C

PROVIDER: S-EPMC3035172 | biostudies-literature | 2011 Jan

REPOSITORIES: biostudies-literature

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Nodes of Ranvier act as barriers to restrict invasion of flanking paranodal domains in myelinated axons.

Thaxton Courtney C Pillai Anilkumar M AM Pribisko Alaine L AL Dupree Jeffrey L JL Bhat Manzoor A MA

Neuron 20110101 2

Accumulation of voltage-gated sodium (Na(v)) channels at nodes of Ranvier is paramount for action potential propagation along myelinated fibers, yet the mechanisms governing nodal development, organization, and stabilization remain unresolved. Here, we report that genetic ablation of the neuron-specific isoform of Neurofascin (Nfasc(NF¹⁸⁶)) in vivo results in nodal disorganization, including loss of Na(v) channel and ankyrin-G (AnkG) enrichment at nodes in the peripheral nervous system (PNS) and ...[more]

PMID: 21262464

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Project description:The mechanisms that govern node of Ranvier organization, stability, and long-term maintenance remain to be fully elucidated. One of the molecular components of the node is the cytoskeletal scaffolding protein, ankyrin G (AnkG), which interacts with multiple members of the nodal complex. The role of AnkG in nodal organization and maintenance is still not clearly defined as to whether AnkG functions as an initial nodal organizer or whether it functions as a nodal stabilizer after the nodal complex has been assembled. Using a mouse model system, we report here that perinatal and juvenile neuronal ablation of AnkG has differential consequences on nodal stability. Early loss of AnkG creates immature nodes with abnormal morphology, which undergo accelerated destabilization within a month, resulting in rapid voltage-gated sodium (NaV) channel and βIV spectrin loss with reduced effects on neurofascin 186. On the other hand, late ablation of AnkG from established nodal complexes leads to slow but progressive nodal destabilization over 10 months, primarily affecting βIV spectrin, followed by NaV channels, with modest impact on neurofascin 186. We also show that ankyrin R and βI spectrin are not sufficient to prevent nodal disorganization after AnkG ablation. Additionally, nodal disorganization in both early and late AnkG mutants is accompanied by axonal pathology and neurological dysfunction. Together, our results suggest that AnkG plays an indispensable role in the maturation and long-term stabilization of the newly assembled nodal complex, and that loss of AnkG after nodal stabilization does not lead to rapid nodal disassembly but to loss of specific nodal components in a time-dependent manner.SIGNIFICANCE STATEMENT Nodes of Ranvier are the myelin-free gaps along myelinated axons that allow fast communication between neurons and their target cells by propagating action potentials in a saltatory manner. The cytoskeletal scaffolding protein ankyrin G (AnkG) has been thought to play an important role in node formation; however, its precise role in nodal assembly, stability, and maintenance is still not clear. By using spatiotemporal ablation of AnkG, we report its differential role in nodal maturation and stabilization. We show that early AnkG-deficient nodes fail to mature and undergo rapid destabilization. In contrast, nodes that assemble with AnkG are much more stable and undergo gradual disintegration with sequential loss of nodal components in the absence of AnkG.

Project description:Myelinated axons are organized into distinct subcellular and molecular regions. Without proper organization, electrical nerve conduction is delayed, resulting in detrimental physiological outcomes. One such region is the paranode where axo-glial septate junctions act as a molecular fence to separate the sodium (Na+) channel-enriched node from the potassium (K+) channel-enriched juxtaparanode. A significant lack of knowledge remains as to cytoskeletal proteins which stabilize paranodal domains and underlying cytoskeleton. Whirlin (Whrn) is a PDZ domain-containing cytoskeletal scaffold whose absence in humans results in Usher Syndromes or variable deafness-blindness syndromes. Mutant Whirlin (Whrn) mouse model studies have linked such behavioral deficits to improper localization of critical transmembrane protein complexes in the ear and eye. Until now, no reports exist about the function of Whrn in myelinated axons.RT-PCR and immunoblot analyses revealed expression of Whrn mRNA and Whrn full-length protein, respectively, in several stages of central and peripheral nervous system development. Comparing wild-type mice to Whrn knockout (Whrn-/-) mice, we observed no significant differences in the expression of standard axonal domain markers by immunoblot analysis but observed and quantified a novel paranodal compaction phenotype in 4 to 8 week-old Whrn-/- nerves. The paranodal compaction phenotype and associated cytoskeletal disruption was observed in Whrn-/- mutant sciatic nerves and spinal cord fibers from early (2 week-old) to late (1 year-old) stages of development. Light and electron microscopic analyses of Whrn knockout mice reveal bead-like swellings in cerebellar Purkinje axons containing mitochondria and vesicles by both. These data suggest that Whrn plays a role in proper cytoskeletal organization in myelinated axons.Domain organization in myelinated axons remains a complex developmental process. Here we demonstrate that loss of Whrn disrupts proper axonal domain organization. Whrn likely contributes to the stabilization of paranodal myelin loops and axonal cytoskeleton through yet unconfirmed cytoskeletal proteins. Paranodal abnormalities are consistently observed throughout development (2 wk-1 yr) and similar between central and peripheral nervous systems. In conclusion, our observations suggest that Whrn is not required for the organization of axonal domains, but once organized, Whrn acts as a cytoskeletal linker to ensure proper paranodal compaction and stabilization of the axonal cytoskeleton in myelinated axons.

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Nodes of Ranvier act as barriers to restrict invasion of flanking paranodal domains in myelinated axons.

Publications

Nodes of Ranvier act as barriers to restrict invasion of flanking paranodal domains in myelinated axons.

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