Search

Propagation of action potentials along axons is optimized through interactions between neurons and myelinating glial cells. Myelination drives division of the axons into distinct molecular domains including nodes of Ranvier. The high density of voltage-gated sodium channels at nodes generates action potentials allowing for rapid and efficient saltatory nerve conduction. At paranodes flanking both sides of the nodes, myelinating glial cells interact with axons, forming junctions that are essential for node formation and maintenance. Recent studies indicate that the disruption of these specialized axonal domains is involved in the pathophysiology of various neurological diseases. Loss of paranodal axoglial junctions due to genetic mutations or autoimmune attack against the paranodal proteins leads to nerve conduction failure and neurological symptoms. Breakdown of nodal and paranodal proteins by calpains, the calcium-dependent cysteine proteases, may be a common mechanism involved in various nervous system diseases and injuries. This chapter reviews recent progress in neurobiology and pathophysiology of specialized axonal domains along myelinated nerve fibers.

Action potential conduction along myelinated axons depends on high densities of voltage-gated Na + channels at the nodes of Ranvier. Flanking each node, paranodal junctions (paranodes) are formed between axons and Schwann cells in the peripheral nervous system (PNS) or oligodendrocytes in the CNS. Paranodal junctions contribute to both node assembly and maintenance. Despite their importance, the molecular mechanisms responsible for paranode assembly and maintenance remain poorly understood. βII spectrin is expressed in diverse cells and is an essential part of the submembranous cytoskeleton. Here, we show that Schwann cell βII spectrin is highly enriched at paranodes. To elucidate the roles of glial βII spectrin, we generated mutant mice lacking βII spectrin in myelinating glial cells by crossing mice with a floxed allele of Sptbn1 with Cnp-Cre mice, and analyzed both male and female mice. Juvenile (4 weeks) and middle-aged (60 weeks) mutant mice showed reduced grip strength and sciatic nerve conduction slowing, whereas no phenotype was observed between 8 and 24 weeks of age. Consistent with these findings, immunofluorescence microscopy revealed disorganized paranodes in the PNS and CNS of both postnatal day 13 and middle-aged mutant mice, but not in young adult mutant mice. Electron microscopy confirmed partial loss of transverse bands at the paranodal axoglial junction in the middle-aged mutant mice in both the PNS and CNS. These findings demonstrate that a spectrin-based cytoskeleton in myelinating glia contributes to formation and maintenance of paranodal junctions. SIGNIFICANCE STATEMENT Myelinating glia form paranodal axoglial junctions that flank both sides of the nodes of Ranvier. These junctions contribute to node formation and maintenance and are essential for proper nervous system function. We found that a submembranous spectrin cytoskeleton is highly enriched at paranodes in Schwann cells. Ablation of βII spectrin in myelinating glial cells disrupted the paranodal cell adhesion complex in both peripheral and CNSs, resulting in muscle weakness and sciatic nerve conduction slowing in juvenile and middle-aged mice. Our data show that a spectrin-based submembranous cytoskeleton in myelinating glia plays important roles in paranode formation and maintenance., (Copyright © 2018 the authors 0270-6474/18/386063-13$15.00/0.)

Nodes of Ranvier and associated paranodal and juxtaparanodal domains along myelinated axons are essential for normal function of the peripheral and central nervous systems. Disruption of these domains as well as increases in the reactive carbonyl species methylglyoxal are implicated as a pathophysiology common to a wide variety of neurological diseases. Here, using an ex vivo nerve exposure model, we show that increasing methylglyoxal produces paranodal disruption, evidenced by disorganized immunostaining of axoglial cell-adhesion proteins, in both sciatic and optic nerves from wild-type mice. Consistent with previous studies showing that increase of methylglyoxal can alter intracellular calcium homeostasis, we found upregulated activity of the calcium-activated protease calpain in sciatic nerves after methylglyoxal exposure. Methylglyoxal exposure altered clusters of proteins that are known as calpain substrates: ezrin in Schwann cell microvilli at the perinodal area and zonula occludens 1 in Schwann cell autotypic junctions at paranodes. Finally, treatment with the calpain inhibitor calpeptin ameliorated methylglyoxal-evoked ezrin loss and paranodal disruption in both sciatic and optic nerves. Our findings strongly suggest that elevated methylglyoxal levels and subsequent calpain activation contribute to the disruption of specialized axoglial domains along myelinated nerve fibers in neurological diseases.

Communication in the central nervous system (CNS) occurs through initiation and propagation of action potentials at excitable domains along axons. Action potentials generated at the axon initial segment (AIS) are regenerated at nodes of Ranvier through the process of saltatory conduction. Proper formation and maintenance of the molecular structure at the AIS and nodes are required for sustaining conduction fidelity. In myelinated CNS axons, paranodal junctions between the axolemma and myelinating oligodendrocytes delineate nodes of Ranvier and regulate the distribution and localization of specialized functional elements, such as voltage-gated sodium channels and mitochondria. Disruption of excitable domains and altered distribution of functional elements in CNS axons is associated with demyelinating diseases such as multiple sclerosis, and is likely a mechanism common to other neurological disorders. This review will provide a brief overview of the molecular structure of the AIS and nodes of Ranvier, as well as the distribution of mitochondria in myelinated axons. In addition, this review highlights important structural and functional changes within myelinated CNS axons that are associated with neurological dysfunction., (Copyright © 2016 Elsevier Ireland Ltd and Japan Neuroscience Society. All rights reserved.)

Rapid action potential propagation along myelinated axons requires voltage-gated Na(+) channel clustering at the axon initial segments (AISs) and nodes of Ranvier. The AIS is intrinsically defined by cytoskeletal proteins expressed in axons, whereas nodes of Ranvier are formed by interaction between neurons and myelinating glia. These axonal domains have long been considered stable structures, but recent studies revealed that they are plastic and contribute to fine adjustment of neuronal activities and circuit function. The AIS changes its distribution and maintains neural circuit activity at a constant level. Morphological changes in myelinated nerve structures presumably modulate the excitability of nodal regions and regulate the timing of activity, thereby optimizing signal processing in a neural circuit. This review highlights recent findings on the structural plasticity of these excitable axonal domains.

Neuron-glia interactions establish functional membrane domains along myelinated axons. These include nodes of Ranvier, paranodal axoglial junctions and juxtaparanodes. Paranodal junctions are the largest vertebrate junctional adhesion complex, and they are essential for rapid saltatory conduction and contribute to assembly and maintenance of nodes. However, the molecular mechanisms underlying paranodal junction assembly are poorly understood. Ankyrins are cytoskeletal scaffolds traditionally associated with Na(+) channel clustering in neurons and are important for membrane domain establishment and maintenance in many cell types. Here we show that ankyrin-B, expressed by Schwann cells, and ankyrin-G, expressed by oligodendrocytes, are highly enriched at the glial side of paranodal junctions where they interact with the essential glial junctional component neurofascin 155. Conditional knockout of ankyrins in oligodendrocytes disrupts paranodal junction assembly and delays nerve conduction during early development in mice. Thus, glial ankyrins function as major scaffolds that facilitate early and efficient paranodal junction assembly in the developing CNS.

The precise and remarkable subdivision of myelinated axons into molecularly and functionally distinct membrane domains depends on axoglial junctions that function as barriers. However, the molecular basis of these barriers remains poorly understood. Here, we report that genetic ablation and loss of axonal βII spectrin eradicated the paranodal barrier that normally separates juxtaparanodal K(+) channel protein complexes located beneath the myelin sheath from Na(+) channels located at nodes of Ranvier. Surprisingly, the K(+) channels and their associated proteins redistributed into paranodes where they colocalized with intact Caspr-labeled axoglial junctions. Furthermore, electron microscopic analysis of the junctions showed intact paranodal septate-like junctions. Thus, the paranodal spectrin-based submembranous cytoskeleton comprises the paranodal barriers required for myelinated axon domain organization.

Rapid action potential propagation in myelinated axons requires Na⁺ channel clustering at nodes of Ranvier. However, the mechanism of clustering at CNS nodes remains poorly understood. Here, we show that the assembly of nodes of Ranvier in the CNS involves three mechanisms: a glia-derived extracellular matrix (ECM) complex containing proteoglycans and adhesion molecules that cluster NF186, paranodal axoglial junctions that function as barriers to restrict the position of nodal proteins, and axonal cytoskeletal scaffolds (CSs) that stabilize nodal Na⁺ channels. We show that while mice with a single disrupted mechanism had mostly normal nodes, disruptions of the ECM and paranodal barrier, the ECM and CS, or the paranodal barrier and CS all lead to juvenile lethality, profound motor dysfunction, and significantly reduced Na⁺ channel clustering. Our results demonstrate that ECM, paranodal, and axonal cytoskeletal mechanisms ensure robust CNS nodal Na⁺ channel clustering., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Guillain-Barré syndrome is divided into two subtypes: acute inflammatory demyelinating polyneuropathy, and acute motor axonal neuropathy. Autoantibodies to gangliosides GM1, GM1b, GD1a, or GalNAc-GD1a were proposed as serologic markers of acute motor axonal neuropathy in adults. In a previous study of Japanese children with Guillain-Barré syndrome, acute motor axonal neuropathy was associated with anti-GM1 immunoglobulin G antibodies. Larger, comprehensive studies are required to confirm this finding. The present study revealed that immunoglobulin G antibodies were against GM1 (34%), GM1b (22%), GD1a (25%), GalNAc-GD1a (13%), and any of these (44%) in 32 Japanese children with Guillain-Barré syndrome. Patients who had the autoantibodies more often manifested previous diarrhea (71% vs 11%, P = 0.001), acute motor axonal neuropathy (64% vs 11%, P = 0.003), and slower recovery (healthy at final follow-up: 29% vs 78%, P = 0.011; able to run with minor signs, 64% vs 11%, P = 0.003) than patients who did not. The clinical features were consistent with those in adults carrying anti-ganglioside antibodies. Anti-ganglioside antibody testing may help predict outcomes in pediatric patients with Guillain-Barré syndrome who prefer not to undergo repeated nerve-conduction studies.

Paranodal junctions of myelinated nerve fibers are important for saltatory conduction and function as paracellular and membrane protein diffusion barriers flanking nodes of Ranvier. The formation of these specialized axoglial contacts depends on the presence of three cell adhesion molecules: neurofascin 155 on the glial membrane and a complex of Caspr and contactin on the axon. We isolated axonal and glial membranes highly enriched in these paranodal proteins and then used mass spectrometry to identify additional proteins associated with the paranodal axoglial junction. This strategy led to the identification of three novel components of the paranodal cytoskeleton: ankyrinB, alphaII spectrin, and betaII spectrin. Biochemical and immunohistochemical analyses revealed that these proteins associate with protein 4.1B in a macromolecular complex that is concentrated at central and peripheral paranodal junctions in the adult and during early myelination. Furthermore, we show that the paranodal localization of ankyrinB is disrupted in Caspr-null mice with aberrant paranodal junctions, demonstrating that paranodal neuron-glia interactions regulate the organization of the underlying cytoskeleton. In contrast, genetic disruption of the juxtaparanodal protein Caspr2 or the nodal cytoskeletal protein betaIV spectrin did not alter the paranodal cytoskeleton. Our results demonstrate that the paranodal junction contains specialized cytoskeletal components that may be important to stabilize axon-glia interactions and contribute to the membrane protein diffusion barrier found at paranodes.

Various ganglioside immunization protocols were examined to refine the procedure for establishing an animal model of acute motor axonal neuropathy. The most effective was subcutaneous injection of an emulsion of 2.5mg of bovine brain ganglioside mixtures, keyhole lympet hemocyanin, and complete Freund's adjuvant to Japanese white rabbits, repeated at 3-week intervals. Under that protocol, all the rabbits developed marked flaccid paralysis associated with plasma anti-GM1 IgG antibody. This acute motor axonal neuropathy rabbit model also could be reproduced by the use of incomplete Freund's adjuvant, methylated bovine serum albumin, and New Zealand white rabbits. These results provide useful information for the confirmation of and further research on the model.

Background: Patients with Guillain-Barré syndrome (GBS) after Mycoplasma pneumoniae infection often have antibodies to galactocerebroside (GalC). Electrodiagnosis may show acute inflammatory demyelinating polyneuropathy (AIDP)., Methods: The authors report a patient with acute motor axonal neuropathy (AMAN) after Mycoplasma infection and review seven cases of Mycoplasma-associated GBS. They investigated anti-GalC serology under various conditions associated with Mycoplasma infection., Results: The patient had immunoglobulin (Ig)G and IgM antibodies against GM1 and GalC, which cross-reacted. During the acute phase, IgM selectively immunostained axons. The cholera toxin B-subunit and rabbit anti-GM1 IgG stained a band in the lipid extract from M pneumoniae, indicative of the presence of a GM1 epitope. Six Mycoplasma-associated GBS patients with anti-GalC antibodies had non-AIDP electrodiagnoses, whereas one with Mycoplasma-associated AIDP had no anti-GalC antibodies. Anti-GalC antibodies were positive in two of five patients who had neurologic diseases other than GBS after Mycoplasma infection and in one of 12 who had acute respiratory disease caused by M pneumoniae not followed by a neurologic disease., Conclusions: Anti-GalC antibodies in Mycoplasma-associated GBS may be an epiphenomenon. In certain cases, anti-GM1 antibodies induced by molecular mimicry with M pneumoniae may cause acute motor axonal neuropathy.

Macrophages in the periaxonal space and surrounding intact myelin sheath are the most prominent pathological feature of acute motor axonal neuropathy (AMAN). We describe this characteristic in nerve roots from paralyzed rabbits immunized with bovine brain ganglioside or GM1. IgG was deposited on nerve root axons. Distal nerve conduction was preserved, and late F wave components were absent during the acute phase. Initial lesions were located mainly on nerve root axons, as in human AMAN. This study thus provides supportive evidence that the rabbits constitute a model of AMAN.

We report two cases of pharyngeal-cervical-brachial (PCB) variant of Guillain-Barré syndrome (GBS). The patients developed dysphagia and weakness of the neck and arms subsequent to Campylobacter jejuni infection. Oropharyngeal palsy recovered poorly. Electrophysiological findings demonstrated axonal conduction failure. Anti-GD1a immunoglobulin G (IgG) antibody was detected in one case, and anti-GM1b IgG antibody in another. Anti-GT1a IgG and immunoglobulin M (IgM) antibodies were negative in both cases. The current cases suggest that the PCB and axonal variants of GBS form a continuous spectrum from the viewpoint of electrophysiological studies as well as antiganglioside serology.

Some humans develop the axonal form of Guillain-Barré syndrome after receiving bovine brain ganglioside. On sensitization with the ganglioside mixture, all of a group of rabbits injected developed high anti-GM1 IgG antibody titers, flaccid limb weakness of acute onset, and a monophasic illness course. Pathological findings for the peripheral nerves showed predominant Wallerian-like degeneration, with neither lymphocytic infiltration nor demyelination. IgG was deposited on the axons of the anterior roots, and GM1 was proved to be present on the axons of peripheral nerves. Sensitization with purified GM1 also induced axonal neuropathy, indicating that GM1 was the immunogen in the mixture. A model of human axonal Guillain-Barré syndrome has been established that uses inoculation with a bovine brain ganglioside mixture or isolated GM1. This model may help to clarify the molecular pathogenesis of the syndrome and to develop new treatments for it.