Axoglial junctions in the mouse mutant Shiverer

Three-dimensional neuron-astrocyte construction on matrigel enhances establishment of functional voltage-gated sodium channels

This study aimed to investigate and compare cell growth manners and functional differences of primary cortical neurons cultured on either poly-d-lysine (PDL) and or Matrigel, to delineate the role of extracellular matrix on providing resemblance to in vivo cellular interactions in nervous tissue. Primary cortical neurons, obtained from embryonic day 15 mice pups, seeded either on PDL- or Matrigel-coated culture ware were investigated by DIC/bright field and fluorescence/confocal microscopy for their morphology, 2D and 3D structure, and distribution patterns. Patch clamp, western blot, and RT-PCR studies were performed to investigate neuronal firing thresholds and sodium channel subtypes Nav1.2 and Nav1.6 expression. Cortical neurons cultured on PDL coating possessed a 2D structure composed of a few numbers of branched and tortuous neurites that contacted with each other in one to one manner, however, neurons on Matrigel coating showed a more complicated dimensional network that depicted tight, linear axonal bundles forming a 3D interacted neuron-astrocyte construction. This difference in growth patterns also showed a significant alteration in neuronal firing threshold which was recorded between 80 < Iinj > 120 pA on PDL and 2 < Iinj > 160 pA on Matrigel. Neurons grown up on Matrigel showed increased levels of sodium channel protein expression of Nav1.2 and Nav1.6 compared to neurons on PDL. These results have demonstrated that a 3D interacted neuron-astrocyte construction on Matrigel enhances the development of Nav1.2 and Nav1.6 in vitro and decreases neuronal firing threshold by 40 times compared to conventional PDL, resembling in vivo neuronal networks and hence would be a better in vitro model of adult neurons.

Preferential conduction block of myelinated axons by nitric oxide

Conduction block by nitric oxide (NO) was examined in myelinated and unmyelinated axons from both the central nervous system and peripheral nervous system. In rat vagus nerves, mouse optic nerves at P12-P23, adult and developing mouse sciatic nerves, and mouse spinal cords, myelinated fibers were preferentially blocked reversibly by concentrations of NO similar to those encountered in inflammatory lesions. The possibility that these differences between myelinated and unmyelinated axons are due to the normal developmental substitution of Na⁺ channel subtype Na_v 1.6 for Na_v 1.2 at nodes of Ranvier was tested by repeating experiments on mice null for Na_v 1.6. Results were unchanged in this mutant. In shiverer optic nerve, which has only scattered regions with nodes of Ranvier, only the fastest component of the compound action potential was reduced. NO was compared with three other methods of blocking conduction: low Na⁺ , high K⁺ , and tetrodotoxin (TTX). In each of these three cases, unmyelinated axons lost conduction simultaneously with myelinated fibers. From changes in conduction velocity in myelinated axons as they were blocked, it was ascertained that NO acted most similarly to TTX. It was concluded that NO likely interacts with axonal Na⁺ channels through an intermediate that is associated with myelin. © 2016 Wiley Periodicals, Inc.

Node of Ranvier disruption as a cause of neurological diseases

Dysfunction and/or disruption of nodes of Ranvier are now recognized as key contributors to the pathophysiology of various neurological diseases. One reason is that the excitable nodal axolemma contains a high density of Nav (voltage-gated Na⁺ channels) that are required for the rapid and efficient saltatory conduction of action potentials. Nodal physiology is disturbed by altered function, localization, and expression of voltage-gated ion channels clustered at nodes and juxtaparanodes, and by disrupted axon–glial interactions at paranodes. This paper reviews recent discoveries in molecular/cellular neuroscience, genetics, immunology, and neurology that highlight the critical roles of nodes of Ranvier in health and disease.

Organization and maintenance of molecular domains in myelinated axons

Over a century ago, Ramon y Cajal first proposed the idea of a directionality involved in nerve conduction and neuronal communication. Decades later, it was discovered that myelin, produced by glial cells, insulated axons with periodic breaks where nodes of Ranvier (nodes) form to allow for saltatory conduction. In the peripheral nervous system (PNS), Schwann cells are the glia that can either individually myelinate the axon from one neuron or ensheath axons of many neurons. In the central nervous system (CNS), oligodendrocytes are the glia that myelinate axons from different neurons. Review of more recent studies revealed that this myelination created polarized domains adjacent to the nodes. However, the molecular mechanisms responsible for the organization of axonal domains are only now beginning to be elucidated. The molecular domains in myelinated axons include the axon initial segment (AIS), where various ion channels are clustered and action potentials are initiated; the node, where sodium channels are clustered and action potentials are propagated; the paranode, where myelin loops contact with the axolemma; the juxtaparanode (JXP), where delayed-rectifier potassium channels are clustered; and the internode, where myelin is compactly wrapped. Each domain contains a unique subset of proteins critical for the domain's function. However, the roles of these proteins in axonal domain organization are not fully understood. In this review, we highlight recent advances on the molecular nature and functions of some of the components of each axonal domain and their roles in axonal domain organization and maintenance for proper neuronal communication.

Structural bases for central nervous system malfunction in the quaking mouse: dysmyelination in a potential model of schizophrenia

The dysmyelinating mouse mutant quaking (qk) is thought to be a model of schizophrenia based on diminution of CNS myelin (Andreone et al., 2007) and downregulation of the Qk gene (Haroutunian et al., 2006) in the brains of schizophrenic patients. The purpose of this study was to identify specific structural defects in the qk mouse CNS that could compromise physiologic function and that in humans might account for some of the cognitive defects characteristic of schizophrenia. Ultrastructural analysis of qk mouse CNS myelinated fibers shows abnormalities in nodal, internodal, and paranodal regions, including marked variation in myelin thickness among neighboring fibers, spotty disruption of paranodal junctions, abnormal distribution of nodal and paranodal ion channel complexes, generalized thinning and incompactness of myelin, and on many axonal profiles complete absence of myelin. These structural defects are likely to cause abnormalities in conduction velocity, synchrony of activation, temporal ordering of signals, and other physiological parameters. We conclude that the structural abnormalities described are likely to be responsible for significant functional impairment both in the qk mouse CNS and in the human CNS with comparable myelin pathology.

Role of transverse bands in maintaining paranodal structure and axolemmal domain organization in myelinated nerve fibers: effect on longevity in dysmyelinated mutant mice

The consequences of dysmyelination are poorly understood and vary widely in severity. The shaking mouse, a quaking allele, is characterized by severe central nervous system (CNS) dysmyelination and demyelination, a conspicuous action tremor, and seizures in approximately 25% of animals, but with normal muscle strength and a normal lifespan. In this study we compare this mutant with other dysmyelinated mutants including the ceramide sulfotransferase deficient (CST-/-) mouse, which are more severely affected behaviorally, to determine what might underlie the differences between them with respect to behavior and longevity. Examination of the paranodal junctional region of CNS myelinated fibers shows that "transverse bands," a component of the junction, are present in nearly all shaking paranodes but in only a minority of CST-/- paranodes. The number of terminal loops that have transverse bands within a paranode and the number of transverse bands per unit length are only moderately reduced in the shaking mutant, compared with controls, but markedly reduced in CST-/- mice. Immunofluorescence studies also show that although the nodes of the shaking mutant are somewhat longer than normal, Na(+) and K(+) channels remain separated, distinguishing this mutant from CST-/- mice and others that lack transverse bands. We conclude that the essential difference between the shaking mutant and others more severely affected is the presence of transverse bands, which serve to stabilize paranodal structure over time as well as the organization of the axolemmal domains, and that differences in the prevalence of transverse bands underlie the marked differences in progressive neurological impairment and longevity among dysmyelinated mouse mutants.

Early events in node of Ranvier formation during myelination and remyelination in the PNS

Action potential conduction velocity increases dramatically during early development as axons become myelinated. Integral to this process is the clustering of voltage-gated Na(+) (Nav) channels at regularly spaced gaps in the myelin sheath called nodes of Ranvier. We show here that some aspects of peripheral node of Ranvier formation are distinct from node formation in the CNS. For example, at CNS nodes, Nav1.2 channels are detected first, but are then replaced by Nav1.6. Similarly, during remyelination in the CNS, Nav1.2 channels are detected at newly forming nodes. By contrast, the earliest Nav-channel clusters detected during developmental myelination in the PNS have Nav1.6. Further, during PNS remyelination, Nav1.6 is detected at new nodes. Finally, we show that accumulation of the cell adhesion molecule neurofascin always precedes Nav channel clustering in the PNS. In most cases axonal neurofascin (NF-186) accumulates first, but occasionally paranodal neurofascin is detected first. We suggest there is heterogeneity in the events leading to Nav channel clustering, indicating that multiple mechanisms might contribute to node of Ranvier formation in the PNS.

Disposition of axonal caspr with respect to glial cell membranes: Implications for the process of myelination

Neurofascin-155 (NF155) and caspr are transmembrane proteins found at discrete locations early during development of the nervous system. NF155 is present in the oligodendrocyte cell body and processes, whereas caspr is on the axonal surface. In mature nerves, these proteins are clustered at paranodes, flanking the node of Ranvier. To understand how NF155 and caspr become localized to the paranodal regions of myelinated nerves, we have studied their distribution over time in myelinating cultures. Our observations indicate that these two proteins are recruited to the cell surface at the contact zone between axons and oligodendrocytes, where they trans-interact. This association explains the early pattern of caspr distribution, a helical coil that winds around the axon, resembling the turns of the myelin sheath. Caspr, an axonal membrane protein, therefore seems to move in register with the overlying myelinating cell via its interactions with myelin proteins. We suggest that NF155 is the glial cell membrane protein responsible for caspr distribution. The pair act as interacting partners on either side of the axoglial contact area. Most likely, there are other proteins on the axonal surface whose distribution is equally influenced by interaction with the nascent myelin sheath. The fact that caspr follows the movement of the spiraling membrane has a direct affect on the interpretation of the way in which myelin is formed.

Differential changes in axonal conduction following CNS demyelination in two mouse models

Transgenic and disease model mice have been used to investigate the molecular mechanisms of demyelinating diseases. However, less attention has been given to elucidating changes in nerve conduction in these mice. We established an experimental system to measure the response latency of cortical neurons and examined changes in nerve conduction in cuprizone-induced demyelinating mice and in myelin basic protein-deficient shiverer mice. Stimulating and recording electrodes were placed in the right and left sensori-motor cortices, respectively. Electrical stimulation of the right cortex evoked antidromic responses in left cortical neurons with a latency of 9.38 +/- 0.31 ms (n = 107; mean +/- SEM). While response latency was longer in mice at 7 days and 4 weeks of cuprizone treatment (12.35 +/- 0.35 ms, n = 102; 11.72 +/- 0.29 ms, n = 103, respectively), response latency at 7 days and 4 weeks after removal of cuprizone was partially restored (10.72 +/- 0.45 ms, n = 106; 10.27 +/- 0.34 ms, n = 107, respectively). Likewise, electron microscopy showed cuprizone-induced demyelination in the corpus callosum and nearly complete remyelination after cuprizone removal. We also examined whether the myelin abnormalities in shiverer mice affected their response latencies. But there were no significant differences in response latencies in shiverer (9.83 +/- 0.24 ms, n = 103) and wild-type (9.33 +/- 0.22 ms, n = 112) mice. The results of these electrophysiological assessments imply that different demyelinating mechanisms, differentially affecting axon conduction, are present in the cuprizone-treated and shiverer mice, and may provide new insights to understanding the pathophysiology of demyelination in animal models in the CNS.

Embryonic stem cell rescue of tremor and ataxia in myelin-deficient shiverer mice

Transplantation of neural precursor cells has been proposed as a possible approach for replacing missing or damaged central nervous system myelin. Neonatal and adult myelin-deficient shiverer (shi) mice, bearing a mutation of the myelin basic protein (MBP) gene, have been used extensively as hosts for testing cell engraftment, migration, and myelination, but relatively little progress has been made in reversing shi motor deficits. Here we describe a prenatal cell replacement strategy, showing that embryonic stem cells injected into shi blastocyst embryos can generate chimeric mice with strong and widespread immunoreactive MBP expression throughout the brain and a behavioral (motor) phenotype that appears essentially rescued.

Myelination of congenitally dysmyelinated spinal cord axons by adult neural precursor cells results in formation of nodes of Ranvier and improved axonal conduction

Emerging evidence suggests that cell-based remyelination strategies may be a feasible therapeutic approach for CNS diseases characterized by myelin deficiency as a result of trauma, congenital anomalies, or diseases. Although experimental demyelination models targeted at the transient elimination of oligodendrocytes have suggested that transplantation-based remyelination can partially restore axonal molecular structure and function, it is not clear whether such therapeutic approaches can be used to achieve functional remyelination in models associated with long-term, irreversible myelin deficiency. In this study, we transplanted adult neural precursor cells (aNPCs) from the brain of adult transgenic mice into the spinal cords of adult Shiverer (shi/shi) mice, which lack compact CNS myelin. Six weeks after transplantation, the transplanted aNPCs expressed oligodendrocyte markers, including MBP, migrated extensively along the white matter tracts of the spinal cord, and formed compact myelin. Conventional and three-dimensional confocal and electron microscopy revealed axonal ensheathment, establishment of paranodal junctional complexes leading to de novo formation of nodes of Ranvier, and partial reconstruction of the juxtaparanodal and paranodal molecular regions of axons based on Kv1.2 and Caspr (contactin-associated protein) expression by the transplanted aNPCs. Electrophysiological recordings revealed improved axonal conduction along the transplanted segments of spinal cords. We conclude that myelination of congenitally dysmyelinated adult CNS axons by grafted aNPCs results in the formation of compact myelin, reconstruction of nodes of Ranvier, and enhanced axonal conduction. These data suggest the therapeutic potential of aNPCs to promote functionally significant myelination in CNS disorders characterized by longstanding myelin deficiency.

Disruption of axo-glial junctions causes cytoskeletal disorganization and degeneration of Purkinje neuron axons

Axo-glial junctions (AGJs) play a critical role in the organization and maintenance of molecular domains in myelinated axons. Neurexin IV/Caspr1/paranodin (NCP1) is an important player in the formation of AGJs because it recruits a paranodal complex implicated in the tethering of glial proteins to the axonal membrane and cytoskeleton. Mice deficient in either the axonal protein NCP1 or the glial ceramide galactosyltransferase (CGT) display disruptions in AGJs and severe ataxia. In this article, we correlate these two phenotypes and show that both NCP1 and CGT mutants develop large swellings accompanied by cytoskeletal disorganization and degeneration in the axons of cerebellar Purkinje neurons. We also show that alphaII spectrin is part of the paranodal complex and that, although not properly targeted, this complex is still formed in CGT mutants. Together, these findings establish a physiologically relevant link between AGJs and axonal cytoskeleton and raise the possibility that some neurodegenerative disorders arise from disruption of the AGJs.

Functional specialization of the axon initial segment by isoform-specific sodium channel targeting

Voltage-dependent sodium channels cluster at high density at axon initial segments, where propagating action potentials are thought to arise, and at nodes of Ranvier. Here, we show that the sodium channel Na(v)1.6 is precisely localized at initial segments of retinal ganglion cells (RGCs), whereas a different isoform, Na(v)1.2, is found in the neighboring unmyelinated axon. During development, initial segments first expressed Na(v)1.2, and Na(v)1.6 appeared later, approximately in parallel with the onset of repetitive RGC firing. In Shiverer mice, Na(v)1.6 localization at the initial segment was unaffected, although Na(v)1.6 expression was severely disrupted in the aberrantly myelinated optic nerve. Targeting or retention of Na(v)1.6 requires molecular interactions that normally occur only at initial segments and nodes of Ranvier. Expression at nodes but not initial segments exhibits an additional requirement for intact myelination. Because of their high density at the initial segment, Na(v)1.6 channels may be crucial in determining neuronal firing properties.

Nodal sodium channel domain integrity depends on the conformation of the paranodal junction, not on the presence of transverse bands

Our understanding of the role that axoglial interactions play in node of Ranvier formation and maintenance remains incomplete. Previous studies of CNS myelinated fibers of CGT-null mice showed abnormalities in the arrangement of paranodal myelin loops and absence of a conspicuous component of the paranodal junction, the ridge-like intercellular transverse bands. Axolemmal sodium channel domains were largely preserved at nodes of Ranvier but displayed some abnormalities in form. Using a combination of freeze-fracture and immunocytochemical methods, we have found additional evidence documenting abnormalities in the size, shape, and location of axolemmal sodium channel clusters in CGT-null mice as well as evidence that these nodal abnormalities are complementary to the organization of paranodal myelin loops, despite the absence of transverse bands. We conclude that the differentiated form of the nodal axolemma and the distribution of axolemmal sodium channels depend on the conformation of paranodal axoglial contacts but not on the presence of transverse bands at the sites of contact.

Molecular constituents of the node of Ranvier

The interaction between neurons and glial cells that results in myelin formation represents one of the most remarkable intercellular events in development. This is especially evident at the primary functional site within this structure, the node of Ranvier. Recent experiments have revealed a surprising level of complexity within this zone, with several components, including ion channels, sequestered with a very high degree of precision and sharply demarcated borders. We discuss the current state of knowledge of the cellular and molecular mechanisms responsible for the formation and maintenance of the node. In normal axons, Na+ channels are present at high density within the nodal gap, and voltage-dependent K+ channels are sequestered on the internodal side of the paranode--a region known as the juxtaparanode. Modifying the expression of certain surface adhesion molecules that have been recently identified, markedly alters this pattern. There is a special emphasis on contactin, a protein with multiple roles in the nervous system. In central nervous system (CNS) myelinated fibers, contactin is localized within both the nodal gap and paranodes, and appears to have unique functions in each zone. New experiments on contactin-null mutant mice help to define these mechanisms.

CNS myelinogenesis in vitro: myelin basic protein deficient shiverer oligodendrocytes

The shiverer mutant mouse is an autosomal recessive mutant characterized by incomplete myelin sheath formation in the central nervous system (CNS). Such mice contain a deletion in the MBP gene, do not produce MBP proteins, and have little or no compact myelin in the CNS. To investigate the myelin sheath formation in shiverer mutant mice resulting from the absence of compact myelin, firstly we developed new methods for generating oligodendrocyte precursor cells (OPCs) from an E17 mouse brain, and examined homozygous shiverer (shi/shi) OPCs with respect to myelinogenesis in vitro. After treatment of shi/shi OPCs in vitro with PDGF or bFGF, proliferation of shi/shi OPCs was enhanced similar to that observed in wild-type OPCs. The majority of cells from the shiverer mutant mouse, however, remained as A2B5-immunoreactive early OPCs. To determine which molecular events affect the differentiation of shi/shi OPCs, we determined the signaling pathway that could be responsible for activating myelin sheath-specific proteins. We found that the developmental schedule of shi/shi OPCs in vitro was accelerated by the addition of cyclic AMP analogs, dibutyryl cAMP (dbcAMP). Treatment of shi/shi OPCs with dbcAMP had significant effect on the differentiation of OPCs that became MAG-expressing oligodendrocytes. To further determine the possible mechanism involved in the activation of MAG by dbcAMP, we examined the cAMP-dependent signaling cascades. The activation of JNK was markedly stimulated by treatment with dbcAMP, and the phosphorylation of transcription factor ATF-2 was also stimulated by dbcAMP. We demonstrated that the MAG-positive shi/shi oligodendrocytes extend processes around axons and finally covered the axon, this was clearly observed by immunocytochemistry of shi/shi oligodendrocyte-DRG cocultures. These results suggest that ATF-2 coupled to specific signal transduction cascades plays an important regulatory role in MAG expression at a specific stage of shi/shi oligodendrocyte differentiation, and OPCs grow to become myelin-forming cells with numerous cell processes that wraps around an axon to form a thin myelin sheath.

Genetic dysmyelination alters the molecular architecture of the nodal region

We have examined the molecular organization of axons in the spinal cords of myelin-deficient (md) rats, which have profound CNS dysmyelination associated with oligodendrocyte cell death. Although myelin sheaths are rare, most large axons are at least partially surrounded by oligodendrocyte processes. At postnatal day 7 (P7), almost all node-like clusters of voltage-gated Na+ channels and ankyrinG are adjacent to axonal segments ensheathed by oligodendrocytes, but at P21, many node-like clusters are found in axonal segments that lack oligodendrocyte ensheathment. In P21 wild-type (WT) rats, the voltage-gated Na+ channels Na(v)1.2, Na(v)1.6, and Na(v)1.8, are found in different subpopulations of myelinated axons, and md rats have a similar distribution. The known molecular components of paranodes--contactin, Caspr, and neurofascin 155--are not clustered in md spinal cords, and no septate-like junctions between oligodendrocyte processes and axons are found by electron microscopy. Furthermore, Kv1.1 and Kv1.2 K+ channels are not spatially segregated from the node-like clusters of Na+ channels in md rats, in contrast to their WT littermates. These results suggest the following: node-like clusters of voltage-gated Na+ channels and ankyrinG form adjacent to ensheathed axonal segments even in the absence of a myelin sheath; these clusters persist after oligodendrocyte cell death; dysmyelination does not alter the expression of different nodal of voltage-gated Na+ channels; the absence of paranodes results in the mislocalization of neurofascin155, contactin, and Caspr, and the aberrant localization of Kv1.1 and Kv1.2.

Contactin orchestrates assembly of the septate-like junctions at the paranode in myelinated peripheral nerve

Rapid nerve impulse conduction depends on specialized membrane domains in myelinated nerve, the node of Ranvier, the paranode, and the myelinated internodal region. We report that GPI-linked contactin enables the formation of the paranodal septate-like axo-glial junctions in myelinated peripheral nerve. Contactin clusters at the paranodal axolemma during Schwann cell myelination. Ablation of contactin in mutant mice disrupts junctional attachment at the paranode and reduces nerve conduction velocity 3-fold. The mutation impedes intracellular transport and surface expression of Caspr and leaves NF155 on apposing paranodal myelin disengaged. The contactin mutation does not affect sodium channel clustering at the nodes of Ranvier but alters the location of the Shaker-type Kv1.1 and Kv1.2 potassium channels. Thus, contactin is a crucial part in the machinery that controls junctional attachment at the paranode and ultimately the physiology of myelinated nerve.

Compact myelin dictates the differential targeting of two sodium channel isoforms in the same axon

Voltage-dependent sodium channels are uniformly distributed along unmyelinated axons, but are highly concentrated at nodes of Ranvier in myelinated axons. Here, we show that this pattern is associated with differential localization of distinct sodium channel alpha subunits to the unmyelinated and myelinated zones of the same retinal ganglion cell axons. In adult axons, Na(v)1.2 is localized to the unmyelinated zone, whereas Na(v)1.6 is specifically targeted to nodes. During development, Na(v)1.2 is expressed first and becomes clustered at immature nodes of Ranvier, but as myelination proceeds, Na(v)1.6 replaces Na(v)1.2 at nodes. In Shiverer mice, which lack compact myelin, Na(v)1.2 is found throughout adult axons, whereas little Na(v)1.6 is detected. Together, these data show that sodium channel isoforms are differentially targeted to distinct domains of the same axon in a process associated with formation of compact myelin.

An oligodendrocyte cell adhesion molecule at the site of assembly of the paranodal axo-glial junction

Two major isoforms of the cell adhesion molecule neurofascin NF186 and NF155 are expressed in the central nervous system (CNS). We have investigated their roles in the assembly of the node of Ranvier and show that they are targeted to distinct domains at the node. At the onset of myelination, NF186 is restricted to neurons, whereas NF155 localizes to oligodendrocytes, the myelin-forming glia of the CNS. Coincident with axon ensheathment, NF155 clusters at the paranodal regions of the myelin sheath where it localizes in apposition to the axonal adhesion molecule paranodin/contactin-associated protein (Caspr1), which is a constituent of the septate junction-like axo-glial adhesion zone. Immunoelectron microscopy confirmed that neurofascin is a glial component of the paranodal axo-glial junction. Concentration of NF155 with Caspr1 at the paranodal junctions of peripheral nerves is also a feature of Schwann cells. In Shiverer mutant mice, which assemble neither compact CNS myelin nor normal paranodes, NF155 (though largely retained at the cell body) is also distributed at ectopic sites along axons, where it colocalizes with Caspr1. Hence, NF155 is the first glial cell adhesion molecule to be identified in the paranodal axo-glial junction, where it likely interacts with axonal proteins in close association with Caspr1.

Ion channel sequestration in central nervous system axons

Na+ and K+ channel localization and clustering are essential for proper electrical signal generation and transmission in CNS myelinated nerve fibres. In particular, Na+ channels are clustered at high density at nodes of Ranvier, and Shaker-type K+ channels are sequestered in juxtaparanodal zones, just beyond the paranodal axoglial junctions. The mechanisms of channel localization at nodes of Ranvier in the CNS during development in both normal and hypomyelinating mutant animals are discussed and reviewed. As myelination proceeds, Na+ channels are initially found in broad zones within gaps between neighbouring oligodendroglial processes, and then are condensed into focal clusters. This process appears to depend on the formation of axoglial junctions. K+ channels are first detected in juxtaparanodal zones, and in mutant mice lacking normal axoglial junctions, these channels fail to cluster. In these mice, despite the presence of numerous oligodendrocytes, Na+ channel clusters are rare, and when present, are highly irregular. A number of molecules have recently been described that are candidates for a role in the neuron-glial interactions driving ion channel clustering. This paper reviews the cellular and molecular events responsible for formation of the mature node of Ranvier in the CNS.

Dependence of nodal sodium channel clustering on paranodal axoglial contact in the developing CNS

Na(+) channel clustering at nodes of Ranvier in the developing rat optic nerve was analyzed to determine mechanisms of localization, including the possible requirement for glial contact in vivo. Immunofluorescence labeling for myelin-associated glycoprotein and for the protein Caspr, a component of axoglial junctions, indicated that oligodendrocytes were present, and paranodal structures formed, as early as postnatal day 7 (P7). However, the first Na(+) channel clusters were not seen until P9. Most of these were broad, and all were excluded from paranodal regions of axoglial contact. The number of detected Na(+) channel clusters increased rapidly from P12 to P22. During this same period, conduction velocity increased sharply, and Na(+) channel clusters became much more focal. To test further whether oligodendrocyte contact directly influences Na(+) channel distributions, nodes of Ranvier in the hypomyelinating mouse Shiverer were examined. This mutant has oligodendrocyte-ensheathed axons but lacks compact myelin and normal axoglial junctions. During development Na(+) channel clusters in Shiverer mice were reduced in numbers and were in aberrant locations. The subcellular location of Caspr was disrupted, and nerve conduction properties remained immature. These results indicate that in vivo, Na(+) channel clustering at nodes depends not only on the presence of oligodendrocytes but also on specific axoglial contact at paranodal junctions. In rats, ankyrin-3/G, a cytoskeletal protein implicated in Na(+) channel clustering, was detected before Na(+) channel immunoreactivity but extended into paranodes in non-nodal distributions. In Shiverer, ankyrin-3/G labeling was abnormal, suggesting that its localization also depends on axoglial contact.

Hypomyelination alters K+ channel expression in mouse mutants shiverer and Trembler

Voltage-gated K+ channels are localized to juxtaparanodal regions of myelinated axons. To begin to understand the role of normal compact myelin in this localization, we examined mKv1.1 and mKv1.2 expression in the dysmyelinating mouse mutants shiverer and Trembler. In neonatal wild-type and shiverer mice, the focal localization of both proteins in axon fiber tracts is similar, suggesting that cues other than mature myelin can direct initial K+ channel localization in shiverer mutants. In contrast, K+ channel localization is altered in hypomyelinated axonal fiber tracts of adult mutants, suggesting that abnormal myelination leads to channel redistribution. In shiverer adult, K+ channel expression is up-regulated in both axons and glia, as revealed by immunocytochemistry, RNase protection, and in situ hybridization studies. This up-regulation of K+ channels in hypomyelinated axon tracts may reflect a compensatory reorganization of ionic currents, allowing impulse conduction to occur in these dysmyelinating mouse mutants.

Sodium channel density in hypomyelinated brain increased by myelin basic protein gene deletion

Trophic control over the expression and membrane distribution of voltage-dependent ion channels is one of the principal organizing events underlying the maturation of excitable cells. The myelin sheath is a major structural determinant of regional ion channel topography in central axons, but the exact molecular signals that mediate local interactions between the oligodendrocyte and axolemma are not known. We have found that large caliber fibre pathways in the brain of the mutant mouse shiverer (shi, gene on chromosome 18), whose developmental fate of myelination is averted by deletion of five exons in the myelin basic protein gene, have a striking excess of sodium channels. As cytoplasmic membranes of shiverer oligodendroglia still adhere to axons, the evidence indicates that myelin basic protein or a myelin basic protein-dependent glial transmembrane signal associated with compact myelin formation, rather than a simple glial-axon contact inhibition or an intrinsic genetic program of neuronal differentiation, could be critical in downregulating sodium channel density in axons. Here we use the shiverer mutant to show that mature central nervous system projection neurons with large caliber unmyelinated fibres sustain functional excitability by increasing sodium channel density. This axon plasticity, triggered by the absence of a single glial protein, contributes to the unexpectedly mild degree of neurological impairment in the mutant brain without myelin, and may be a potentially inducible mechanism determining the recovery of function from dysmyelinating disease.

Axolemmal abnormalities in myelin mutants

Evidence is reviewed that the paranodal axoglial junction plays important roles in the differentiation and function of myelinated axons. In myelin-deficient axons, ion flux across the axolemma is greater than that in myelinated fibers because a larger proportion of the axolemma is active during continuous, as opposed to saltatory, conduction. In addition, older myelin-deficient rats that have developed spontaneous seizures display small foci of node-like E-face particle accumulations in CNS axons as well as more diffuse regions of increased particle density and number. Assuming that the E-face particles represent sodium channels, such regions could underlie high sodium current density during activity, low threshold for excitation, and increased extracellular potassium accumulation. Depending on the degree of spontaneous channel opening, they could also represent sites of spontaneous generation of activity. The appearance of seizures and their gradual increase in frequency and severity could represent an increase in the number of such regions. In addition, diminution in the dimensions of the extracellular space during maturation would result in increased extracellular resistance, which, together with increasing axonal diameter, would tend to increase the likelihood of ephaptic interaction among neighboring axons as well as the likelihood of extracellular potassium rises to levels that could cause spontaneous activity.

The dysmyelinating mouse mutations shiverer (shi) and myelin deficient (shimld)

Shiverer (shi/shi) is an autosomal recessive mouse mutation that produces a shivering phenotype in affected mice. A shivering gait can be seen from a few weeks after birth until their early death, which occurs between 50 and 100 days. The central nervous system of the mutant mouse is hypomyelinated but the peripheral nervous system appears normal. The myelin of the CNS, wherever present, is not well compacted and lacks the major dense line. Myelin basic protein (MBP), which is associated with the major dense line, is absent, and this is due to a deletion of the major part of the gene encoding MBP. Transgenic shiverer mice that have integrated and express the wild-type mouse MBP transgene no longer shiver and have normal life spans. Conversely, normal mice that have integrated an antisense MBP transgene, shiver. Myelin deficient shimld/shimld is allelic to shiverer (shi/shi) but the mutant mouse is less severely affected. Although MBP is present in the CNS, it is low in quantity and is not developmentally regulated. The gene encoding MBP has been both duplicated and inverted. Transgenic shimld/shimld mice with the wild-type MBP transgene have normal phenotypes.

Patchy myelination pattern in the jimpy mouse brain: immunohistochemical study

The jimpy (jp) mutation of the mouse leads to a dramatic decrease of myelination in the hemizygous mutant central nervous system (CNS). Several descriptions based on classical histology, immunohistochemistry, and electron microscopy (EM) have demonstrated the scarcity of myelin formation in the different parts of the CNS. The immunohistochemical study presented here showed a very singular patchy pattern of myelin distribution in the different areas of the whole mutant brain. The myelin patches are randomly dispersed without bilateral symmetry, and their density and location vary from one animal to another. No reproducible pattern of myelination could be found among the population observed. This distribution has been compared with observations on young heterozygotes and wild-type homozygotes from the same strain. A similar patchy and random distribution of myelin could be observed in heterozygotes, which present an intermediate level of myelination. This strongly suggests that a migration of precursors or immature oligodendrocytes (ODCs) from the periventricular zone followed by local multiplication of colonies of ODCs before myelination is a general feature in normal as well as pathological conditions.

Role of glial cells in the differentiation and function of myelinated axons

Myelinated axons are highly differentiated in the vicinity of the node of Ranvier, both structurally and with respect to ion channel distribution. Evidence is reviewed showing that axonal differentiation depends upon two distinct types of interaction between glial cells and the axolemma, one at the node itself, with astrocyte processes, and the second, more extensive one, in the paranodal region, with oligodendrocyte processes. In the peripheral nervous system, Schwann cells fulfill both roles. Glial or Schwann cell abnormalities, due to genetic deficiencies, diseases or experimental procedures, result in corresponding abnormalities in the axolemma and can have devastating effects on nerve fiber function. An example, the myelin-deficient mutant rat, is presented, and the defects underlying the profound and ultimately lethal neurological abnormalities seen in this mutant are discussed in relation to abnormalities in its axoglial interactions.

Abnormal axoglial junctions in the myelin-deficient rat mutant

Although the myelin-deficient rat displays a gross deficiency of myelin in the CNS, occasional myelin segments of moderate thickness can be found. The typical lamellar pattern, consisting of alternating major dense and intermediate lines, is present in some regions of such segments, but the pattern is abnormal elsewhere. Redundant folds are common, and astrocyte processes occur frequently between the myelin sheath and axolemma or within the sheath. In the paranodal region, myelin lamellae occasionally form a palisade of 'terminal loops' against the axolemma, but discrete transverse bands occur only rarely and regular arrays of transverse bands over an extended length have not been seen. 'Reversed' paranodal junctions occur more often. Here the outermost layer of myelin, instead of being closest to the node, is furthest from it, and successive layers form terminal loops that approach the node progressively. These loops face away from the axon and do not contact it. At paranodal junctions of this kind only the innermost loop, or a small number of inner loops, adjoins the axolemma and, as a result, the size of the paranodal axoglial junction is markedly restricted. These defects in the paranodal junction may underlie the intrusion of astrocyte processes from either end of a myelin segment into the internodal periaxonal space and between myelin lamellae. Thus, one of the normal functions of the paranodal junction may be to restrict extension of astrocyte processes into and beneath myelin segments. The myelin-deficient rat also exhibits node-like specializations of the axolemma in association with glial cell processes.

Myelination by transplanted fetal and neonatal oligodendrocytes in a dysmyelinating mutant

Demyelination is a major feature of CNS injury and disease, including multiple sclerosis. To examine the potential for myelination by transplanted oligodendrocytes, initially described by Gumpel et al., we have transplanted neonatal cortex of mice with normal myelin into a dysmyelinating mutant, the shiverer mouse. We have found that oligodendrocyte precursors mature and synthesize myelin following transplantation. Immunostaining with antibodies to myelin basic protein (MBP), neurofilament protein and glial fibrillary acidic protein, demonstrates myelination both within the graft and extending out into the host, axonal sprouting from the graft which parallels the MBP-reactivity, and minimal astrocytic proliferation in response to the transplant.

Dependence of axolemmal differentiation on contact with glial cells in chronically demyelinated lesions of cat spinal cord

Chronically demyelinated lesions of cat dorsal columns were created by focal injection of the glial toxin ethidium bromide. Freeze-fracture studies show that the center of the lesion, which is devoid of glial cells and processes, contains axons having neither node-like nor paranodal-type membrane specializations. Near the margin of the lesion, however, where axons are in contact with glial cells, the axolemma sometimes displays focal accumulations of E- and P-face particles resembling those at nodes of Ranvier. In cases where the adjacent cell could be identified, it had the characteristics of an astrocyte. Linear indentations of the axolemma displaying a paracrystalline pattern like that of the paranodal axolemma also occur in the marginal region. Here, the adjacent cell had the characteristics of an oligodendrocyte. These specializations may be closely associated with each other or spatially separate. Normal nodal and paranodal specializations were absent throughout the lesion at all time periods examined. These findings support the view that both the formation and the maintenance of nodal and paranodal axon membrane specializations require contact with glial cells.

Axo-glial relations in the retina-optic nerve junction of the adult rat: freeze-fracture observations on axon membrane structure

The axolemmal ultrastructure of nerve fibres within the retina-optic nerve junction (ROJ) from adult rats was examined by freeze-fracture electron microscopy. In the juxtaocular (proximal) region of the ROJ, all fibres are unmyelinated. The axons generally have a membrane ultrastructure similar to that of retinal nerve fibre layer axons, with a high density of intramembranous particles (IMPs) on the P-fracture face and a low density of IMPs on the E-face. However, along some axons in this region of the ROJ, localized aggregations of E-face IMPs are observed. At levels of the ROJ closer to the optic nerve proper, the unmyelinated fibres enter a transition zone in which the axons acquire myelin sheaths. By the distal boundary of the transitional zone (optic nerve proper), virtually all fibres are myelinated. Within the transitional zone, conventional axo-glial associations and axolemmal ultrastructure is present at nodes of Ranvier. In addition, atypical axo-glial relationships and atypical nodal segments are observed in this region. At some nodes, an isolated oligodendroglial process, the axolemma usually displays a paranodal-like ultrastructure. Finger-like oligodendroglial processes were also observed in association with non-nodal unmyelinated axon membrane. At these sites of association, the axon membrane tends to be indented and may have a paranodal-like morphology. Nodal axolemma may exhibit several atypical forms in the transition zone. At some nodes, the nodal axolemma has a low density of E-face particles. Also, nodes of extended linear length (approximately 2 micron) exhibit a lower-than-normal density of P-face IMPs. At heminodes, the axolemma immediately adjacent to the terminal loops lacks the usual nodal characteristics of high IMP density and high percentage of large particles. The results show that aberrant axo-glial associations accompanied by unusual ultrastructural characteristics of the axolemma are present in the ROJ of normal adult rats.

Reversible diabetic nerve dysfunction: structural correlates to electrophysiological abnormalities

Structural alterations of the nodal and paranodal areas were examined in the posterior tibial nerve in insulin-depleted and insulin-treated diabetic BB rats. The early metabolic phase of the distal symmetrical polyneuropathy was characterized by paranodal axonal swellings and nodal bulgings of the axon. These alterations correlate with intraaxonal sodium accumulation and decreased sodium equilibrium potentials which account for the early nerve conduction defect. Both the structural and electrophysiological abnormalities were completely normalized after vigorous insulin therapy. In the chronic diabetic polyneuropathy the paranodal area showed loss of paranodal axoglial junctions and paranodal myelin retraction. These changes may be partially responsible for the impaired electrical activity at the node as exemplified by irreversibly impaired sodium permeability and nerve conduction.

Structural specializations in cat of chronically demyelinated spinal cord axons as seen in freeze-fracture replicas

Axons in chronically demyelinated spinal cord lesions, induced by ethidium bromide injection, display patches of intramembranous particles and indentations resembling nodal and paranodal axolemmal specializations respectively. Both occur in the marginal region of the lesions where the demyelinated axons are intimately associated with astrocytic and oligodendrocytic processes, and probably correspond to the aberrant node-like and paranodal junctional complexes seen in thin sections of this region. Demyelinated axons in the center of the lesion, which are not in contact with glial processes, do not display these membrane specializations.

'Shaking pups': a disorder of central myelination in the spaniel dog. IV. Freeze-fracture electron microscopic studies of axons, oligodendrocytes and astrocytes in the spinal cord white matter

Large axons identified in freeze-fracture replicas of the spinal cord white matter of 'shaking pups' were encircled by spiral processes at the paranodes that appeared to arise from oligodendrocytes. In the axolemma adjacent to some of these processes there were paranode-like intramembranous specializations and node-like accumulations of E-face particles; in other instances, no special contacts formed between axons and the spiral processes. The outer surfaces of the spiral processes, which often apposed astrocytes, frequently contained gap junctions. Although many of the abnormalities identified in this dysmyelinating mutant by freeze-fracture electron microscopy are probably secondary to a more fundamental defect of myelin formation, the prominence of the spiral processes suggests that the encircling of axons by oligodendrocytes may be an independent state of ensheathment and not a passive effect of myelin formation.

Development of nodal and paranodal membrane specializations in amphibian peripheral nerves

Peripheral nerves from the hind legs of frog tadpoles were examined in order to ascertain the pattern of development of nodal and paranodal specializations in myelinated fibers. In thin sections the earliest detectable node-related specializations resemble "intermediate" junctions between axons and Schwann cell processes. These occur in individually ensheathed axons near the edges of the sheath segments and could represent early nodal or paranodal components or transient structures. The characteristic nodal "undercoating" is indistinct and highly variable in thickness in immature fibers and its density is lower in developing nodes than in adult nodes. Corresponding freeze-fracture replicas of developing axons demonstrate aggregates of nodal E face particles whose concentration is lower than that in the adult. Such aggregates usually occur immediately adjacent to Schwann cell indentations, even though early in development the latter may not exhibit the paracrystalline pattern seen in the adult paranodal axolemma. On rare occasions, node-like particle aggregates and presumptive nodal undercoatings have been observed without recognizable paranodal junctions or indentations nearby. However, neither specialization has been found in axons not individually ensheathed by Schwann cells. Paranodal Schwann cell loops are widely separated and irregularly arranged in the developing nodes, and the paranodal regions flanking a node usually mature asymmetrically. Differentiated paranodal junctions appear early in axons ensheathed by only a few loose Schwann cell lamellae. However, such junctions are not formed by all paranodal loops; they consistently appear first in the loops close to the node and only later in those further removed. No junctional specialization has been observed in either the axolemma or the Schwann cell membrane without the close association of the other.

Increased axonal proteolysis in myelin-deficient mutant mice

Protein degradation within retinal ganglion cell axons in vitro is 50 to 110 percent faster than normal in mutant mice exhibiting deficiencies of myelin in the central nervous system. Proteolysis is increased proximally and distally within retinal ganglion cell axons of mice carrying the jumpy mutation or its allele, myelin synthesis deficiency, and is increased distally within those axons of quaking mice. The proteolytic defect is axon (neuron)-specific since the rate of protein degradation within glial cells is normal. Increased axonal proteolysis does not bear a simple relation to hypomyelination since shiverer, another mouse mutant deficient in central myelin, displayed normal rates of axonal protein degradation under the same conditions. These observations suggest an abnormal axon-glial interaction in mice with primary glial defects and raise the possibility that the functioning of histologically normal axons (neurons) may be altered in dysmyelinating diseases.