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. © 2013 Wiley Periodicals, Inc.     

Multiply myelinated axons in the optic nerve of mice deficient for the myelin-associated glycoprotein

We recently reported that some retinal ganglion cell axons in mice deficient for the myelin-associated glycoprotein are concentrically surrounded by more than one myelin sheath. In the present study, we demonstrate that myelin sheaths displaced from the axon reveal a normal ultrastructure of compact myelin, with the only exception that multiple myelination of axons frequently correlates with the presence of unfused regions of major dense lines. Supernumerary sheaths terminated on other sheaths or on astrocyte cell surfaces in a pattern closely resembling the morphology of a true paranode. The thickness of compact myelin of multiply myelinated axons was significantly increased when compared with axons of similar caliber surrounded by a single myelin sheath. Our observations demonstrate that maintenance of compact myelin and paranodal regions is not dependent on direct axonal contact and that the presence of more than one concentric myelin sheath around an axon results in dysregulation of the axon-to-fiber ratio. © 1995 Wiley-Liss, Inc.     

Myelination-dependent axonal membrane specializations demonstrated in insufficiently myelinated nerves of the dystrophic mouse

‘Dystrophic’ mice of the 129/ReJ-Dy strain have a genetic defect affecting Schwann cell proliferation. Spinal nerve roots of these animals contain myelinated and unmyelinated axons in addition to groups of large ‘amyelinated’ axons. In affected regions of the spinal roots, myelinated axons are missing their myelin sheaths. Where the myelination terminates or begins, half-nodes are created. Freeze-fracture analysis of these half-nodes shows that only the myelinated side contains rows of dimeric particles in the axonal P-face of the paranode. The P-face on the amyelinated side of a half-node, and the remainder of the amyelinated axon, contains a dense even distribution of particles, many of which are the size of dimeric-particle subunits, but only a few of which are arranged into short rows. As the long circumferential rows are not found on the unmyelinated side of the half-node we conclude that the paranodal rows of dimeric particles are dependent upon myelination for their organization.     

Rapid disruption of axon-glial integrity in response to mild cerebral hypoperfusion

Myelinated axons have a distinct protein architecture essential for action potential propagation, neuronal communication, and maintaining cognitive function. Damage to myelinated axons, associated with cerebral hypoperfusion, contributes to age-related cognitive decline. We sought to determine early alterations in the protein architecture of myelinated axons and potential mechanisms after hypoperfusion. Using a mouse model of hypoperfusion, we assessed changes in proteins critical to the maintenance of paranodes, nodes of Ranvier, axon-glial integrity, axons, and myelin by confocal laser scanning microscopy. As early as 3 d after hypoperfusion, the paranodal septate-like junctions were damaged. This was marked by a progressive reduction of paranodal Neurofascin signal and a loss of septate-like junctions. Concurrent with paranodal disruption, there was a significant increase in nodal length, identified by Nav1.6 staining, with hypoperfusion. Disruption of axon-glial integrity was also determined after hypoperfusion by changes in the spatial distribution of myelin-associated glycoprotein staining. These nodal/paranodal changes were more pronounced after 1 month of hypoperfusion. In contrast, the nodal anchoring proteins AnkyrinG and Neurofascin 186 were unchanged and there were no overt changes in axonal and myelin integrity with hypoperfusion. A microarray analysis of white matter samples indicated that there were significant alterations in 129 genes. Subsequent analysis indicated alterations in biological pathways, including inflammatory responses, cytokine-cytokine receptor interactions, blood vessel development, and cell proliferation processes. Our results demonstrate that hypoperfusion leads to a rapid disruption of key proteins critical to the stability of the axon-glial connection that is mediated by a diversity of molecular events.     

Recent progress on the molecular organization of myelinated axons

The structure of myelinated axons was well described 100 years ago by Ramón y Cajal, and now their molecular organization is being revealed. The basal lamina of myelinating Schwann cells contains laminin-2, and their abaxonal/outer membrane contains two laminin-2 receptors, alpha6beta4 integrin and dystroglycan. Dystroglycan binds utrophin, a short dystrophin isoform (Dp116), and dystroglycan-related protein 2 (DRP2), all of which are part of a macromolecular complex. Utrophin is linked to the actin cytoskeleton, and DRP2 binds to periaxin, a PDZ domain protein associated with the cell membrane. Non-compact myelin--found at incisures and paranodes--contains adherens junctions, tight junctions, and gap junctions. Nodal microvilli contain F-actin, ERM proteins, and cell adhesion molecules that may govern the clustering of voltage-gated Na+ channels in the nodal axolemma. Na(v)1.6 is the predominant voltage-gated Na+ channel in mature nerves, and is linked to the spectrin cytoskeleton by ankyrinG. The paranodal glial loops contain neurofascin 155, which likely interacts with heterodimers composed of contactin and Caspr/paranodin to form septate-like junctions. The juxtaparanodal axonal membrane contains the potassium channels Kv1.1 and Kv1.2, their associated beta2 subunit, as well as Caspr2. Kv1.1, Kv1.2, and Caspr2 all have PDZ binding sites and likely interact with the same PDZ binding protein. Like Caspr, Caspr2 has a band 4.1 binding domain, and both Caspr and Caspr2 probably bind to the band 4.1 B isoform that is specifically found associated with the paranodal and juxtaparanodal axolemma. When the paranode is disrupted by mutations (in cgt-, contactin-, and Caspr-null mice), the localization of these paranodal and juxtaparanodal proteins is altered: Kv1.1, Kv1.2, and Caspr2 are juxtaposed to the nodal axolemma, and this reorganization is associated with altered conduction of myelinated fibers. Understanding how axon-Schwann interactions create the molecular architecture of myelinated axons is fundamental and almost certainly involved in the pathogenesis of peripheral neuropathies.     

Paranodal dysmyelination in peripheral nerves of Trembler mice

Subtle defects in paranodes of myelinated nerve fibers can cause significant physiological malfunction. We have investigated myelinated fibers in the peripheral nervous system (PNS) of the Trembler mouse, a model of CMT‐1A neuropathy, for evidence of such defects. Ultrastructural analysis shows that the “transverse bands,” which attach the myelin sheath to the axon at the paranodal axoglial junction, are grossly diminished in number in Trembler nerve fibers. Although paranodes often appear to be greatly elongated, it is only a short region immediately adjacent to the node of Ranvier that displays transverse bands. Where transverse bands are missing, the junctional gap widens, thus reducing resistance to short circuiting of nodal action currents during saltatory conduction and increasing the likelihood that axonal K⁺ channels under the myelin sheath will be activated. In addition, we find evidence that structural domains in Trembler axons are incompletely differentiated, consistent with diminution in nodal Na channel density, which could further compromise conduction. Deficiency of transverse bands may also increase susceptibility to disruption of the paranodal junction and retraction of the myelin sheath. We conclude that Trembler PNS myelinated fibers display subtle defects in paranodal and nodal regions that could contribute significantly to conduction defects and increased risk of myelin detachment. © 2014 Wiley Periodicals, Inc.     

Spatiotemporal ablation of myelinating glia‐specific neurofascin (NfascNF155) 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. © 2009 Wiley‐Liss, Inc.     

Ultrastructural concomitants of anoxic injury and early post-anoxic recovery in rat optic nerve

To study the effects of anoxia on CNS white matter, we examined the ultrastructure of axons and glial cells in a white matter tract, the rat optic nerve, that was subjected to a standardized anoxic insult in vitro. Previous electrophysiological studies showed that in this model, action potential conduction is rapidly abolished by anoxia, and conduction is restored after reoxygenation in about 30% of axons following a 60-min anoxic period. The present study examined the ultrastructural correlates of anoxic injury and early post-anoxic recovery in this model. Optic nerves examined immediately following 60 min of anoxia displayed numerous large, apparently empty zones located within myelin sheaths adjacent to the axon. The myelin remained compact and retained its periodicity. In some regions, the extracellular space was enlarged. There was mitochondrial swelling with loss of normal cristae. There was also loss of microtubules and, to a smaller degree, of neurofilaments in large-diameter axons. Some nodes of Ranvier in anoxic optic nerves displayed detachment of terminal oligodendroglial loops or retraction of the myelin from the node; the presence of tongue-like processes, extending from nearby cells under the detached myelin loops, suggested a possible role of cell-mediated damage to the paranodal myelin. Bundles of dense astrocyte processes were present, and there was vesicular degeneration of perinodal astrocyte processes. In optic nerves that had been permitted to recover for 60 min in oxygenated Ringers following the anoxic period, empty zones were only rarely observed within myelin sheaths and, when present, were smaller than in optic nerves immediately following 60 min of anoxia. The axoplasm of large fibers continued to show loss of microtubules and neurofilaments, as well as mitochondrial swelling. Myelin appeared normal, and only rare paranodal oligodendroglial processes remained unattached from the axon membrane. These results provide support for the idea that, during anoxia, myelinated axons are damaged with significant injury to cytoskeletal elements, probably due to an influx of calcium. The ultrastructural results, together with our earlier observations on the physiological correlates of anoxia and re-oxygenation, suggest that the development of intramyelinic spaces or damage to paranodes lead to conduction block in the anoxic optic nerve. These results also suggest that repair of these structural abnormalities may provide a morphological basis for the early recovery of conduction that occurs after re-oxygenation.     

Effects of aging on myelinated nerve fibers in monkey primary visual cortex

In monkeys, myelin sheaths of the axons in the vertical bundles of nerve fibers passing through the deeper layers of primary visual cortex show age-related alterations in their structure. These alterations have been examined by comparing the myelin sheaths in young monkeys, 5-10 years old, with those in old monkeys, between 25 and 33 years of age. The age-related alterations are of four basic types. In some sheaths, there is local splitting of the major dense line to accommodate dense cytoplasm derived from the oligodendrocytes. Other sheaths balloon out, and in these locations, the intraperiod line in that part of the sheath opens up to surround a fluid-filled space. Other alterations are the formation of redundant myelin so that a sheath is too large for the enclosed axon and the formation of double sheaths in which one layer of compact myelin is surrounded by another one. These alterations in myelin increase in frequency with the ages of the monkeys, and there is a significant correlation between the breakdown of the myelin and the impairments in cognition exhibited by individual monkeys. This correlation also holds even when the old monkeys, 25 to 33 years of age, are considered as a group. It is suggested that the correlation between the breakdown of myelin in the old monkeys and their impairments in cognition has not to do specifically with visual function but to the role of myelin in axonal conduction throughout the brain. The breakdown of myelin could impair cognition by leading to a change in the conduction rates along axons, resulting in a loss of synchrony in cortical neuronal circuits.     

The node of Ranvier in CNS pathology

Healthy nodes of Ranvier are crucial for action potential propagation along myelinated axons, both in the central and in the peripheral nervous system. Surprisingly, the node of Ranvier has often been neglected when describing CNS disorders, with most pathologies classified simply as being due to neuronal defects in the grey matter or due to oligodendrocyte damage in the white matter. However, recent studies have highlighted changes that occur in pathological conditions at the node of Ranvier, and at the associated paranodal and juxtaparanodal regions where neurons and myelinating glial cells interact. Lengthening of the node of Ranvier, failure of the electrically resistive seal between the myelin and the axon at the paranode, and retraction of myelin to expose voltage-gated K⁺ channels in the juxtaparanode, may contribute to altering the function of myelinated axons in a wide range of diseases, including stroke, spinal cord injury and multiple sclerosis. Here, we review the principles by which the node of Ranvier operates and its molecular structure, and thus explain how defects at the node and paranode contribute to neurological disorders.     

Effects of IDPN-induced axonal swellings on conduction in motor nerve fibers

Paranodal demyelination produces a reduction of conduction velocity and conduction block. The relative proportions of these changes appear to vary among different demyelinating disorders. In this study we have examined the effects on conduction of paranodal demyelination produced by giant axonal swellings. The axonal swellings were induced in rats by administration of beta, beta'-iminodipropionitrile (IDPN). In this experimental model synchronous axonal swellings occur in the proximal region of virtually every alpha-motorneuron without evidence of segmental demyelination or fiber loss. Conduction across the motor neuron was evaluated by two methods: a monosynaptic reflex pathway and intracellular recording from single motor neurons. Increases in the delay across the central region of the monosynaptic reflex pathway began between 2 and 4 days after toxin administration. Intracellular studies confirmed that the slowing occurred across the proximal regions of the motor axons; more distal regions of the motor axons were unaffected. The substantial reduction in conduction velocity over the swollen segment occurs with only moderate evidence of conduction block, as assayed by a reduction in the H-reflex/M-response amplitude ratio. Parallel morphological studies showed that in the enlarged fibers the myelin terminal loops maintained contact with the axon but were displaced from the paranodal region into the internode. The appearance of this "passive" paranodal demyelination correlated closely with the increase in conduction delay. We suggest that the contact maintained by the displaced myelin terminal loops with the axolemma allows saltatory conduction to continue, and explains the paucity of conduction block in this model despite the prominent conduction slowing.     

Cation-binding sites in trigeminal ganglia and maxillary nerve: unusual reactivity of perikarya, stem axons and satellite cells

We have used the cupric/ferrocyanide reaction to study cation-binding in trigeminal ganglia and maxillary nerve of adult rats. Unmyelinated axons did not react, whereas myelinated axons were stained at nodal, paranodal or cleft sites. At 'nodal' sites, metallic deposits were found in the axoplasm, along the axolemma, and at the extracellular interfaces of the paranodal myelin. At 'paranodal' sites, particles were concentrated in the paranodal axoplasm and in the intracellular spaces of the myelin loops. Most maxillary axons examined at successive sites had all nodal or all paranodal staining, but 13 of 51 had a mixture. In trigeminal ganglia there was no staining of perineurial sheath, endoneurial cells or mast cells. Satellite cells and their basal laminae were prominently stained, with those around small neurons more reactive than those of large neurons. Patches of neuronal membrane on cell bodies were stained, more often for small than large neurons. The axon hillock and proximal stem axon were not stained in some cases, but approximately half the neurons had staining of perikaryal cytoplasm at the axon hillock or a dense asymmetric band in the proximal stem axon. Strong intraaxonal staining was found at the junction between unmyelinated proximal and myelinated distal stem axon. In distal stem axons, staining was found at the first myelin segment and at each successively thicker myelin segment; staining was mostly weak and paranodal, with intensity proportional to myelin thickness. The T-junction between stem and main myelinated axon had nodal or paranodal patterns; unmyelinated T-junctions were not stained. The varied cation-binding patterns in trigeminal ganglia show unusual properties of satellite cells and important differences between stem and main axons. The results that the cell membrane of axon hillock and proximal stem regions of many sensory large and small neurons may have numerous sodium channels and could affect signal propagation.     

Distribution of sodium channels in chronically demyelinated spinal cord axons: immuno-ultrastructural localization and electrophysiological observations

The immuno-ultrastructural localization of voltage-sensitive sodium channels was demonstrated within a central demyelinating lesion induced in the rat spinal cord by ethidium bromide/irradiation using polyclonal antibody 7493. Antibody 7493 has previously been shown to immunostain intensely axon membrane at nodes of Ranvier, and also perinodal astrocyte processes. At 25–35 days post injection/irradiation, the central portion of the demyelinating lesion is populated with chronically demyelinated axons and there is an absence of glial processes. Sodum channel immunoreactivity was not observed on the chronically demyelinated axolemma within this central portion of the lesion. Within the peripheral portion of the lesion demyelinated axons were occasionally abutted by astrocyte and Schwann cell processes. At these focal sites of apposition, the axon membrane displayed intense sodium channel immunoreactivity, while the abutting astrocyte and Schwann cell processes did not exhibit immunostaining. Also in the periphery of the lesion, some axons become ensheathed and myelinated by oligodendrocytes and Schwann cells. The axon membrane of circumferentially ensheathed axons displayed antibody 7493 immunostaining, and this immunoreactivity persisted on the axolemma until the ensheathing cytoplasmic processes compacted into myelin. Internodal axon membrane beneath the myelin sheath did not display sodium channel immunoreactivity, though (putative) developing nodal axon membrane adjacent to terminal paranodal loops exhibited robust sodium channel staining. Electrophysiological recordings within the ethidium bromide/irradiation lesion demonstrated that at least some axons conducted action potentials within the lesion, while others exhibited conduction block. These results indicate that there is a reorganization of sodium channels within the axon membrane of chronically demyelinated central axons.     

The node of ranvier in early wallerian degeneration: A freeze-fracture study

Summary Using the freeze-freeze-fracture technique, the sciatic nerve of the rat and rabbit was examined distally at 24 h after crush, with particular reference to the node of Ranvier and paranode. The paranodes, in the majority of myelinated fibres, showed a loss of the cytoplasmic circumferential bands and longitudinal columns and their associated membrane pores which characterise the normal Schwann cell surface. Axonal changes consisting of accumulations of axoplasmic organelles occurred at both the node and paranode. At the nodes large intramembraneous partiles in the axolemma (E face) appeared unchanged. Nodal Schwann cell microvilli and paranodal myelin terminal loops were generally unaffected. The findings are discussed in terms of the decrease in amplitude of the action potential which occurs in early Wallerian degeneration.     

Influence of aging on peripheral nerve function and regeneration

Aging deeply influences several morphologic and functional features of the peripheral nervous system (PNS). Morphologic studies have reported a loss of myelinated and unmyelinated nerve fibers in elderly subjects, and several abnormalities involving myelinated fibers, such as demyelination, remyelination and myelin balloon figures. The deterioration of myelin sheaths during aging may be due to a decrease in the expression of the major myelin proteins (P0, PMP22, MBP). Axonal atrophy, frequently seen in aged nerves, may be explained by a reduction in the expression and axonal transport of cytoskeletal proteins in the peripheral nerve. Aging also affects functional and electrophysiologic properties of the PNS, including a decline in nerve conduction velocity, muscle strength, sensory discrimination, autonomic responses, and endoneurial blood flow. The age-related decline in nerve regeneration after injury may be attributed to changes in neuronal, axonal, Schwann cell and macrophage responses. After injury, Wallerian degeneration is delayed in aged animals, with myelin remnants accumulated in the macrophages being larger than in young animals. The interaction between Schwann cells and regenerative axons takes longer, and the amount of trophic and tropic factors secreted by reactive Schwann cells and target organs are lower in older subjects than they are in younger subjects. The rate of axonal regeneration becomes slower and the density of regenerating axons decrease in aged animals. Aging also determines a reduction in terminal and collateral sprouting of regenerated fibers, further limiting the capabilities for target reinnervation and functional restitution. These age-related changes are not linearly progressive with age; the capabilities for axonal regeneration and reinnervation are maintained throughout life, but tend to be delayed and less effective with aging.     

Sulfatide decrease in myelin influences formation of the paranodal axo‐glial junction and conduction velocity in the sciatic nerve

Cerebroside sulfotransferase (CST) catalyzes the production of sulfatide, which is one of the major glycolipids in myelin. Homozygous CST knockout mice were shown to be completely deficient in sulfatide. They were born healthy but began to display progressive neurological deficits from 6 weeks of age. Severe abnormalities of paranodal regions and changes in axonal ion channel distribution were prominent in both the central and peripheral nervous systems. But whether partial decreases in myelin sulfatide levels influence paranodal formation, as well as nerve conduction velocity (NCV), is largely unknown. To determine the functional significance of sulfatide content in myelin, we performed electrophysiological, morphological, and biochemical analyses using heterozygote, homozygote, and wild‐type mouse peripheral nerves and compared the results with individual sulfatide content. NCVs were significantly reduced in homozygote animals compared with wild‐type mice. In contrast, these values were markedly varied in individual heterozygote mice. On the basis of NCV values, we divided heterozygous mice into two groups: mice with mild but significant reduction of NCV and those with normal NCV. Teased nerve fibers obtained from individual mouse sciatic nerves were immunostained, and Na⁺ channel and Caspr cluster lengths were measured to determine abnormal levels of junctional formation at the paranode. Furthermore, sulfatide content in each sciatic nerve was examined by thin layer chromatography. The results demonstrated significant correlations among sulfatide level, severity of paranodal abnormality, and reduction of NCV. Thus, the fine regulation of myelin sulfatide content by CST is important for normal function of myelinated axons. © 2013 Wiley Periodicals, Inc.     

The neuroglial population in the primary visual cortex of the aging rhesus monkey

The effects of age on neuroglial cells have been examined in the primary visual cortices of rhesus monkeys that had been behaviorally tested. The assessment of changes in the neuroglial populations was made on the basis of the frequency of occurrence of profiles of neuroglial cells in semithick sections of osmicated tissue stained with toluidine blue. No changes were found in the numbers of astrocytes and microglial cells with age, but the numbers of oligodendrocytes increased by about 50%. The myelinated nerve bundles at the level of layer 4 were also examined by electron microscopy to assess the effects of age on the nerve fibers. The numbers of nerve fiber profiles showing age-related alterations in their myelin sheaths increase with age. There was also an age-related increase in the frequency of profiles of nerve fibers sectioned through paranodes, indicating that shorter lengths of myelin are being produced by remyelination. These changes in sheaths both correlate significantly with the frequency of oligodendrocyte profiles, suggesting that with age additional oligodendrocytes are required to remyelinate nerve fibers whose sheaths have broken down, probably by death of the original parent oligodendroglial cell. Also the most cognitively impaired monkeys had the greatest numbers of oligodendrocytes, but this is probably a secondary correlation, reflecting the fact that altered myelin slows down the rate of conduction along nerve fibers, which leads to cognitive decline.     

Freeze-fracture characterization of isolated myelin and axolemma membrane fractions

The macromolecular organization of membranes isolated from the rabbit optic nerve and tract was analyzed using the freese-fracture technique. A myelin fraction and two axolemma-enriched fractions were prepared from a preparation of myelinated axons isolated by flotation in a buffered salt-sucrose medium.In the myelinated axon preparation, axolemma and myelin membranes were easily identified. Large areas of the axon membrane and myelin membrane totally lacked intramembronous particles. The particles remaining on the myelin membrane formed patches of evenly distributed elongated and globular particles. In contrast, the particles remaining on the axolemma were globular in shape and tightly clustered. Particle clustering and particle-free areas were not characteristic of either the axolemma or myelin membrane of whole nerves fixed in situ and processed for freeze-fracture.The isolated myelin membrane fraction contained a large number of vesicles completely lacking intramembronous particles. Of the remaining membrane vesicles, profiles with dispersed elongated and globular particles predominated. A small percentage of vesicles displayed intramembranous particles of the same size, shape and clustering pattern as that seen on the axolemma of the myelinated axon preparation. The two axolemma fractions were enriched in membrane containing tightly clustered globular particles. Particle-free vesicles as well as some myelin membrane vesicles were also seen in the axolemma fractions.     

Functions and distribution of voltage-gated sodium and potassium channels in mammalian Schwann cells.

Recent patch-clamp studies on freshly isolated mammalian Schwann cells suggest that voltage-gated sodium and potassium channels, first demonstrated in cells under culture conditions, are present in vivo. The expression of these channels, at least at the cell body region, appears to be dependent on the myelinogenic and proliferative states of the Schwann cell. Specifically, myelin elaboration is accompanied by a down regulation of functional potassium channel density at the cell body. One possibility to account for this is a progressive regionalization of ion channels on a Schwann cell during myelin formation. In adult myelinating Schwann cells, voltage-gated potassium channels appear to be localized at the paranodal region. Theoretical calculations have been made of activity-dependent potassium accumulations in various compartments of a mature myelinated nerve fibre; the largest potassium accumulation occurs not at the nodal gap but rather at the adjacent 2-4 microns length of periaxonal space at the paranodal junction. Schwann cell potassium channels at the paranode may contribute to ionic regulation during nerve activities.     

Nouveautés sur les mécanismes de formation des nœuds de Ranvier

Myelin plays a crucial role in the rapid and saltatory conduction of the nerve impulse along myelinated axons. In addition, myelin closely regulates the organization of the axonal compartments. This organization involves several complex mechanisms including axo-glial contact, diffusion barriers, the cytoskeletal network, and the extracellular matrix. In peripheral nerves, the axo-glial contact dictates the formation of the nodes and the clustering of the voltage-gated sodium channels (Nav). The axo-glial contact at nodes implicates adhesion molecules expressed by the Schwann cell (gliomedin and NrCAM), which binds a partner, neurofascin-186, on the axonal side. This complex is essential for the recruitment of ankyrin-G, a cytoskeletal scaffolding protein, which binds and concentrates Nav channels at nodes. The paranodal junctions flanking the nodes also play a complementary function in node formation. These junctions are formed by the association of contactin-1/caspr-1/neurofascin-155 and create a diffusion barrier, which traps proteins at the nodes and dampens their diffusion along the internode. In the central nervous system, the mechanisms of node formation are different and the formation of the paranodal junctions precedes the aggregation of Nav channels at nodes. However, node formation can still happen in absence of paranodal junctions in the CNS. One explanation is that NF186 interacts with components of the extracellular matrix around the node and thereby stabilizes the aggregation of nodal proteins. It is likely that many other proteins are also implicated in the signaling pathways that regulate the differentiation of the axonal compartments. The nature and function of these proteins are yet to be identified.