Rows of dimeric-particles within the axolemma and juxtaposed particles within glia, incorporated into a new model for the paranodal glial-axonal junction at the node of Ranvier

Excitable domains of myelinated nerves: axon initial segments and nodes of Ranvier

Neurons are highly polarized cells. They can be subdivided into at least two structurally and functionally distinct domains: somatodendritic and axonal domains. The somatodendritic domain receives and integrates upstream input signals, and the axonal domain generates and relays outputs in the form of action potentials to the downstream target. Demand for quick response to the harsh surroundings prompted evolution to equip vertebrates' neurons with a remarkable glia-derived structure called myelin. Not only Insulating the axon, myelinating glia also rearrange the axonal components and elaborate functional subdomains along the axon. Proper functioning of all theses domains and subdomains is vital for a normal, efficient nervous system.

Electron tomography of paranodal septate-like junctions and the associated axonal and glial cytoskeletons in the central nervous system

The polarized domains of myelinated axons are specifically organized to maximize the efficiency of saltatory conduction. The paranodal region is directly adjacent to the node of Ranvier and contains specialized septate-like junctions that provide adhesion between axons and glial cells and that constitute a lateral diffusion barrier for nodal components. To complement and extend earlier studies on the peripheral nervous system, electron tomography was used to image paranodal regions from the central nervous system (CNS). Our three-dimensional reconstructions revealed short filamentous linkers running directly from the septate-like junctions to neurofilaments, microfilaments, and organelles within the axon. The intercellular spacing between axons and glia was measured to be 7.4 ± 0.6 nm, over twice the value previously reported in the literature (2.5-3.0 nm). Averaging of individual junctions revealed a bifurcated structure in the intercellular space that is consistent with a dimeric complex of cell adhesion molecules composing the septate-like junction. Taken together, these findings provide new insight into the structural organization of CNS paranodes and suggest that, in addition to providing axo-glial adhesion, cytoskeletal linkage to the septate-like junctions may be required to maintain axonal domains and to regulate organelle transport in myelinated axons.

Molecular disruptions of the panglial syncytium block potassium siphoning and axonal saltatory conduction: pertinence to neuromyelitis optica and other demyelinating diseases of the central nervous system

The panglial syncytium maintains ionic conditions required for normal neuronal electrical activity in the central nervous system (CNS). Vital among these homeostatic functions is "potassium siphoning," a process originally proposed to explain astrocytic sequestration and long-distance disposal of K(+) released from unmyelinated axons during each action potential. Fundamentally different, more efficient processes are required in myelinated axons, where axonal K(+) efflux occurs exclusively beneath and enclosed within the myelin sheath, precluding direct sequestration of K(+) by nearby astrocytes. Molecular mechanisms for entry of excess K(+) and obligatorily-associated osmotic water from axons into innermost myelin are not well characterized, whereas at the output end, axonally-derived K(+) and associated osmotic water are known to be expelled by Kir4.1 and aquaporin-4 channels concentrated in astrocyte endfeet that surround capillaries and that form the glia limitans. Between myelin (input end) and astrocyte endfeet (output end) is a vast network of astrocyte "intermediaries" that are strongly inter-linked, including with myelin, by abundant gap junctions that disperse excess K(+) and water throughout the panglial syncytium, thereby greatly reducing K(+)-induced osmotic swelling of myelin. Here, I review original reports that established the concept of potassium siphoning in unmyelinated CNS axons, summarize recent revolutions in our understanding of K(+) efflux during axonal saltatory conduction, then describe additional components required by myelinated axons for a newly-described process of voltage-augmented "dynamic" potassium siphoning. If any of several molecular components of the panglial syncytium are compromised, K(+) siphoning is blocked, myelin is destroyed, and axonal saltatory conduction ceases. Thus, a common thread linking several CNS demyelinating diseases is the disruption of potassium siphoning/water transport within the panglial syncytium. Continued progress in molecular identification and subcellular mapping of glial ion and water channels will lead to a better understanding of demyelinating diseases of the CNS and to development of improved treatment regimens.

Analysis of peripheral nerve expression profiles identifies a novel myelin glycoprotein, MP11

The myelin sheath insulates axons and allows for rapid salutatory conduction in the nervous system of all vertebrates. The formation of peripheral myelin requires expression of the transcription factor Egr2, which is responsible for inducing such essential myelin-associated genes as Mpz, Mbp, Pmp22, and Mag. Using microarray analysis to compare gene expression patterns in peripheral nerve during development, during remyelination after nerve injury, and in a congenital hypomyelinating mouse model, we identified an evolutionarily conserved novel component of myelin called Mp11 (myelin protein of 11 kDa). The Mp11 genomic locus contains multiple conserved Egr binding sites, and Mp11 induction is regulated by the expression of Egr2. Similar to other Egr2-dependent genes, it is induced during developmental myelination and remyelination after nerve injury. Mp11 is a glycoprotein expressed preferentially in the myelin of the peripheral nervous system versus CNS and is specifically localized to the Schmidt-Lanterman incisures and paranodes of peripheral nerve. The Mp11 protein contains no identifiable similarity to other known protein domains or motifs. However, like other myelin genes, strict Mp11 expression levels are a requirement for the in vitro myelination of DRG neurons, indicating that this previously uncharacterized gene product is a critical component of peripheral nervous system myelin.

Electron tomographic analysis of cytoskeletal cross-bridges in the paranodal region of the node of Ranvier in peripheral nerves

The node of Ranvier is a site for ionic conductances along myelinated nerves and governs the saltatory transmission of action potentials. Defects in the cross-bridging and spacing of the cytoskeleton are a prominent pathological feature in diseases of the peripheral nerve. Electron tomography was used to examine cytoskeletal-cytoskeletal, membrane-cytoskeletal, and heterologous cell connections in the paranodal region of the node of Ranvier in peripheral nerves. Focal attachment of cytoskeletal filaments to each other and to the axolemma and paranodal membranes of the Schwann cell via narrow cross-bridges was visualized in both neuronal and glial cytoplasm. A subset of intermediate filaments associates with the cytoplasmic surfaces of supramolecular complexes of transmembrane structures that are presumed to include known and unknown junctional proteins. Mitochondria were linked to both microtubules and neurofilaments in the axoplasm and to neighboring smooth endoplasmic reticulum by narrow cross-bridges. Tubular cisternae in the glial cytoplasm were also linked to the paranodal glial cytoplasmic loop juxtanodal membrane by short cross-bridges. In the extracellular matrix between axon and Schwann cell, junctional bridges formed long cylinders linking the two membranes. Interactions between cytoskeleton, membranes, and extracellular matrix associations in the paranodal region are likely critical not only for scaffolding, but also for intracellular and extracellular communication.

Development of a model for microphysiological simulations: small nodes of ranvier from peripheral nerves of mice reconstructed by electron tomography

The node of Ranvier is a complex structure found along myelinated nerves of vertebrate animals. Specific membrane, cytoskeletal, junctional, extracellular matrix proteins and organelles interact to maintain and regulate associated ion movements between spaces in the nodal complex, potentially influencing response variation during repetitive activations or metabolic stress. Understanding and building high resolution three dimensional (3D) structures of the node of Ranvier, including localization of specific macromolecules, is crucial to a better understanding of the relationship between its structure and function and the macromolecular basis for impaired conduction in disease. Using serial section electron tomographic methods, we have constructed accurate 3D models of the nodal complex from mouse spinal roots with resolution better than 7.5 nm. These reconstructed volumes contain 75-80% of the thickness of the nodal region. We also directly imaged the glial axonal junctions that serve to anchor the terminal loops of the myelin lamellae to the axolemma. We created a model of an intact node of Ranvier by truncating the volume at its midpoint in Z, duplicating the remaining volume and then merging the new half volume with mirror symmetry about the Z-axis. We added to this model the distribution and number of Na+ channels on this reconstruction using tools associated with the MCell simulation program environment. The model created provides accurate structural descriptions of the membrane compartments, external spaces, and formed structures enabling more realistic simulations of the role of the node in modulation of impulse propagation than have been conducted on myelinated nerve previously.

Association of TAG-1 with Caspr2 is essential for the molecular organization of juxtaparanodal regions of myelinated fibers

Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier. Here, we show the role in this process of TAG-1, a glycosyl-phosphatidyl-inositol-anchored cell adhesion molecule. In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems. In contrast, the localization of protein 4.1B, an axoplasmic partner of Caspr2, was only moderately altered. TAG-1, which is expressed in both neurons and glia, was able to associate in cis with Caspr2 and in trans with itself. Thus, a tripartite intercellular protein complex, comprised of these two proteins, appears critical for axo-glial contacts at juxtaparanodes. This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo-glial interactions.

Protein 4.1B associates with both Caspr/paranodin and Caspr2 at paranodes and juxtaparanodes of myelinated fibres

Caspr/paranodin, a neuronal transmembrane glycoprotein, is essential for the structure and function of septate-like paranodal axoglial junctions at nodes of Ranvier. A closely related protein, Caspr2, is concentrated in juxtaparanodal regions where it associates indirectly with the shaker-type potassium channels. Although ultrastructural studies indicate that paranodal complexes are linked to the cytoskeleton, the intracellular partners of Caspr/paranodin, as well as those of Caspr2, are poorly characterized. We show that the conserved intracellular juxtamembrane regions (GNP motif) of Caspr/paranodin and Caspr2 bind proteins 4.1R and 4.1B. 4.1B is known to be enriched in paranodal and juxtaparanodal regions. 4.1B immunoreactivity accumulates progressively at paranodes and juxtaparanodes during postnatal development, following the concentration of Caspr/paranodin and Caspr2, respectively, in central and peripheral myelinated axons. These two proteins coimmunoprecipitated with 4.1B in brain homogenates. Our results provide strong evidence for the association of 4.1B with Caspr/paranodin at paranodes and with Caspr2 at juxtaparanodes. We propose that 4.1B anchors these axonal proteins to the actin-based cytoskeleton in these two regions.

Development of nodes of Ranvier

The architecture and function of the nodes of Ranvier depend on several specialized cell contacts between the axon and myelinating glial cells. These sites contain highly organized multimolecular complexes of ion channels and cell adhesion molecules, closely connected with the cytoskeleton. Recent findings are beginning to reveal how this organization is achieved during the development of myelinated nerves. The role of membrane proteins involved in axoglial interactions and of associated cytoplasmic molecules is being elucidated, while studies of mutant mice have underlined the importance of glial cells and the specific role of axonal proteins in the organization of axonal domains.

Retention of a cell adhesion complex at the paranodal junction requires the cytoplasmic region of Caspr

An axonal complex of cell adhesion molecules consisting of Caspr and contactin has been found to be essential for the generation of the paranodal axo-glial junctions flanking the nodes of Ranvier. Here we report that although the extracellular region of Caspr was sufficient for directing it to the paranodes in transgenic mice, retention of the Caspr-contactin complex at the junction depended on the presence of an intact cytoplasmic domain of Caspr. Using immunoelectron microscopy, we found that a Caspr mutant lacking its intracellular domain was often found within the axon instead of the junctional axolemma. We further show that a short sequence in the cytoplasmic domain of Caspr mediated its binding to the cytoskeleton-associated protein 4.1B. Clustering of contactin on the cell surface induced coclustering of Caspr and immobilized protein 4.1B at the plasma membrane. Furthermore, deletion of the protein 4.1B binding site accelerated the internalization of a Caspr-contactin chimera from the cell surface. These results suggest that Caspr serves as a "transmembrane scaffold" that stabilizes the Caspr/contactin adhesion complex at the paranodal junction by connecting it to cytoskeletal components within the axon.

Epithelial cell polarity and cell junctions in Drosophila

The polarized architecture of epithelial cells and tissues is a fundamental determinant of animal anatomy and physiology. Recent progress made in the genetic and molecular analysis of epithelial polarity and cellular junctions in Drosophila has led to the most detailed understanding of these processes in a whole animal model system to date. Asymmetry of the plasma membrane and the differentiation of membrane domains and cellular junctions are controlled by protein complexes that assemble around transmembrane proteins such as DE-cadherin, Crumbs, and Neurexin IV, or other cytoplasmic protein complexes that associate with the plasma membrane. Much remains to be learned of how these complexes assemble, establish their polarized distribution, and contribute to the asymmetric organization of epithelial cells.

A molecular view on paranodal junctions of myelinated fibers

The axoglial paranodal junctions, flanking the Ranvier nodes, are specialized adhesion sites between the axolemma and myelinating glial cells. Unraveling the molecular composition of paranodal junctions is crucial for understanding the mechanisms involved in the regulation of myelination, and positioning and segregation of the voltage-gated Na+ and K+ channels, essential for the generation and conduction of action potentials. Paranodin/Caspr was the first neuronal transmembrane glycoprotein identified at the paranodal junctions. Paranodin/Caspr is associated on the axonal membrane with contactin/F3, a glycosylphosphatidylinositol-anchored protein, essential for its correct targeting. The extra and intracellular regions of paranodin encompass multiple domains which can be involved in protein-protein interactions with other axonal proteins and glial proteins. Thus, paranodin plays a central role in the assembly of multiprotein complexes necessary for the formation and maintenance of paranodal junctions.

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.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

Imbalances in N-CAM, SAM and polysialic acid may underlie the paranodal ion channel barrier defect in diabetic neuropathy

Breakdown of protective tissue barrier systems characterizes the chronic diabetic complications affecting the retina, and peripheral and central nerve tracts. The progressive damages to the blood-retina-, blood-nerve-, and paranodal ion channel barriers have pathophysiological consequences for the relentless progression of these complications. The continuing damage to the paranodal ion channel barrier in the spontaneously diabetic BB/W rat is associated with an increasingly irreversible nerve conduction defect, due to impaired nodal Na+ currents associated with displacement of nodal Na+ channels across the damaged paranodal barrier. The structural substrate for the mechanical barrier of the paranode is provided by electron-dense junctional complexes made up by a moiety of neural cell adhesive-(N-CAM), neural-glial adhesive (Ng-CAM), substrate adhesive molecules (SAMs) and polysialic acid (PSA). To further explore the mechanism underlying the protective barrier defect in diabetic neuropathy we examined the expression and immunolocalization of these molecules in peripheral nerve. In 6-month diabetic BB/W rats, direct and indirect ELISAs revealed significantly up-regulated N-CAM (P < 0.05), tenascin (Ng-CAM), (P < 0.001) and N-cadherin (A-CAM) (P < 0.03). On the other hand, SAMs showed little change, except for PSA which showed a significantly (P < 0.03) decreased concentration in the diabetic nerve. Immunocytochemical identification of these molecules revealed no visually detectable differences between diabetic and control rats. In conclusion, these data suggest that imbalances between highly interactive molecules responsible for the adhesiveness between terminal Schwann cell loops and the paranodal axolemma may underlie the critical paranodal barrier defect in diabetic neuropathy.

The axonal membrane protein Caspr, a homologue of neurexin IV, is a component of the septate-like paranodal junctions that assemble during myelination

We have investigated the potential role of contactin and contactin-associated protein (Caspr) in the axonal-glial interactions of myelination. In the nervous system, contactin is expressed by neurons, oligodendrocytes, and their progenitors, but not by Schwann cells. Expression of Caspr, a homologue of Neurexin IV, is restricted to neurons. Both contactin and Caspr are uniformly expressed at high levels on the surface of unensheathed neurites and are downregulated during myelination in vitro and in vivo. Contactin is downregulated along the entire myelinated nerve fiber. In contrast, Caspr expression initially remains elevated along segments of neurites associated with nascent myelin sheaths. With further maturation, Caspr is downregulated in the internode and becomes strikingly concentrated in the paranodal regions of the axon, suggesting that it redistributes from the internode to these sites. Caspr expression is similarly restricted to the paranodes of mature myelinated axons in the peripheral and central nervous systems; it is more diffusely and persistently expressed in gray matter and on unmyelinated axons. Immunoelectron microscopy demonstrated that Caspr is localized to the septate-like junctions that form between axons and the paranodal loops of myelinating cells. Caspr is poorly extracted by nonionic detergents, suggesting that it is associated with the axon cytoskeleton at these junctions. These results indicate that contactin and Caspr function independently during myelination and that their expression is regulated by glial ensheathment. They strongly implicate Caspr as a major transmembrane component of the paranodal junctions, whose molecular composition has previously been unknown, and suggest its role in the reciprocal signaling between axons and glia.

Grid-mapped freeze-fracture analysis of gap junctions in gray and white matter of adult rat central nervous system, with evidence for a "panglial syncytium" that is not coupled to neurons

In white matter regions of the brain and spinal cord of adult mammals, gap junctions previously were observed linking astrocytes to astrocytes, as well as to oligodendrocytes and ependymacytes. The resulting "functional syncytium" was proposed to modulate the ion fluxes that occur during electrical activity of the associated axons. Gap junctions also have been reported linking neurons with glia, and functional neuronal-glial coupling has been postulated. To investigate the glial syncytium and the neuron-to-glial coupling hypotheses, we used "grid-mapped freeze fracture," conventional thin-section electron microscopy, and light microscope immunocytochemistry to examine and characterize neurons and glia in gray and white matter of adult rat brain and spinal cord. We have obtained quantitative evidence for the abundance and widespread distribution of gap junctions interlinking the three primary types of macroglia throughout both gray and white matter of the mammalian central nervous system (CNS), thereby extending the concept to that of a functional panglial syncytium. In contrast to previous reports, we show that of more than 400 gap junctions in which both participating cells were identified, none were between neurons and glia. Thus, neuronal coupling and glial coupling involved separate and distinct pathways. Finally, putative water channels (i.e., "square arrays") were confirmed to be abundant and in close association with gap junctions in astrocytes and ependymacytes. Because the astrocyte "intermediaries" extend cytoplasmic conduits throughout gray and white matter of brain and spinal cord, from the ependymal layer to the pia-glial limitans, and from oligodendrocytes surrounding axons to astrocyte endfeet surrounding capillaries, the proposed panglial syncytium, with its abundance of water channels and intercellular ion channels, is optimally positioned and equipped to modulate water and ion fluxes across broad regions of the CNS.

Paranodin, a glycoprotein of neuronal paranodal membranes

Ranvier nodes are flanked by paranodal regions, at the level of which oligodendrocytes or Schwann cells interact closely with axons. Paranodes play a critical role in the physiological properties of myelinated nerve fibers. Paranodin, a prominent 180 kDa transmembrane neuronal glycoprotein, was purified and cloned from adult rat brain, and found to be highly concentrated in axonal membranes at their junction with myelinating glial cells, in paranodes of central and peripheral nerve fibers. The large extracellular domain of paranodin is related to neurexins, and its short intracellular tail binds protein 4.1, a cytoskeleton-anchoring protein. Paranodin may be a critical component of the macromolecular complex involved in the tight interactions between axons and myelinating glial cells characteristic of the paranodal region.

Glucose transporters in rat peripheral nerve: paranodal expression of GLUT1 and GLUT3

Peripheral nerve depends on glucose oxidation to energize the repolarization of excitable axonal membranes following impulse conduction, hence requiring high-energy demands by the axon at the node of Ranvier. To enter the axon at this site, glucose must be transported from the endoneurial space across Schwann cell plasma membranes and the axolemma. Such transport is likely to be mediated by facilitative glucose transporters. Although immunohistochemical studies of peripheral nerves have detected high levels of the transporter GLUT1 in endoneurial capillaries and perineurium, localization of glucose transporters to Schwann cells or peripheral axons in vivo has not been documented. In this study, we demonstrate that the GLUT1 transporter is expressed in the plasma membrane and cytoplasm of myelinating Schwann cells around the nodes of Ranvier and in the Schmidt-Lanterman incisures, making them potential sites of transcellular glucose transport. No GLUT1 was detected in axonal membranes. GLUT3 mRNA was expressed only at low levels, but GLUT3 polypeptide was barely detected by immunocytochemistry or immunoblotting in peripheral nerve from young adult rats. However, in 13-month-old rats, GLUT3 polypeptide was present in myelinated fibers, endoneurial capillaries, and perineurium. In myelinated fibers, GLUT3 appeared to be preferentially expressed in the paranodal regions of Schwann cells and nodal axons, but was also present in the internodal aspects of these structures. The results of the present study suggest that both Schwann cell GLUT1 and axonal and Schwann cell GLUT3 are involved in the transport of glucose into the metabolically active regions of peripheral axons.

An orderly development of paranodal axoglial junctions and bracelets of Nageotte in the rat sural nerve

The present study was designed to assess the normal development of the paranodal apparatus with particular emphasis on axoglial junctions (AGJs) which constitute the paranodal barrier system. The sural nerve was examined in 10- and 31-day-old rats. During the early phase of myelination AGJ attachment of terminal myelin loops to the axolemma proceeded from the node to the internode. The frequency of terminal loops with AGJ attachment increased with fiber growth. As myelination advanced internodal-most loops became almost 100% attached to the axolemma by AGJs, whereas at the same time an increasing number of nodal-most loops were unattached, suggesting a lack of AGJ formation at this site. The formation of bracelets of Nageotte increased with the progressive addition of myelin loops. They formed most frequently at the juxtanodal interface between unattached and attached loops, probably reflecting crowding of terminal loops along the unchanged length of the paranodal axolemma. The findings suggest a complex but orderly age- and fiber size-dependent maturation process of the paranode and its structural barrier system. The present data will serve as a basis for the evaluation of this anatomical region in regenerating and remyelinating fibers in various neuropathies.

Development of nodes of Ranvier in feline nerves: an ultrastructural presentation

The ultrastructure of developing nodes of Ranvier and adjacent paranodes of future large myelinated fibers in feline lumbar spinal roots is described. The development starts before birth concurrent with myelination and is finished at the end of the first postnatal month when the nodal regions of future large fibers, now 4-5 microns of diameter, for the first time appear like miniatures of those of their 4 times thicker and fully mature counterparts. At this stage the fibers also begin to show mature functional properties. The latent maturation process is denoted "nodalization" and includes two major events: (1) the formation of a narrow node gap bordered by compact myelin segments and filled with Schwann cell microvilli that interconnect an undercoated nodal axolemma with rapidly increasing accumulations of mitochondria lodging in the longitudinal cords of Schwann cell cytoplasm that is distributed outside a more and more crenated paranodal myelin sheath; (2) the setting of a fixed number of nodes along the axons; an event that includes segmental axonal and myelin sheath degeneration and is concluded by the elimination of supernumerary Schwann cells.

Nodal Na(+)-channel displacement is associated with nerve-conduction slowing in the chronically diabetic BB/W rat: prevention by aldose reductase inhibition

Chronic nerve conduction showing in experimental diabetic neuropathy has been associated with decreased nodal Na+ permeability and an ultrastructurally identifiable loss of axo-glial junctions, which comprise the paranodal voltage channel barrier separating nodal Na+ channels from paranodal K+ channels. In human and experimental diabetic neuropathy these structural changes of the paranodal apparatus correlate closely with the nerve conduction defect. The present immunocytochemical study of the alpha-subunit of the Na+ channel examined whether the breach of the voltage channel barrier may account for a shift in the distribution of Na+ channels explaining decreased nodal Na+ permeability. Biobreeding Wistar (BB/W) rats diabetic for 4-8 months showed a progressive redistribution of nodal Na+ channels across the paranodal barrier into the paranodal and internodal domains which was associated with chronic nerve conduction slowing. The present data suggest that structural damage to the paranodal barrier system in diabetic nerve facilitates the lateral displacement of Na+ channels from the nodal axolemma thereby diminishing their nodal density and the nodal Na+ permeability associated with the chronic nerve conduction defect in experimental diabetes. These abnormalities were prevented by the treatment with an aldose reductase inhibitor, belonging to a class of drugs that, in neuropathic patients, improves nerve-conduction velocity and repairs axo-glial dysjunction of the paranodal apparatus.

Molecular anatomy and genetics of myelin proteins in the peripheral nervous system

Myelin contains a number of proteins, the major examples of which are protein zero (Po), P2 protein, peripheral myelin protein 22 (PMP22), myelin basic proteins (MBPs), myelin-associated glycoprotein (MAG) and the recently described connexin 32 (Cx32). This list is probably still incomplete. The localisation and possible functions of these proteins are reviewed. In the past few years a number of inherited demyelinating neuropathies in mice and the human have been shown to be due to mutations affecting the genes PMP22, Po and Cx32 so that it has become possible to characterise the molecular pathology of the majority of these disorders. This has provided important insights into the relationships between the structure of myelin and the function of its constituent proteins.

Molecular dissection of the myelinated axon

The membrane of the myelinated axon expresses a rich repertoire of physiologically active molecules: (1) Voltage-sensitive NA+ channels are clustered at high density (approximately 1,000/microns 2) in the nodal axon membrane and are present at lower density (< 25/microns 2) in the internodal axon membrane under the myelin. Na+ channels are also present within Schwann cell processes (in peripheral nerve) and perinodal astrocyte processes (in the central nervous system) which contact the Na+ channel-rich axon membrane at the node. In some demyelinated fibers, the bared (formerly internodal) axon membrane reorganizes and expresses a higher-than-normal Na+ channel density, providing a basis for restoration of conduction. The presence of glial cell processes, adjacent to foci of Na+ channels in immature and demyelinated axons, suggests that glial cells participate in the clustering of Na+ channels in the axon membrane. (2) "Fast" K+ channels, sensitive to 4-aminopyridine, are present in the paranodal or internodal axon membrane under the myelin; these channels may function to prevent reexcitation following action potentials, or participate in the generation of an internodal resting potential. (3) "Slow" K+ channels, sensitive to tetraethylammonium, are present in the nodal axon membrane and, in lower densities, in the internodal axon membrane; their activation produces a hyperpolarizing afterpotential which modulates repetitive firing. (4) The "inward rectifier" is activated by hyperpolarization. This channel is permeable to both Na+ and K+ ions and may modulate axonal excitability or participate in ionic reuptake following activity. (5) Na+/K(+)-ATPase and (6) Ca(2+)-ATPase are also present in the axon membrane and function to maintain transmembrane gradients of Na+, K+, and Ca2+. (7) A specialized antiporter molecule, the Na+/Ca2+ exchanger, is present in myelinated axons within central nervous system white matter. Following anoxia, the Na+/Ca2+ exchanger mediates an influx of Ca2+ which damages the axon. The molecular organization of the myelinated axon has important pathophysiological implications. Blockade of fast K+ channels and Na+/K(+)-ATPase improves action potential conduction in some demyelinated axons, and block of the Na+/Ca2+ exchanger protects white matter axons from anoxic injury. Modification of ion channels, pumps, and exchangers in myelinated fibers may thus provide an important therapeutic approach for a number of neurological disorders.

Clustering of voltage-dependent sodium channels on axons depends on Schwann cell contact

In myelinated nerves, segregation of voltage-dependent sodium channels to nodes of Ranvier is crucial for saltatory conduction along axons. As sodium channels associate and colocalize with ankyrin at nodes of Ranvier, one possibility is that sodium channels are recruited and immobilized at axonal sites which are specified by the subaxolemmal cytoskeleton, independent of glial cell contact. Alternatively, segregation of channels at distinct sites along the axon may depend on glial cell contact. To resolve this question, we have examined the distribution of sodium channels, ankyrin and spectrin in myelination-competent cocultures of sensory neurons and Schwann cells by immunofluorescence, using sodium channel-, ankyrin- and spectrin-specific antibodies. In the absence of Schwann cells, sodium channels, ankyrin and spectrin are homogeneously distributed on sensory axons. When Schwann cells are introduced into these cultures, the distribution of sodium channels dramatically changes so that channel clusters on axons are abundant, but ankyrin and spectrin remain homogeneously distributed. Addition of latex beads or Schwann cell membranes does not induce channel clustering. Our results suggest that segregation of sodium channels on axons is highly dependent on interactions with active Schwann cells and that continuing axon-glial interactions are necessary to organize and maintain channel distribution during differentiation of myelinated axons.

Three-dimensional fine structure of cytoskeletal-membrane interactions at nodes of Ranvier

Cytoskeleton-membrane-extracellular matrix interactions at the node of Ranvier were examined in both central and peripheral axons by combining three different methods for tissue preparation with three different electron microscopic techniques for imaging supramolecular structure. Conventional and three-dimensional high voltage electron microscopy of thin and semithick sections of tissues stained en bloc with ferric chloride revealed the presence of transcellular structures across the nodal gap traversing the paranodal glial-axonal junction. These structures penetrate both axonal and glial membranes and are further traced to the cortical axoplasm. This observation was verified by an examination of similar regions in rapidly-frozen freeze-substituted fresh axons. The filamentous nature of these structures, their focal attachment to the external true surface of the nodal and paranodal axolemma and their association with membrane particles were visualized in deep etch rotary-shadow replicas. At the node, both extracellular gap-crossing filaments and membrane-cytoskeletal linkers in the nodal axoplasm are joined to one of the prominent membrane particles of the nodal axolemma. At the paranodal axo-glial junction, the anchoring site of these membrane-cytoskeleton linkers are found on the linear arrays of 16 nm particles. Thus, cytoplasmic filaments and extracellular filaments or bridge structures are involved in the membrane-cytoskeletal interaction at the node and paranode. Some of these membrane particles are known to play a role in ionic conductances known to occur at this site. An additional role in cell adhesion or maintenance of the membrane specialization of this functionally important site of axolemma is now indicated.

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.

Distribution and lateral mobility of voltage-dependent sodium channels in neurons

Voltage-dependent sodium channels are distributed nonuniformly over the surface of nerve cells and are localized to morphologically distinct regions. Fluorescent neurotoxin probes specific for the voltage-dependent sodium channel stain the axon hillock 5-10 times more intensely than the cell body and show punctate fluorescence confined to the axon hillock which can be compared with the more diffuse and uniform labeling in the cell body. Using fluorescence photobleaching recovery (FPR) we measured the lateral mobility of voltage-dependent sodium channels over specific regions of the neuron. Nearly all sodium channels labeled with specific neurotoxins are free to diffuse within the cell body with lateral diffusion coefficients on the order of 10(-9) cm2/s. In contrast, lateral diffusion of sodium channels in the axon hillock is restricted, apparently in two different ways. Not only do sodium channels in these regions diffuse more slowly (10(-10)-10(-11) cm2/s), but also they are prevented from diffusing between axon hillock and cell body. No regionalization or differential mobilities were observed, however, for either tetramethylrhodamine-phosphatidylethanolamine, a probe of lipid diffusion, or FITC-succinyl concanavalin A, a probe for glycoproteins. During the maturation of the neuron, the plasma membrane differentiates and segregates voltage-dependent sodium channels into local compartments and maintains this localization perhaps either by direct cytoskeletal attachments or by a selective barrier to channel diffusion.

Freeze-fracture studies on unmyelinated axolemma of rat cervical sympathetic trunk: correlation with saxitoxin binding

The density and diameter distributions of intramembranous particles (IMPs) within unmyelinated axolemma from rat cervical sympathetic trunk were examined with freeze-fracture electron microscopy. The axolemma displays a highly asymmetrical partitioning of IMPs with ca. 1200 IMPs microns-2 on P-faces and ca. 100 IMPs microns-2 on E-faces. Particle sizes (diameters) are unimodally distributed on both fracture faces, with a range from 2.4 nm to 15.6 nm. Approximately 16% of the particles on P-faces and 28% of particles on E-faces are of a large (greater than 9.6 nm) diameter. On both fracture faces, the IMPs appear to be randomly distributed; no aggregations of particles were observed. The results indicate that there are ca. 230 large IMPs microns-2 of unmyelinated axolemma from rat cervical sympathetic trunk. The density of these IMPs is similar to the density of saxitoxin binding sites on unmyelinated axolemma from rat cervical sympathetic trunk (Pellegrino et al. 1984 (Brain Res. 305, 357-360)), which suggests that many of the large diameter particles may be the morphological correlate of voltage-sensitive Na+ channels.

Freeze-fracture studies of reactive myelinated nerve fibres after diffuse axonal injury

We have studied the axonal and myelin sheath response in diffuse axonal injury after angular acceleration using the freeze-fracture and thin section techniques. It was found that the glial-axonal junction was intact until 1 h after injury. But upon loss of the nodal axolemma specialisations, after 3 to 4 h, the dimeric particles of the glial-axonal junction (GAJ) were lost and, by 6 h, the myelin lamellae became separated from the axonal remnant. There was a correlated loss of glial membrane specialisations of the GAJ during this separation. In the internodal region a suggestion of membrane damage occurred after 20 min but discrete myelin dislocations (particle-free areas) were not found until 1-h survival and were extensive by 6 h. Areas of loosely organised myelin occurred between intact axons at 7-28 days after injury. No evidence for growth cone formation was obtained.

Diagnostic levels of ultrasound may disrupt myelination

Neonatal rats 3 to 5 days of age were exposed to the ultrasound beam from a medical ultrasound imaging system. Dorsal nerve roots were examined by electron microscopy. Comparison between exposed and sham-exposed controls revealed disruption of the nodes of Ranvier attributable to ultrasound. Morphologic changes ranged from vacuole formation in the paranodal region to frank demyelination and were still evident after 24 h of recovery. Rats of this age are at a stage of myelination similar to that of a human fetus 4 to 5 months. The ultrasound intensities used in this study are consistent with those used for human imaging (SPTA 0.135 mW/cm2, SATA 0.045 mW/cm2, SPTP 8.7 W/cm2, SPPA 1.9 W/cm2), but the relevance of these findings to clinical ultrasound will require further study.

Node of Ranvier formation along fibres regenerating through silicone tube implants: a freeze-fracture and thin-section electron microscopic study

Thin-section and freeze-fracture electron microscopy have been used to examine the morphogenesis of the node of Ranvier in peripheral nerves regenerating through silicone tubes. A major question posed by this study is whether node formation in fibres regenerating across a gap recapitulates that occurring in normal development. Node formation occurs concurrently with myelination and follows a similar spatial gradient of progression from a proximal to distal direction along the regenerated nerve. Presumptive nodal sites appear prior to myelin formation and are identified as a prominent subaxolemmal density in thin sections and axonal particle patches in freeze-fracture. Following the appearance of presumptive nodes in regenerating fibres, dimeric particles are inserted into the axolemma adjacent to the node. These particles are in close apposition to the overlying Schwann cell terminal processes and with maturity adopt the same circumferential orientation seen in adult nodes. The nodal axolemma of regenerating fibres shows a characteristic increase in the prominence of its subaxolemmal densification and number of heterogeneously sized particles. Mature regenerated nodes demonstrate a complete annulus of nodal particles indistinguishable from control nodes. The results of the present study show that the nodal architecture of regenerating fibres is a faithful reconstruction of normal mature nodes, thus indicating that the morphological correlates associated with saltatory conduction at the node are present in regenerated nodes.

Freeze-fracture characterization of proteolipid protein and basic protein of central nervous system myelin incorporated in liposomes

Proteolipid protein (PLP) and basic protein (BP) of central nervous system myelin were purified from calf brain white matter and incorporated in liposomes of L-dimyristoyl-alpha-phosphatidylcholine (DML) or in liposomes formed with an extract of natural lipids from myelin. Freeze-fracture replicas of the liposomes were prepared to study the number and size of intramembrane protein particles (IMP) in the fracture faces of the lipid bilayer. Globular and elongated IMP were observed in the freeze-fracture liposome membranes after incorporation of proteolipid protein. Globular IMP were the most frequently found (91-96% of the total IMP), and some of them showed a tiny black spot or pit on the top, suggesting the presence of hydrophilic channels in these particles. Globular and elongated IMP were also observed in the fractured membranes when basic protein was incorporated in liposomes. Again, globular IMP were the most frequent (92-95%) but no spots were present on the top. In addition, both globular and elongated IMP generated by basic protein were significantly larger than IMP generated by PLP. The proportion, size and form of globular and elongated particles generated by PLP and BP were unaffected by the amount of protein incorporated in liposomes (0.13-0.75 protein/lipid, w/w) nor by the type of lipid matrix used (DML or myelin natural lipid mixture). Intramembrane particles were absent from membranes of liposomes of pure lipid.

Freeze-fracture study of the mechanoreceptive digital corpuscles of mice

The freeze-fracture replication technique was used to study the mechanoreceptive digital corpuscles in toe pads of mice. The axon terminal plasmalemma had intramembranous particles (IMPs) at a density of 2367 +/- 517 microns-2 (mean +/- S.E.M.) in the P-face and 84 +/- 4 microns-2 in the E-face. Particles were 10 +/- 1.8 nm in diameter in the P-face and 10 +/- 1.5 nm (mean +/- S.D.) in the E-face. Particle-rich and particle-free areas were noted in the P-face. The lamellar cell plasmalemma had IMPs at a density of 3359 +/- 224 microns-2 in the P-face and 265 +/- 95 microns-2 in the E-face. Particles were 10 +/- 1.4 nm in diameter in the P-face and 10 +/- 1.6 nm in the E-face. Non-terminal unmyelinated fibres in the connective tissue compartment of toe pads were also examined: the P-faces of the axolemma and Schwann cell plasmalemma had IMPs at a density of 1356 +/- 283 microns-2 and 1514 +/- 514 microns-2, respectively, while the E-face of these membranes had only a few particles. Particles were 9 +/- 1.2 nm and 10 +/- 1.6 nm in diameter in the P-faces of axon and Schwann cell plasmalemmata, respectively. The results show that the IMPs in terminal axolemma and in lamellar cell plasmalemma have a much higher density than those of non-terminal axons or Schwann cells in myelinated and unmyelinated fibres. In addition, IMPs in the terminal axolemma are larger than those in non-terminal axolemma except for the nodal axolemma. It can be said that plasmalemmata of both the axon terminals and lamellar cells of digital corpuscles are specialized in terms of IMPs, suggesting that they have specific physiological properties in mechanoreceptive functions including mechano-electric transduction.

Internodal microvillus-like Schwann cell fingers in myelinated fibres in mouse spinal roots

Collections of microvillus-like Schwann cell fingers identical to those described previously in the nodal gap substance were commonly found along the internodes of large myelinated fibres in the spinal roots of adult mice. They were covered by Schwann cell basal lamina and focally protruded from the outer cytoplasmic Schwann cell compartment. Unlike nodal Schwann cell fingers, these internodal fingers had no contact with the axolemma, but were directed toward the endoneurium. These were not recognized in the distal peripheral nerves. The frequent occurrence of internodal Schwann cell fingers in the spinal root fibres suggests that these structures may be involved in some electrophysiological regulatory mechanism in this particular region of the nervous system.

Nodes of Ranvier and Schmidt-Lanterman incisures: an in vivo lanthanum tracer study

The permeability of the tight junctional system of myelin, at the juxtanodal myelin terminal loops and Schmidt-Lanterman incisures, was investigated using the ionic tracer lanthanum (a) in vivo followed by fixation, (b) concurrently with fixation, (c) following fixation. Employing the same methods the juxtanodal membrane complex formed between myelin loops and axolemma was also tested. The results of this study demonstrate that the periaxonal space (between axon and Schwann cell) is apparently accessible to lanthanum via the myelin loop-axolemmal junction, irrespective of the mode of exposure of myelinated fibres to the tracer. Similarly, the tight junctions between adjacent myelin terminal loops apparently do not prevent lanthanum penetration either in living or in fixed nerves. By contrast the tracer obtained access to the extracellular space within incisures only in vivo. The results are interpreted in terms of the permeability of nodes and incisures in vivo to physiologically important ions and related to current concepts of the electrophysiology of the myelinated nerve fibre.

Axolemmal differentiation in myelinated fibers of rat peripheral nerves

In developing rat peripheral fibers, nodal specialization appears early, prior to myelin compaction, and is first detected as a junction between the axon and the overhanging Schwann cell process characterized by a uniformly wide (approximately 18 nm) intercellular gap containing a patchy dense substance and a cytoplasmic undercoating subjacent to the axolemma. The gap width is rather consistent but the axolemmal undercoating is more variable and lower in density than that found at more mature nodes of Ranvier, and it is also highly variable in length, ranging from 0.5 to 3 micron. The outermost Schwann cell layer is usually prominent with a large volume of cytoplasm and many organelles. In freeze-fracture replicas, modal specializations are characterized by accumulations of large (approximately 10 nm) particles in the axolemma, especially the E face, but immature nodes generally have a lower particle concentration than mature nodes. No node-like particle aggregates have been found in axons not intimately associated with Schwann cells. Mature paranodal axon-Schwann cell junctions are usually formed first by the loops closest to the node and are characterized by a 2-3 nm gap between the apposed membranes, periodic intercellular densities (transverse bands) in the gap and cisternae flattened against the junctional Schwann cell membrane. The loops further removed from the node display a wider gap containing irregularly spaced or diffuse intercellular densities, or none. Mature junctions appear relatively late in the rat, and it is not unusual to find developing nodes with several Schwann cell loops present that do not indent the axolemma significantly and are not associated with the paracrystalline pattern characteristic of the mature junctional axolemma. In such instances, the nodal particle aggregates do not have sharply circumscribed boundaries. The majority of the developing nodes are asymmetric with one paranodal segment more mature than the other.

Electron microscopic serial section analysis of nodes of Ranvier in lumbar spinal roots of the cat: a morphometric study of nodal compartments in fibres of different sizes

Serially sectioned nodes of Ranvier from nerve fibres 2-20 micron in diameter of feline ventral and dorsal spinal roots were examined electron microscopically, reconstructed to scale and analysed morphometrically. The assumed 'fresh-state' value of several structural variables, considered to be of functional significance, were calculated by the use of compensation factors. The compensated data were plotted against fibre and axon diameters. It was calculated that the membranous area of the 'fresh-state' nodal axon segment increased more or less exponentially from less than 5 micron2 to 30 micron2 with increasing fibre diameter (D). Most variables associated with the nodal gap and the Schwann cell initially increased rapidly with D and then levelled out or even decreased in fibres with a D value greater than 8-12 micron. The area open for communication between the nodal axolemma and the endoneurial space was 30-100 times smaller than the membrane area of the nodal axolemma. The volume of the extracellular space in the nodal gap, outside the nodal axolemma, increased linearly from less than 0.1 micron3 to about 0.6 micron3 with increasing fibre size. The Schwann cell membrane area facing the nodal gap outnumbered the membrane area of the nodal axon by 10-15 times in nerve fibres with a D value between 5 and 15 microns. Some functional implications of the 'fresh-state' nodal model are discussed.

Electron microscopic serial section analysis of nodes of Ranvier in lumbosacral spinal roots of the cat: ultrastructural organization of nodal compartments in fibres of different sizes

The general ultrastructural organization of nodes of Ranvier in peripheral nerve fibres from 2 to 20 microns in diameter (D) was investigated in the adult cat using serially sectioned ventral and dorsal spinal roots. The study was performed in order to collect and systematize information considered necessary for a morphometric analysis of the node of Ranvier. In all cases a node of Ranvier could be divided into a central nodal axon segment and a surrounding nodal Schwann cell compartment. The latter included a nodal gap matrix substance, more or less overlapping nodal Schwann cell collars and, as a rule, also a Schwann cell brush-border emanating from the nodal Schwann cell collars and occupying the nodal gap. The relative size and the organization level of the nodal Schwann cell compartment increased with increasing fibre size up to a fibre diameter of 8-10 microns. At this fibre size the nodal gap was of a fairly even height (1 micron) all around the nodal axon and contained a thick brush-border of densely packed, more or less radially arranged Schwann cell microvilli. In very small fibres (D less than 3 microns) the nodal gap was low (less than 0.1 microns) and contained no or few microvilli. In fibres greater than 10 microns in diameter the relative size and the degree of structural order of the nodal Schwann cell compartment decreased with increasing fibre size. Drastic sectorial variations in nodal gap height and local thinning-out of the brush-border became prominent features in the largest fibres. The possible in vivo organization of the nodal Schwann cell compartment is discussed. Preliminary calculations indicate that the extracellular space directly surrounding the nodal axon might be quite small and that the area open for free communication between this extracellular space and the endoneurial space might be very much restricted, measuring as little as 2% of the area of the nodal axolemma. Algorithms for calculating various nodal structural parameters are discussed.

Distortions of the nodes of Ranvier from axonal distension by filamentous masses in hexacarbon intoxication

A study has been made of the structural changes of nodal and paranodal regions of the nodes of Ranvier of peripheral nerves of rats in which marked accumulations of neurofilaments have occurred within axons under the influence of 2,5-hexanediol over 10 weeks. The neurofilamentous masses caused distension of the axon at two points of apparent weakness as they attempted to slide through the axonal constriction at the nodes. Principally, a spiral axonal protrusion pushed into the zone of unattached myelin loops in the proximal paranodal spinous bracelet of Nageotte. This led to a conical widening of the paranodal constriction and considerable attenuation of the overlying myelin. No degeneration of the myelin occurred however. Alternatively, or additionally, a protrusion occurred of the axon at the nodal region which increased the nodal gap width and occasionally compressed and displaced the adjacent distal paranodal constriction which could have led to some obstruction of axoplasmic flow. Swelling of distal paranodal regions occurred later and was usually associated with proximal swelling. It was also accompanied by evidence suggesting transnodal passage of filamentous material. Sometimes, however, striking nodal constriction occurred in association with symmetrical paranodal swelling. These observations suggest that the spiral glial-axonal relationships at nodes of Ranvier are capable of marked deformation that might allow the intra-axonal neurofilamentous masses to move distally. These findings are discussed in relation to the structural features of the paranodal constrictions.

Immunocytochemical localization of sodium channel distributions in the excitable membranes of Electrophorus electricus

The tetrodotoxin binding protein, a major component of the Na+ channel, has been purified from the electric organ of the South American eel Electrophorus electricus. Antibodies to this protein were raised in rabbits and their specificity was demonstrated by a highly sensitive radioimmunoassay and by immunoprecipitation procedures. These antibodies were used to examine the distribution of the binding protein in the eel electroplax membranes and along myelinated nerve axons. The distribution of the antigen was determined by using the peroxidase-antiperoxidase technique at both the light and electron microscopic levels. In the electrocytes of the electric organ, only the innervated face showed staining in experimental material. The stained regions of electroplax plasmalemma included the caveolae of the innervated surface while caveolae of the non-innervated surface did not stain. Thus, the innervated surface including caveolae exclusively contains the Na+ channels. Along myelinated axons, staining was limited to the nodal zone of the node of Ranvier. The paranodal and internodal zones did not stain for the binding protein. Limited diffusion of primary IgG and subsequent reactants into the paranodal and internodal sites was eliminated as a possible source of focal staining at nodes because mechanically demyelinated preparations also exhibited focal nodal staining. Thus, this tetrodotoxin binding protein component of the Na+ channel is located solely within the nodal zone of the node of Ranvier.

Rat optic nerve: freeze-fracture studies during development of myelinated axons

This freeze-fracture study examines the development of myelinated fibers in the rat optic nerve. Axolemma of optic nerve fibers were studied before, during, and after myelination. At birth, the optic nerve is composed entirely of non-myelinated (premyelinated) axons, while in the adult, virtually all fibers acquire compact myelin. Myelination begins at 6-8 days postparturition and proceeds rapidly, such that by 28 days of age approximately 85% of the axons are myelinated. The axolemma of premyelinated fibers from 2-day-old animals exhibits an asymmetrical partitioning of intramembranous particles (IMPs) between E- and P-fracture faces; the E-face had approximately 125 particles/micron2 and the P-face approximately 550 particles/micron2. Particle densities for premyelinated axolemma from 8, 12, 14, 16, and 28-day-old nerves were similar to those observed at 2 days. Beginning at 8-12 days postnatal, definitive association between oligodendroglial processes and axons (termed 'ensheathed' fibers) was observed. At the time of glial ensheathment, there was a 50-100% increase in the number of P-face particles; in contrast, the E-face did not display an overall increase in particle density. In certain regions, however, localized aggregations of E-face particles were observed. IMPs on P-faces of ensheathed axons had a greater mean particle size and higher percentage of 'large' (greater than 9.6 nm) particles than did IMPs on the corresponding fracture face of premyelinated fibers. Myelinated axons from 14-16 day optic nerves displayed several differences from adult myelinated fibers. The P-face of the internodal axolemma had approximately 45% fewer particles than that of adult internodal membrane, and the percentage of large IMPs on the P-face of the younger internodal membrane was approximately 50% of the value for adult internodal axolemma. E-faces of internodal axolemma from 14-16-day-old and adult animals had equivalent IMP densities and size distributions. The nodal region of myelinated axons from 14-16-day-old rats had fewer large particles on both E- and P-faces than did adult fibers, though particle densities on both fracture faces were similar for the two age groups. These studies demonstrate a clear reorganization of axon membrane structure concomitant with axo-glial ensheathment, followed by continued gradual axolemmal changes as myelination progresses.