Glial regulation of the axonal membrane at nodes of Ranvier

Optical Read-out of Neural Activity in Mammalian Peripheral Axons: Calcium Signaling at Nodes of Ranvier

Current neural interface technologies have serious limitations for advanced prosthetic and therapeutic applications due primarily to their lack of specificity in neural communication. An optogenetic approach has the potential to provide single cell/axon resolution in a minimally invasive manner by optical interrogation of light-sensitive reporters and actuators. Given the aim of reading neural activity in the peripheral nervous system, this work has investigated an activity-dependent signaling mechanism in the peripheral nerve. We demonstrate action potential evoked calcium signals in mammalian tibial nerve axons using an in vitro mouse model with a dextran-conjugated fluorescent calcium indicator. Spatial and temporal dynamics of the signal are presented, including characterization of frequency-modulated amplitude. Pharmacological experiments implicate T-type CaV channels and sodium-calcium exchanger (NCX) as predominant mechanisms of calcium influx. This work shows the potential of using calcium-associated optical signals for neural activity read-out in peripheral nerve axons.

Glial Cells and Integrity of the Nervous System

Today, there is enormous progress in understanding the function of glial cells, including astroglia, oligodendroglia, Schwann cells, and microglia. Around 150 years ago, glia were viewed as a glue among neurons. During the course of the twentieth century, microglia were discovered and neuroscientists' views evolved toward considering glia only as auxiliary cells of neurons. However, over the last two to three decades, glial cells' importance has been reconsidered because of the evidence on their involvement in defining central nervous system architecture, brain metabolism, the survival of neurons, development and modulation of synaptic transmission, propagation of nerve impulses, and many other physiological functions. Furthermore, increasing evidence shows that glia are involved in the mechanisms of a broad spectrum of pathologies of the nervous system, including some psychiatric diseases, epilepsy, and neurodegenerative diseases to mention a few. It appears safe to say that no neurological disease can be understood without considering neuron-glia crosstalk. Thus, this book aims to show different roles played by glia in the healthy and diseased nervous system, highlighting some of their properties while considering that the various glial cell types are essential components not only for cell function and integration among neurons, but also for the emergence of important brain homeostasis.

Myelin damage and repair in pathologic CNS: challenges and prospects

Injury to the central nervous system (CNS) results in oligodendrocyte cell death and progressive demyelination. Demyelinated axons undergo considerable physiological changes and molecular reorganizations that collectively result in axonal dysfunction, degeneration and loss of sensory and motor functions. Endogenous adult oligodendrocyte precursor cells and neural stem/progenitor cells contribute to the replacement of oligodendrocytes, however, the extent and quality of endogenous remyelination is suboptimal. Emerging evidence indicates that optimal remyelination is restricted by multiple factors including (i) low levels of factors that promote oligodendrogenesis; (ii) cell death among newly generated oligodendrocytes, (iii) inhibitory factors in the post-injury milieu that impede remyelination, and (iv) deficient expression of key growth factors essential for proper re-construction of a highly organized myelin sheath. Considering these challenges, over the past several years, a number of cell-based strategies have been developed to optimize remyelination therapeutically. Outcomes of these basic and preclinical discoveries are promising and signify the importance of remyelination as a mechanism for improving functions in CNS injuries. In this review, we provide an overview on: (1) the precise organization of myelinated axons and the reciprocal axo-myelin interactions that warrant properly balanced physiological activities within the CNS; (2) underlying cause of demyelination and the structural and functional consequences of demyelination in axons following injury and disease; (3) the endogenous mechanisms of oligodendrocyte replacement; (4) the modulatory role of reactive astrocytes and inflammatory cells in remyelination; and (5) the current status of cell-based therapies for promoting remyelination. Careful elucidation of the cellular and molecular mechanisms of demyelination in the pathologic CNS is a key to better understanding the impact of remyelination for CNS repair.

Molecular disorganization of axons adjacent to human lacunar infarcts

Cerebral microvascular disease predominantly affects brain white matter and deep grey matter, resulting in ischaemic damage that ranges from lacunar infarcts to white matter hyperintensities seen on magnetic resonance imaging. These lesions are common and result in both clinical stroke syndromes and accumulate over time, resulting in cognitive deficits and dementia. Magnetic resonance imaging studies suggest that these lesions progress over time, accumulate adjacent to prior lesions and have a penumbral region susceptible to further injury. The pathological correlates of this adjacent injury in surviving myelinated axons have not been previously defined. In this study, we sought to determine the molecular organization of axons in tissue adjacent to lacunar infarcts and in the regions surrounding microinfarcts, by determining critical elements in axonal function: the morphology and length of node of Ranvier segments and adjacent paranodal segments. We examined post-mortem brain tissue from six patients with lacunar infarcts and tissue from two patients with autosomal dominant retinal vasculopathy and cerebral leukoencephalopathy (previously known as hereditary endotheliopathy with retinopathy, nephropathy and stroke) who accumulate progressive white matter ischaemic lesions in the form of lacunar and microinfarcts. In axons adjacent to lacunar infarcts yet extending up to 150% of the infarct diameter away, both nodal and paranodal length increase by ∼20% and 80%, respectively, reflecting a loss of normal cell-cell adhesion and signalling between axons and oligodendrocytes. Using premorbid magnetic resonance images, brain regions from patients with retinal vasculopathy and cerebral leukoencephalopathy that harboured periventricular white matter hyperintensities were selected and the molecular organization of axons was determined within these regions. As in regions adjacent to lacunar infarcts, nodal and paranodal length in white matter of these patients is increased. Myelin basic protein and neurofilament immunolabelling demonstrates that axons in these adjacent regions have preserved axonal cytoskeleton organization and are generally myelinated. This indicates that the loss of normal axonal microdomain architecture results from disrupted axoglial signalling in white matter adjacent to lacunar and microinfarcts. The loss of the normal molecular organization of nodes and paranodes is associated with axonal degeneration and may lead to impaired conduction velocity across surviving axons after stroke. These findings demonstrate that the degree of white matter injury associated with cerebral microvascular disease extends well beyond what can be identified using imaging techniques and that an improved understanding of the neurobiology in these regions can drive new therapeutic strategies for this disease entity.

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.

Membrane domain organization of myelinated axons requires βII spectrin

The spectrin-based cytoskeleton functions as a barrier to restrict axonal proteins, such as juxtaparanodal K⁺ channels, to specific membrane domains.

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

Axon-glia interaction and membrane traffic in myelin formation

In vertebrate nervous systems myelination of neuronal axons has evolved to increase conduction velocity of electrical impulses with minimal space and energy requirements. Myelin is formed by specialized glial cells which ensheath axons with a lipid-rich insulating membrane. Myelination is a multi-step process initiated by axon-glia recognition triggering glial polarization followed by targeted myelin membrane expansion and compaction. Thereby, a myelin sheath of complex subdomain structure is established. Continuous communication between neurons and glial cells is essential for myelin maintenance and axonal integrity. A diverse group of diseases, from multiple sclerosis to schizophrenia, have been linked to malfunction of myelinating cells reflecting the physiological importance of the axon-glial unit. This review describes the mechanisms of axonal signal integration by oligodendrocytes emphasizing the central role of the Src-family kinase Fyn during central nervous system (CNS) myelination. Furthermore, we discuss myelin membrane trafficking with particular focus on endocytic recycling and the control of proteolipid protein (PLP) transport by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Finally, PLP mistrafficking is considered in the context of myelin diseases.

The axonal endoplasmic reticulum and protein trafficking: Cellular bootlegging south of the soma

Neurons are responsible for the generation and propagation of electrical impulses, which constitute the central mechanism of information transfer between the nervous system and internal or external environments. Neurons are large and polarized cells with dendrites and axons constituting their major functional domains. Axons are thin and extremely long specializations that mediate the conduction of these electrical impulses. Regulation of the axonal proteome is fundamental to generate and maintain neural function. Although classical mechanisms of protein transport have been around for decades, a variety newly identified mechanisms to control the abundance of axonal proteins have appeared in recent years. Here we briefly describe the classical models of axonal transport and compare them to the emerging concepts of axonal biosynthesis centered on the endoplasmic reticulum. We review the structure of the axonal endoplasmic reticulum, and its role in diffusion and trafficking of axonal proteins. We also analyze the contribution of other secretory organelles to axonal trafficking and evaluate the potential consequences of axonal endoplasmic reticulum malfunction in neuropathology.

Three mechanisms assemble central nervous system nodes of Ranvier

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

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.

Culturing oligodendrocyte lineage cells from neonatal rats

The use of enriched oligodendrocyte lineage cell cultures has yielded insight into functions of these cells and regulatory mechanisms. This chapter details methods that result in such cultures.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

Differential susceptibility to axonopathy in necrotic and non-necrotic perinatal white matter injury

BACKGROUND AND PURPOSE

White matter injury (WMI) is the leading cause of brain injury in preterm survivors and results in myelination failure. Although axonal degeneration occurs in necrotic lesions, the role of axonopathy in myelination failure remains controversial for diffuse non-necrotic WMI, which is currently the major form of WMI. We determined the burden of axonopathy in diffuse lesions.

METHODS

We analyzed WMI in a preterm fetal sheep model of global cerebral ischemia that replicates the relative burden of necrotic and non-necrotic human WMI. WMI was analyzed at 1 or 2 weeks after ischemia and identified by ex vivo high-field (11.7 Tesla) magnetic resonance imaging of fixed brain tissue. Axonal integrity was analyzed by immunohistochemical detection of axon injury markers and by transmission electron microscopy to quantify axon loss and degeneration in magnetic resonance imaging-defined lesions.

RESULTS

Axonal degeneration, defined by staining for neurofilament protein and β-amyloid precursor protein, was restricted to discrete necrotic foci with robust microglial activation. Unexpectedly, axonal degeneration was not visualized in the major form of WMI, which comprised large non-necrotic lesions with diffuse reactive astrogliosis. In these major lesions, quantitative electron microscopy studies confirmed no significant differences in the density of intact and degenerating axons or in the distribution of axon diameters relative to controls.

CONCLUSIONS

The mechanism of myelination failure differs significantly in perinatal WMI dependent on the burden of necrosis. Axonopathy is associated with focal necrotic injury but not with primary diffuse non-necrotic lesions, which supports that intact axons in the primary lesions are potential targets for myelination.

Molecular microdomains in a sensory terminal, the vestibular calyx ending

Many primary vestibular afferents form large cup-shaped postsynaptic terminals (calyces) that envelope the basolateral surfaces of type I hair cells. The calyceal terminals both respond to glutamate released from ribbon synapses in the type I cells and initiate spikes that propagate to the afferent's central terminals in the brainstem. The combination of synaptic and spike initiation functions in these unique sensory endings distinguishes them from the axonal nodes of central neurons and peripheral nerves, such as the sciatic nerve, which have provided most of our information about nodal specializations. We show that rat vestibular calyces express an unusual mix of voltage-gated Na and K channels and scaffolding, cell adhesion, and extracellular matrix proteins, which may hold the ion channels in place. Protein expression patterns form several microdomains within the calyx membrane: a synaptic domain facing the hair cell, the heminode abutting the first myelinated internode, and one or two intermediate domains. Differences in the expression and localization of proteins between afferent types and zones may contribute to known variations in afferent physiology.

Accumulation of Kv7.2 channels in putative ectopic transduction zones of mice nerve-end neuromas

Background

Modulation of M-type currents has been proposed as a new strategy for the treatment of neuropathic pain due to their role in regulating neuronal excitability. Using electrophysiological techniques we showed previously that the opening of Kv7 channels with retigabine, blocked ectopic discharges from axotomized fibers but did not alter transduction at intact skin afferents. We hypothesized that after nerve damage, accumulation of Kv7 channels in afferent fibers may increase M-type currents which then acquired a more important role at regulating fiber excitability.

Findings

In this study, we used an immunohistochemical approach to examine patterns of expression of Kv7.2 channels in afferent fibers after axotomy and compared them to patterns of expression of voltage gated Na⁺ channels (Nav) which are key electrogenic elements in peripheral axons known to accumulate in experimental and human neuromas.

Axotomy induced an enlargement and narrowing of the nodes of Ranvier at the proximal end of the neuroma together with a dramatic demyelination and loss of structure at its distal end in which naked accumulations of Nav were present. In addition, axotomy also induced accumulations of Kv7.2 that co-localized with those of Nav channels.

Conclusions

Whilst Nav channels are mandatory for initiation of action potentials, (i.e. responsible for the generation/propagation of ectopic discharges) an increased accumulation of Kv7.2 channels after axotomy may represent a homeostatic compensation to over excitability in axotomized fibers, opening a window for a peripheral action of M-current modulators under conditions of neuropathy.

Comparing peripheral glial cell differentiation in Drosophila and vertebrates

In all complex organisms, the peripheral nerves ensure the portage of information from the periphery to central computing and back again. Axons are in part amazingly long and are accompanied by several different glial cell types. These peripheral glial cells ensure electrical conductance, most likely nature the long axon, and establish and maintain a barrier towards extracellular body fluids. Recent work has revealed a surprisingly similar organization of peripheral nerves of vertebrates and Drosophila. Thus, the genetic dissection of glial differentiation in Drosophila may also advance our understanding of basic principles underlying the development of peripheral nerves in vertebrates.

Claudin family proteins in Caenorhabditis elegans

In the last decade, the claudin family of integral membrane proteins has been identified as the major protein component of the tight junctions in all vertebrates. The claudin superfamily proteins also function to regulate channel activity, intercellular signaling, and cell morphology. Subsequently, claudin homologues have been identified in invertebrates, including Drosophila and Caenorhabditis elegans. Recent studies demonstrate that the C. elegans claudins, clc-1 to clc-5, and similar proteins in the greater PMP22/EMP/claudin/calcium channel γ subunit family, including nsy-1-nsy-4 and vab-9, while highly divergent at a sequence level from each other and from the vertebrate claudins, in some cases play roles similar to those traditionally assigned to their vertebrate homologues. These include regulating cell adhesion and passage of small molecules through the paracellular space. The claudin superfamily proteins also function to regulate channel activity, intercellular signaling, and cell morphology. Study of claudin superfamily proteins in C. elegans should continue to provide clues as to how core claudin protein function can be modified to serve various specific roles at regions of cell-cell contact in metazoans.

Proteomic analysis of optic nerve lipid rafts reveals new paranodal proteins

Neuron-glia interactions at paranodal junctions play important roles in action potential propagation. Among their many functions, they contribute to the passive electrical properties of myelinated nerve fibers and actively regulate the polarized distribution of ion channels along axons. Despite their importance, relatively little is known about the molecules responsible for paranode formation and function. Paranodal junction formation apparently depends on interactions among three cell adhesion molecules: caspr and contactin on the axon and neurofascin 155 (NF-155) on the glial membrane. Using Caspr-null paranodal mutant mice, we demonstrate that loss of paranodal junctions causes failure of NF-155 to partition into lipid rafts, indicating that proteins located at paranodal junctions have biochemical characteristics of lipid raft-associated proteins. Based on this property of paranodal junctions, mass spectrometry of lipid rafts isolated from a pure white matter tract (optic nerve) was used to search for new paranodal proteins. Because we used a relatively crude biochemical preparation, we identified several hundred different proteins. Among these, we found all previously described paranodal proteins. Further analysis based on antibody staining of central and peripheral nerves revealed beta-adducin, septin 2, and sh3p8 as putative paranodal proteins. We describe the localization of these proteins in relation to other markers of nodes, paranodes, and juxtaparanodes in adult and developing nerve fibers. Finally, we describe their distribution in dysmyelinating TremblerJ mice, a model for the peripheral neuropathy Charcot-Marie-Tooth disease.

Are BMPs involved in normal nerve and following transection?: a pilot study

Bone morphogenic proteins (BMPs) may have neurotrophic functions but there is limited evidence of these functions in the peripheral nervous system. We therefore investigated the expression of BMPs and BMP receptors (BMPRs) in normal and injured peripheral nerves. In 10 of 15 Sprague-Dawley rats, a 3-mm segment of sciatic nerve was resected at the trifurcation in the thigh. One day (n = 5) and 7 days (n = 5) after transection, proximal and distal stumps were removed and immunohistochemically analyzed for BMP-2, -7, BMPR-1A, -1B, and -2. The other five animals served as normal controls. In normal nerves, BMP-2 expression was localized at Ranvier's node, and BMP-7 and BMPR-1B were expressed in several axon-Schwann cell units, whereas other receptors were not expressed. After nerve transection, BMP-7 expression was upregulated at both proximal and distal stumps along with Schwann cell columns during Wallerian degeneration. BMPRs were also upregulated compared with the normal nerve. The upregulation in BMP expression after nerve transection suggests that BMPs may play a role in the healing response of the peripheral nerve.

Oligodendrocyte development and myelin biogenesis: parsing out the roles of glycosphingolipids

The myelin sheath is an extension of the oligoddendrocyte (OL) plasma membrane enriched in lipids that ensheaths the axons of the central and peripheral nervous system. Here, we review the involvement of glycosphingolipids in myelin/OL functions, including the regulation of OL differentiation, lipid raft-mediated trafficking and signaling, and neuron-glia interactions.

boudin is required for septate junction organisation in Drosophila and codes for a diffusible protein of the Ly6 superfamily

The Ly6 superfamily, present in most metazoan genomes, codes for different cell-surface proteins and secreted ligands containing an extracellular motif called a Ly6 domain or three-finger domain. We report the identification of 36 novel genes coding for proteins of this family in Drosophila. One of these fly Ly6 proteins, coded by the gene boudin (bou), is essential for tracheal morphogenesis in the fly embryo and contributes to the maintenance of the paracellular barrier and the organisation of the septate junctions in this tissue. Bou, a glycosylphosphatidylinositol anchored membrane protein, is also required for septate junction organisation in epithelial tissues and in the chordotonal organ glial cells, but not in the central nervous system. Our study reveals interesting parallelisms between the Ly6 proteins of flies and vertebrates, such as the CD59 antigen. Similarly to this human protein, Bou travels from cell to cell associated with extracellular particles and, consistently, we show that it is required in a non-cell-autonomous fashion. Our work opens the way for future studies addressing the function of Ly6 proteins using Drosophila as a model system.

Axonal targeting of Caspr2 in hippocampal neurons via selective somatodendritic endocytosis

Contactin-associated protein 2 (Caspr2) is a neuronal membrane protein that is mutated in autism and related disorders. Although it is highly enriched at juxtaparanodes of Ranvier where it is essential for Shaker-type K(+) channel clustering, little is known about its function and regulation. In the present study, we examined the polarized expression of Caspr2 in hippocampal neurons using extracellular hemagglutinin (HA)-tagged Caspr2 constructs. We found that Caspr2 was targeted to the axonal surface, but colocalized with early endosomes in the somatodendritic compartment. The inhibition of endocytosis using a Dynamin-1 mutant or treatment with Dynasore prevented Caspr2 internalization from the dendrites and cell body. We identified a short sequence included into the 4.1B-binding domain that is required for the endocytosis of Caspr2. This sequence contains a protein kinase C (PKC) substrate motif on Thr1292, and point mutation of this residue or treatment with a PKC inhibitor prevented the somatodendritic internalization of Caspr2. Thus, the PKC-dependent trafficking of Caspr2 underlies its polarized expression in hippocampal neurons.

Neonatal chimerization with human glial progenitor cells can both remyelinate and rescue the otherwise lethally hypomyelinated shiverer mouse

Congenitally hypomyelinated shiverer mice fail to generate compact myelin and die by 18-21 weeks of age. Using multifocal anterior and posterior fossa delivery of sorted fetal human glial progenitor cells into neonatal shiverer x rag2(-/-) mice, we achieved whole neuraxis myelination of the engrafted hosts, which in a significant fraction of cases rescued this otherwise lethal phenotype. The transplanted mice exhibited greatly prolonged survival with progressive resolution of their neurological deficits. Substantial myelination in multiple regions was accompanied by the acquisition of normal nodes of Ranvier and transcallosal conduction velocities, ultrastructurally normal and complete myelination of most axons, and a restoration of a substantially normal neurological phenotype. Notably, the resultant mice were cerebral chimeras, with murine gray matter but a predominantly human white matter glial composition. These data demonstrate that the neonatal transplantation of human glial progenitor cells can effectively treat disorders of congenital and perinatal hypomyelination.

Myelin, DIGs, and membrane rafts in the central nervous system

Over the past 40 years our understanding of the organization of cell membranes has changed dramatically. Membranes are no longer viewed as a homogenous sea of phospholipids studded with randomly positioned islands of proteins. Our current view of the membrane involves the formation of small lipid clusters, comprised mainly of cholesterol and sphingolipids, known as membrane rafts. These lipid clusters apparently include and exclude specific proteins leading to the hypothesis that these domains (1) regulate cellular polarity and compartmentalization through trafficking and sorting, (2) provide platforms for cellular signaling and adhesion, and (3) function as cellular gate keepers. Tremendous controversy surrounds the concept of membrane rafts primarily because these small, highly dynamic entities are too small to be observed with traditional microscopic methods and the most utilized approach for raft analysis relies on poorly quantified, inconsistent biochemical extractions. New analytical approaches are being developed and applied to the study of membrane rafts and these techniques provide great promise for furthering our understanding of these enigmatic domains. In this review we will provide a brief summary of the current understanding of membrane rafts, utilizing the CNS myelin literature for illustrative purposes, and present caveats that should be considered when studying these domains.

Multiple molecular interactions determine the clustering of Caspr2 and Kv1 channels in myelinated axons

Clustering of Kv1 channels at the juxtaparanodal region (JXP) in myelinated axons depends on their association with the Caspr2/TAG-1 adhesion complex. The interaction between these channels and Caspr2 was suggested to depend on PDZ (PSD-95/Discs large/zona occludens-1) scaffolding proteins. Here, we show that at a subset of the JXP, PSD-93 colocalizes with Caspr2, K(+) channels and its related protein postsynaptic density protein-95 (PSD-95). The localization of PSD-93 and PSD-95 depends on the presence of Caspr2, as both scaffolding proteins failed to accumulate at the JXP in mice lacking either Caspr2 or TAG-1. In contrast, Caspr2 and K(+) channels still colocalized and associated in PSD-93, PSD-95 or double PSD-93/PSD-95 null mice. To directly evaluate the role of PDZ domain proteins in the function of Caspr2, we examined the ability of transgenic Caspr2 molecules lacking either their cytoplasmic domain (Caspr2dCT), or their PDZ-binding sequence (Caspr2dPDZ), to restore Kv1 channel clustering in Caspr2 null mice. We found that while Kv1 channels were distributed throughout internodes in nerves expressing Caspr2dCT, they were clustered at the JXP in axons expressing a full-length Caspr2 (Caspr2FL) or the Caspr2dPDZ transgene. Further proteomic analysis revealed that Caspr2 interacts with a distinct set of scaffolding proteins through its PDZ- and protein 4.1-binding sequences. These results demonstrate that while the molecular assembly of the JXP requires the cytoplasmic domain of Caspr2, its carboxy-terminal PDZ-binding motif is dispensable for Kv1 channel clustering. This mechanism is clearly distinct from the one operating at the axon initial segment, which requires PSD-93 for Kv1 channel clustering.

Small Rho GTPases are key regulators of peripheral nerve biology in health and disease

A thorough knowledge of the cellular and molecular basis of the structure and function of peripheral nerves is of paramount importance not only for a better understanding of the fascinating biology of the peripheral nervous system but also for providing critical insights into the various diseases affecting peripheral nerves as the firm foundation of potential treatments. Genetic approaches in model organisms, in combination with research on hereditary forms of neuropathies, have contributed significantly to our progress in this field. In this review, we will focus on recent advances using these synergistic approaches that led to the identification of small Rho GTPases and their regulators as crucial functional players in proper development and function of myelinated peripheral nerves, with a particular emphasis on the cell biology of Schwann cells in health and disease.

Axonal domains: role for paranodal junction in node of Ranvier assembly

A new study shows that communication between axons and glia at the paranodal junction can orchestrate the formation of the node of Ranvier.

Molecular mechanisms of node of Ranvier formation

Action potential propagation along myelinated nerve fibers requires high-density protein complexes that include voltage-gated Na(+) channels at the nodes of Ranvier. Several complementary mechanisms may be involved in node assembly including: (1) interaction of nodal cell adhesion molecules with the extracellular matrix; (2) restriction of membrane protein mobility by paranodal junctions; and (3) stabilization of ion channel clusters by axonal cytoskeletal scaffolds. In the peripheral nervous system, a secreted glial protein at the nodal extracellular matrix interacts with axonal cell adhesion molecules to initiate node formation. In the central nervous system, both glial soluble factors and paranodal axoglial junctions may function in a complementary manner to contribute to node formation.

GFAP aggregates in the cochlear nerve increase the noise vulnerability of sensory cells in the organ of Corti in the murine model of Alexander disease

Outer hair cell (OHC) loss in the auditory sensory epithelium is a primary cause of noise-induced sensory-neural hearing loss (SNHL). To clarify the participation of glial cells in SNHL, we used an Alexander disease (AxD) mouse model. These transgenic mice harbor the AxD causal mutant of the human glial fibrillary acidic protein (GFAP) under the control of the mouse GFAP promoter. It is thought that GFAP aggregates compromise the function of astrocytes. In the auditory pathway, the formation of GFAP aggregates was observed only in GFAP-positive cells of the cochlear nerve. The presence of GFAP aggregates did not change auditory function at the threshold level. To assess the change in vulnerability to auditory excitotoxicity, both transgenic and control mice were treated with intense noise exposure. Auditory threshold shifts were assessed by auditory brainstem responses (ABR) at 1 and 4 weeks after noise exposure, and OHC damage was analyzed by quantitative histology at 4 weeks after exposure. Transgenic mice showed more severe ABR deficits and OHC damage, suggesting that cochlear nerve glial cells with GFAP aggregates play a role in noise susceptibility. Thus, we should focus more on the roles of cochlear nerve glial cells in SNHL.

Surface expression and distribution of voltage-gated potassium channels in neurons (Review)

The last decade has witnessed an exponential increase in interest in one of the great mysteries of nerve cell biology: Specifically, how do neurons know where to place the ion channels that control their excitability? Many of the most important insights have been gleaned from studies on the voltage-gated potassium channels (Kvs) which underlie the shape, duration and frequency of action potentials. In this review, we gather recent evidence on the expression, trafficking and maintenance mechanisms which control the surface density of Kvs in different subcellular compartments of neurons and how these may be regulated to control cell excitability.

Spectrin and ankyrin-based cytoskeletons at polarized domains in myelinated axons

In myelinated nerve fibers, action potential initiation and propagation requires that voltage-gated ion channels be clustered at high density in the axon initial segments and nodes of Ranvier. The molecular organization of these subdomains depends on specialized cytoskeletal and scaffolding proteins such as spectrins, ankyrins, and 4.1 proteins. These cytoskeletal proteins are considered to be important for 1) formation, localization, and maintenance of specific integral membrane protein complexes, 2) a barrier restricting the diffusion of both cytoplasmic and membrane proteins to distinct regions or compartments of the cell, and 3) stabilization of axonal membrane integrity. Increased insights into the role of the cytoskeleton could provide important clues about the pathophysiology of various neurological disorders.

Nectin-like proteins mediate axon Schwann cell interactions along the internode and are essential for myelination

Axon-glial interactions are critical for the induction of myelination and the domain organization of myelinated fibers. Although molecular complexes that mediate these interactions in the nodal region are known, their counterparts along the internode are poorly defined. We report that neurons and Schwann cells express distinct sets of nectin-like (Necl) proteins: axons highly express Necl-1 and -2, whereas Schwann cells express Necl-4 and lower amounts of Necl-2. These proteins are strikingly localized to the internode, where Necl-1 and -2 on the axon are directly apposed by Necl-4 on the Schwann cell; all three proteins are also enriched at Schmidt-Lanterman incisures. Binding experiments demonstrate that the Necl proteins preferentially mediate heterophilic rather than homophilic interactions. In particular, Necl-1 on axons binds specifically to Necl-4 on Schwann cells. Knockdown of Necl-4 by short hairpin RNA inhibits Schwann cell differentiation and subsequent myelination in cocultures. These results demonstrate a key role for Necl-4 in initiating peripheral nervous system myelination and implicate the Necl proteins as mediators of axo-glial interactions along the internode.

No effect of genetic deletion of contactin-associated protein (CASPR) on axonal orientation and synaptic plasticity

Myelinated axons are endowed with a specialized domain structure that is essential for saltatory action potential conduction. The paranodal domain contains the axoglial junctions and displays a unique ultrastructure that resembles the invertebrate septate junctions (SJs). Biochemical characterizations of the paranodal axoglial SJs have identified several molecular components that include Caspr and contactin (Cont) on the axonal side and neurofascin 155 kDa (NF155) isoform on the glial side. All these proteins are essential for the formation of the axoglial SJs. Based on the interactions between Caspr and Cont and their colocalization in the CA1 synaptic areas, it was proposed that the synaptic function of Cont requires Caspr. Here we have extended the phenotypic analysis of CASPR mutants to address further the role of Caspr at the axoglial SJs and also in axonal orientation and synaptic plasticity. We report that, in CASPR mutants, the smooth endoplasmic reticulum (SER) forms elongated membranous complexes that accumulate at the nodal/paranodal region and stretch into the juxtaparanodal region, a defect that is consistent with the paranodal disorganization. We show that the cerebellar microorganization is unaffected in CASPR mutants. We also demonstrate that Caspr function is not essential for normal CA1 synaptic transmission and plasticity. Taken together with previous findings, our results highlight that the Caspr/Cont complex is essential for the formation of axoglial SJs, whereas Cont may regulate axonal orientation and synaptic plasticity independent of its association with Caspr.

Proteomic strategies in multiple sclerosis and its animal models

The early and precise diagnosis, the prognosis, and the clinical management of multiple sclerosis, remain a considerable challenge. In recent years, the development of novel and powerful proteomic techniques prompted the use of these approaches for the search of unique biomarkers in the cerebrospinal fluid of multiple sclerosis patients. A few studies have also utilized proteomics to delineate the profile of differentially expressed proteins in animal models of the human disease in order to gain global insights into affected pathways. The identification of differentially expressed proteins may be an initial step in the discovery of novel targets and mechanisms that play critical roles in the pathology of multiple sclerosis. Based on these findings, future investigations may elucidate the events leading to demyelination, axonal damage, and neurodegeneration, providing better insights into mechanisms governing the onset and progression of the disease. Although these proteomic studies provide valuable information, they are also faced with a number of challenges. The present review discusses some of the strengths and limitations of proteomic investigations as applied to multiple sclerosis.

Wrapping it up: the cell biology of myelination

During nervous system development, oligodendroglia in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) synthesise large amounts of specific proteins and lipids to generate myelin, a specialised membrane that spirally ensheathes axons and facilitates fast conduction of the action potential. Myelination is initiated after glial processes have attached to the axon and polarisation of the plasma membrane has been triggered. Myelin assembly is a multi-step process that occurs in spatially distinct regions of the cell. We propose that assembly of myelin proteins and lipids starts during their transport through the biosynthetic pathway and continues at the plasma membrane aided by myelin-basic protein (MBP). These sequential processes create the special lipid and protein composition necessary for myelin to perform its insulating function during nerve conduction.

Neurofascin assembles a specialized extracellular matrix at the axon initial segment

Action potential initiation and propagation requires clustered Na⁺ (voltage-gated Na⁺ [Nav]) channels at axon initial segments (AIS) and nodes of Ranvier. In addition to ion channels, these domains are characterized by cell adhesion molecules (CAMs; neurofascin-186 [NF-186] and neuron glia–related CAM [NrCAM]), cytoskeletal proteins (ankyrinG and βIV spectrin), and the extracellular chondroitin-sulfate proteoglycan brevican. Schwann cells initiate peripheral nervous system node formation by clustering NF-186, which then recruits ankyrinG and Nav channels. However, AIS assembly of this protein complex does not require glial contact. To determine the AIS assembly mechanism, we silenced expression of AIS proteins by RNA interference. AnkyrinG knockdown prevented AIS localization of all other AIS proteins. Loss of NF-186, NrCAM, Nav channels, or βIV spectrin did not affect other neuronal AIS proteins. However, loss of NF-186 blocked assembly of the brevican-based AIS extracellular matrix, and NF-186 overexpression caused somatodendritic brevican clustering. Thus, NF-186 assembles and links the specialized brevican-containing AIS extracellular matrix to the intracellular cytoskeleton.

Nodes of Ranvier and axon initial segments are ankyrin G-dependent domains that assemble by distinct mechanisms

Axon initial segments (AISs) and nodes of Ranvier are sites of action potential generation and propagation, respectively. Both domains are enriched in sodium channels complexed with adhesion molecules (neurofascin [NF] 186 and NrCAM) and cytoskeletal proteins (ankyrin G and βIV spectrin). We show that the AIS and peripheral nervous system (PNS) nodes both require ankyrin G but assemble by distinct mechanisms. The AIS is intrinsically specified; it forms independent of NF186, which is targeted to this site via intracellular interactions that require ankyrin G. In contrast, NF186 is targeted to the node, and independently cleared from the internode, by interactions of its ectodomain with myelinating Schwann cells. NF186 is critical for and initiates PNS node assembly by recruiting ankyrin G, which is required for the localization of sodium channels and the entire nodal complex. Thus, initial segments assemble from the inside out driven by the intrinsic accumulation of ankyrin G, whereas PNS nodes assemble from the outside in, specified by Schwann cells, which direct the NF186-dependent recruitment of ankyrin G.

Secreted gliomedin is a perinodal matrix component of peripheral nerves

The interaction between gliomedin and the axonodal cell adhesion molecules (CAMs) neurofascin and NrCAM induces the clustering of Na⁺ channels at the nodes of Ranvier. We define new interactions of gliomedin that are essential for its clustering activity. We show that gliomedin exists as both transmembrane and secreted forms that are generated by proteolytic cleavage of the protein, and that only the latter is detected at the nodes of Ranvier. The secreted extracellular domain of gliomedin binds to Schwann cells and is incorporated into the extracellular matrix (ECM) in a heparin-dependent manner, suggesting the involvement of heparan sulfate proteoglycans (HSPGs). Furthermore, we show that the N-terminal region of gliomedin serves as an oligomerization domain that mediates self-association of the molecule, which is required for its binding to neurofascin and NrCAM. Our results indicate that the deposition of gliomedin multimers at the nodal gap by binding to HSPGs facilitates the clustering of the axonodal CAMs and Na⁺ channels.

Rho regulates membrane transport in the endocytic pathway to control plasma membrane specialization in oligodendroglial cells

Differentiation of oligodendrocytes is associated with dramatic changes in plasma membrane structure, culminating in the formation of myelin membrane sheaths. Previous results have provided evidence that regulation of endocytosis may represent a mechanism to control myelin membrane growth. Immature oligodendrocytes have a high rate of clathrin-independent endocytosis for the transport of membrane to late endosomes/lysosomes (LE/Ls). After maturation and receiving signals from neurons, endocytosis is reduced and transport of membrane from LE/Ls to the plasma membrane is triggered. Here, we show that changes in Rho GTPase activity are responsible for switching between these two modes of membrane transport. Strikingly, Rho inactivation did not only reduce the transport of cargo to LE/L but also increased the dynamics of LE/L vesicles. Furthermore, we provide evidence that Rho inactivation results in the condensation of the plasma membrane in a polarized manner. In summary, our data reveal a novel role of Rho: to regulate the flow of membrane and to promote changes in cell surface structure and polarity in oligodendroglial cells. We suggest that Rho inactivation is required to trigger plasma membrane specialization in oligodendrocytes.

betaIV spectrin is recruited to axon initial segments and nodes of Ranvier by ankyrinG

High densities of ion channels at axon initial segments (AISs) and nodes of Ranvier are required for initiation, propagation, and modulation of action potentials in axons. The organization of these membrane domains depends on a specialized cytoskeleton consisting of two submembranous cytoskeletal and scaffolding proteins, ankyrinG (ankG) and βIV spectrin. However, it is not known which of these proteins is the principal organizer, or if the mechanisms governing formation of the cytoskeleton at the AIS also apply to nodes. We identify a distinct protein domain in βIV spectrin required for its localization to the AIS, and show that this domain mediates βIV spectrin's interaction with ankG. Dominant-negative ankG disrupts βIV spectrin localization, but does not alter endogenous ankG or Na⁺ channel clustering at the AIS. Finally, using adenovirus for transgene delivery into myelinated neurons, we demonstrate that βIV spectrin recruitment to nodes of Ranvier also depends on binding to ankG.