Differentiation of axon-related Schwann cells in vitro. I. Ascorbic acid regulates basal lamina assembly and myelin formation

Neurosteroids: synthesis and functions in the central and peripheral nervous systems

Some steroids are synthesized within the central and peripheral nervous systems, mostly by glial cells. These are known as neurosteroids. In the brain, neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3 alpha,5 alpha-reduced metabolite 3 alpha,5 alpha-tetrahydroprogesterone (allopregnanolone) activates the type A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phophodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.

Myelin-associated glycoprotein and galactosylcerebroside expression in Schwann cells during myelination

The temporo-spatial expression profiles of the myelin-associated glycoprotein (MAG) and galactosylcerebrosides (Gal-CBs) were analysed during Schwann cell differentiation and myelination in Schwann cell cultures and in dorsal root ganglion/Schwann cell cocultures. The temporal expression profiles demonstrate that the mechanisms triggering and regulating MAG and Gal-CBs are different and independent. They also provide further support for a role of MAG, but not Gal-CBs, in the establishment of the Schwann cell/axon interaction and in the formation of the primary mesaxon, while both MAG and Gal-CBs may participate in the further maturation of the mesaxon. The spatial expression profiles demonstrate that although Gal-CBs are concentrated in the same structures (mesaxonal spirals, paranodes and Schmidt-Lanterman incisures) as MAG in both developing and mature sheaths, where it is therefore possible that the two might have complementary functions, they are also present in regions from which MAG is excluded.

Role of Nitric Oxide as Mediator of Nerve Injury in Inflammatory Neuropathies

Different lines of evidence suggest that nitric oxide (NO) plays a key role in the pathogenesis of inflammatory neuropathies; however, it is still unclear which structures in the peripheral nerve are the primary targets of NO-mediated nerve injury. To address this issue, we determined the expression of NO metabolites in sural nerve biopsies and in cerebrospinal fluid from patients with inflammatory neuropathies and studied the pathologic effects of NO in an in vitro model of myelinated Schwann cell-neuron cocultures. In cerebrospinal fluid samples, nitrite levels remained unaltered; however, nitrotyrosine, a marker for peroxynitrite formation, could be identified in nerve biopsies from patients with inflammatory neuropathies. In an in vitro model of Schwann cell neuron cocultures, high concentrations of NO induced robust demyelination, which was the result of NO-mediated axonal injury, whereas Schwann cell viability remained unaffected. These findings suggest that in contrast to Schwann cells, sensory neurons are the primary target of NO-mediated cytotoxicity and the loss of myelin is the result of selective damage to axons rather than a direct harmful effect to Schwann cells. Our findings imply that NO contributes to the pathologic changes seen in the inflamed peripheral nervous system, which is characterized by the features of axonal injury and subsequent myelin degradation, previously described as Wallerian-like degeneration.

Developmental abnormalities in the nerves of peripheral myelin protein 22-deficient mice

Peripheral myelin protein 22 (PMP22) is a tetraspan glycoprotein whose misexpression is associated with a family of hereditary peripheral neuropathies. In a recent report, we have characterized a novel PMP22-deficient mouse model in which the first two coding exons were replaced by the lacZ reporter. To investigate further the myelin abnormalities in the absence of PMP22, sciatic nerves and dorsal root ganglion (DRG) neuron explant cultures from PMP22-deficient mice were studied at various stages of myelination. Throughout the first 3 months of postnatal development, myelin protein and beta4 integrin levels are dramatically reduced, whereas p75 and beta1 integrin remain elevated. By immunostaining, the distributions of several glial proteins, including beta4 integrin, the voltage-gated potassium channel Kv1.1, and E-cadherin, are altered. Schwann cells from PMP22-deficient mice are able to produce limited amounts of myelin in DRG explant cultures, yet the internodal segments are dramatically fewer and shorter. The comparison of PMP22-deficient mice with other PMP22 mutant models reveals that the decrease in beta4 integrin is specific to an absence of PMP22. Furthermore, whereas lysosome-associated membrane protein 1 and ubiquitin are notably up-regulated in nerves of PMP22-deficient mice, heat shock protein 70 levels remain constant or decrease compared with wild-type or PMP22 mutant samples. Together these results support a role for PMP22 in the early events of peripheral nerve myelination. Additionally, although myelin abnormalities are a commonality among PMP22 neuropathic models, the underlying subcellular mechanisms are distinct and depend on the specific genetic abnormality.

Novel aspects of vitamin C: how important is glypican-1 recycling?

The reduced form of vitamin C, ascorbic acid, is well known for its function as an antioxidant and as a protective agent against scurvy. However, many recent studies indicate other functions for vitamin C in mammalian cells. Novel findings provide possible explanations for observed beneficial effects of a high intake of vitamin C on cell growth, gene transcription, host resistance to infection, uptake of polyamines and clearance of misfolded proteins. Vitamin C exerts its effects indirectly via hypoxia-inducible factor, nitric oxide synthase and the heparan sulfate proteoglycan glypican-1, which is deglycanated in a vitamin C- and copper-dependent reaction.

Ascorbic acid inhibits PMP22 expression by reducing cAMP levels

Charcot-Marie-Tooth [CMT] syndrome is the most common hereditary peripheral neuropathy. CMT1A, which accounts for 50% of all CMT cases, usually results from triploidy of the PMP22 gene. Preclinical trials using an animal model show that disabled mice force-fed with high doses of ascorbic acid partially recover muscular strength after a few months of treatment, and suggest that high doses of ascorbic acid repress PMP22 expression. In this study, we demonstrated that ascorbic acid represses PMP22 gene expression by acting on intracellular cAMP levels and adenylate cyclase activity. This action is dose dependent and specific to ascorbic acid, since repression is not observed after treatment with other antioxidants. The new properties of ascorbic acid are discussed, along with the implications of these findings for CMT disease treatment.

Asymmetric ERM activation at the Schwann cell process tip is required in axon-associated motility

Axon-associated Schwann cell (SC) motility and process dynamics are crucial in the development and regeneration of the peripheral nervous system (PNS). The bipolar morphology of SCs represents an unexplored conundrum in terms of directed motility. Using fluorescence time-lapse microscopy of transfected SCs within myelinating dorsal root ganglion (DRG) explants, we demonstrate cycling of SCs between bipolar and highly motile, unipolar morphologies as a result of asymmetric process retraction and extension. Unipolar SC motility appears nucleotaxic in nature, similar to the movement of neurons on radial glia during cortical development. We also show that asymmetric process retraction is associated with the activation of ERM (ezrin/radixin/moesin) proteins and subsequent recruitment of ezrin-binding phospho-protein 50 kDa (EBP50) at the retracting process tip. This activation occurs in response to localized synthesis of phosphatidylinositol (4,5)-bisphosphate (PIP2) at this site. Finally, we demonstrate that the activation of ERM proteins at the SC process tip is essential for motility and the maintenance of SC polarity, as ERM disruption yields a dysfunctional, multi-polar cell. These results demonstrate that specializations at the tips of SC processes regulate their dynamics, which in turn is associated with directed motility in these cells.

Laminin alpha1 chain improves laminin alpha2 chain deficient peripheral neuropathy

Absence of laminin alpha2 chain leads to a severe form of congenital muscular dystrophy (MDC1A) associated with peripheral neuropathy. Hence, future therapies should be aimed at alleviating both muscle and neurological dysfunctions. Pre-clinical studies in animal models have mainly focused on ameliorating the muscle phenotype. Here we show that transgenic expression of laminin alpha1 chain in muscles and the peripheral nervous system of laminin alpha2 chain deficient mice reduced muscular dystrophy and largely corrected the peripheral nerve defects. The presence of laminin alpha1 chain in the peripheral nervous system resulted in near-normal myelination, restored Schwann cell basement membranes and improved rotarod performance. In summary, we postulate that laminin alpha1 chain is an excellent substitute for laminin alpha2 chain in multiple tissues and suggest that treatment with laminin alpha1 chain may be beneficial for MDC1A in humans.

Skin-derived precursors generate myelinating Schwann cells for the injured and dysmyelinated nervous system

Although neural stem cells hold considerable promise for treatment of the injured or degenerating nervous system, their current human sources are embryonic stem cells and fetally derived neural tissue. Here, we asked whether rodent and human skin-derived precursors (SKPs), neural crest-related precursors found in neonatal dermis, represent a source of functional, myelinating Schwann cells. Specifically, cultured SKPs responded to neural crest cues such as neuregulins to generate Schwann cells, and these Schwann cells proliferated and induced myelin proteins when in contact with sensory neuron axons in culture. Similar results were obtained in vivo; 6 weeks after transplantation of naive SKPs or SKP-derived Schwann cells into the injured peripheral nerve of wild-type or shiverer mutant mice (which are genetically deficient in myelin basic protein), the majority of SKP-derived cells had associated with and myelinated axons. Naive rodent or human SKPs also generated Schwann cells that myelinated CNS axons when transplanted into the dysmyelinated brain of neonatal shiverer mice. Thus, neonatal SKPs generate functional neural progeny in response to appropriate neural crest cues and, in so doing, provide a highly accessible source of myelinating cells for treatment of nervous system injury, congenital leukodystrophies, and dysmyelinating disorders.

Signaling axis in schwann cell proliferation and differentiation

Recent progress in molecular biology has markedly expanded our knowledge of the molecular mechanism behind the proliferation and differentiation processes of Schwann cells, the myelin-forming cells in peripheral nervous systems. Intracellular signaling molecules participate in integrating various stimuli from cytokines and other humoral factors and control the transcriptional activities of the genes that regulate mitosis or differentiation. This article provides an overview of the roles played by the intracellular pathways regulating Schwann cell functions. In Schwann cell proliferation, cyclic adenosine monophosphate signals and mitogen-activated protein kinase pathways play pivotal roles and may also interact with each other. Regarding differentiation, myelin formation is regulated by various cytokines and extracellular matrix molecules. Specifically, platelet-derived growth factor, neuregulin, and insulin-like growth factor-I all are classified as ligands for receptor-type tyrosine kinase and activate common intracellular signaling cascades, mitogen-activated protein kinase pathways, and phosphatidylinositol-3-kinase pathways. The balance of activities between these two pathways appears crucial in regulating Schwann cell differentiation, in which phosphatidylinositol-3-kinase pathways promote myelin formation. Analysis of these signaling molecules in Schwann cells will enable us not only to understand their physiological development but also to innovate new approaches to treat disorders related to myelination.

Peripheral myelin protein 22 is in complex with alpha6beta4 integrin, and its absence alters the Schwann cell basal lamina

Peripheral myelin protein 22 (PMP22) is a tetraspan membrane glycoprotein, the misexpression of which is associated with hereditary demyelinating neuropathies. Myelinating Schwann cells (SCs) produce the highest levels of PMP22, yet the function of the protein in peripheral nerve biology is unresolved. To investigate the potential roles of PMP22, we engineered a novel knock-out (-/-) mouse line by replacing the first two coding exons of pmp22 with the lacZ reporter. PMP22-deficient mice show strong beta-galactosidase reactivity in peripheral nerves, cartilage, intestines, and lungs, whereas phenotypically they display the characteristics of tomaculous neuropathy. In the absence of PMP22, myelination of peripheral nerves is delayed, and numerous axon-SC profiles show loose basal lamina, suggesting altered interactions of the glial cells with the extracellular matrix. The levels of beta4 integrin, a molecule involved in the linkage between SCs and the basal lamina, are severely reduced in nerves of PMP22-deficient mice. During early stages of myelination, PMP22 and beta4 integrin are coexpressed at the cell surface and can be coimmunoprecipitated together with laminin and alpha6 integrin. In agreement, in clone A colonic carcinoma cells, epitope-tagged PMP22 forms a complex with beta4 integrin. Together, these data indicate that PMP22 is a binding partner in the integrin/laminin complex and is involved in mediating the interaction of SCs with the extracellular environment.

Microanatomy of axon/glial signaling during Wallerian degeneration

How do myelinated axons signal to the nuclei of cells that enwrap them? The cell bodies of oligodendrocytes and Schwann cells are segregated from axons by multiple layers of bimolecular lipid leaflet and myelin proteins. Conventional signal transduction strategies would seem inadequate to the challenge without special adaptations. Wallerian degeneration provides a model to study axon-to-Schwann cell signaling in the context of nerve injury. We show a hitherto undetected rapid, but transient, activation of the receptor tyrosine kinase erbB2 in myelinating Schwann cells after sciatic nerve axotomy. Deconvolving microscopy using phosphorylation state-specific antibodies shows that erbB2 activation emanates from within the microvilli of Schwann cells, in direct contact with the axons they enwrap. To define the functional role of this transient activation, we used a small molecule antagonist of erbB2 activation (PKI166). The response of myelinating Schwann cells to axotomy is inhibited by PKI166 in vivo. Using neuron/Schwann cell cocultures prepared in compartmentalized cell culture chambers, we show that even transient activation of erbB2 is sufficient to initiate Schwann cell demyelination and that the initiating functions of erbB2 are localized to Schwann cells.

Glypican-1 and alpha4(V) collagen are required for Schwann cell myelination

Schwann cell myelination requires interactions with the extracellular matrix (ECM) mediated by cell surface receptors. Previously, we identified a type V collagen family member, alpha4(V) collagen, which is expressed by Schwann cells during peripheral nerve differentiation. This collagen binds with high affinity to heparan sulfate through a unique binding motif in the noncollagenous N-terminal domain (NTD). The principal alpha4(V) collagen-binding protein on the Schwann cell surface is the heparan sulfate proteoglycan glypican-1. We investigated the role of alpha4(V) collagen and glypican-1 in Schwann cell terminal differentiation in cultures of Schwann cells and dorsal root ganglion neurons. Small interfering RNA-mediated suppression of glypican-1 expression decreased binding of alpha4(V)-NTD to Schwann cells, adhesion and spreading of Schwann cells on alpha4(V)-NTD, and incorporation of alpha4(V) collagen into Schwann cell ECM. In cocultures, alpha4(V) collagen coassembles with laminin on the surface of polarized Schwann cells to form tube-like ECM structures that are sites of myelination. Suppression of glypican-1 or alpha4(V) collagen expression significantly inhibited myelination. These results demonstrate an important role for these proteins in peripheral nerve terminal differentiation.

Laminins and their receptors in Schwann cells and hereditary neuropathies

This review focuses on the influence of laminins, mediated through laminin receptors present on Schwann cells, on peripheral nerve development and pathology. Laminins influence multiple aspects of cell differentiation and tissue morphogenesis, including cell survival, proliferation, cytoskeletal rearrangements, and polarity. Peripheral nerves are no exception, as shown by the discovery that defective laminin signals contribute to the pathogenesis of diverse neuropathies such as merosin-deficient congenital muscular dystrophy and Charcot-Marie-Tooth 4F, neurofibromatosis, and leprosy. In the last 5 years, advanced molecular and cell biological techniques and conditional mutagenesis in mice began revealing the role of different laminins and receptors in developing nerves. In this way, we are starting to explain morphological and pathological observations beginning at the start of the last century. Here, we review these recent advances and show how the roles of laminins and their receptors are surprisingly varied in both time and place.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

Defining the role of olfactory ensheathing cells in facilitating axon remyelination following damage to the spinal cord

Olfactory ensheathing cells (OECs) are unique cells that are responsible for the successful regeneration of olfactory axons throughout the life of adult mammals. More than a decade of research has shown that implantation of OECs may be a promising therapy for damage to the nervous system, including spinal cord injury. Based on this research, several clinical trials worldwide have been initiated that use autologous transplantation of olfactory tissue containing OECs into the damaged spinal cord of humans. However, research from several laboratories has challenged the widely held belief that OECs are directly responsible for myelinating axons and promoting axon regeneration. The purpose of this review is to provide a working hypothesis that integrates several current ideas regarding the mechanisms of the beneficial effects of OECs. Specifically, OECs promote axon regeneration and functional recovery indirectly by augmenting the endogenous capacity of host Schwann cells to invade the damaged spinal cord. Together with Schwann cells, OECs create a 3-dimensional matrix that provides a permissive microenvironment for successful axon regeneration in the adult mammalian central nervous system.

Vitamin C uptake and recycling among normal and tumor cells from the central nervous system

Specialized cells transport vitamin C in its reduced form using sodium-dependent cotransporters (SVCT1 and SVCT2). Additionally, different cells transport the oxidized form of vitamin C, dehydroascorbic acid, through glucose transporters (GLUTs). We have proposed recently a model for vitamin C uptake that resolves the apparent contradiction that although only ascorbic acid is detectable in vivo, there are cells that transport only dehydroascorbic acid. We carried out a detailed kinetic analysis to compare the mechanisms of vitamin C uptake in normal human melanocytes, neurons isolated from brain cortex, hypothalamic ependymal-glial cells, and astrocytes. Uptake of ascorbic acid was also analyzed in the human oligodendroglioma cell line TC620, in human choroid plexus papilloma cells (HCPPC-1), and in the neuroblastoma cell line Neuro-2a. Melanocytes were used to carry out a detailed analysis of vitamin C uptake. Analysis of the transport data by the Lineweaver-Burk plot revealed the presence of one functional component (K(m) 20 microM) involved in ascorbic acid transport by melanocytes. Vitamin C sodium-dependent saturable uptake was also observed in neurons and hypothalamic tanycytes. We confirmed SVCT2 expression in neurons by in situ hybridization; however, SVCT2 expression was not detected in astrocytes in situ. Functional data indicate that astrocytes transport mainly dehydroascorbic acid, using the glucose transporter GLUT1. Our functional uptake analyses support the hypothesis that astrocytes are involved in vitamin C recycling in the nervous system. This recycling model may work as an efficient system for the salvage of vitamin C by avoiding the hydrolysis of dehydroascorbic acid produced by antioxidative protection.

Erythropoietin is neuroprotective in models of HIV sensory neuropathy

HIV-associated sensory neuropathy (HIV-SN) is the most common neurological complication of HIV infection. Presently, there are no effective therapies for this painful neuropathy. The pathology of HIV-SN is characterized by 'dying back' sensory axonal degeneration and a more modest loss of dorsal root ganglion (DRG) sensory neurons. It has been hypothesized that HIV-SN results from neurotoxicity by secreted viral proteins, such as the HIV envelope glycoprotein gp120. Furthermore, neurotoxicity by dideoxynucleoside (DDX) agents, results in the observed higher incidence of HIV-SN in HIV-infected patients taking these antiretroviral drugs. In this study we show that administration of picomolar amounts of the hormone erythropoietin (EPO) prevents sensory axonal degeneration and in vitro DRG neuronal death by both gp120 and ddC (a neurotoxic DDX drug). Our results suggest that EPO may be useful in the treatment of HIV-SN.

A novel endogenous erythropoietin mediated pathway prevents axonal degeneration

Clinically relevant peripheral neuropathies (such as diabetic and human immunodeficiency virus sensory neuropathies) are characterized by distal axonal degeneration, rather than neuronal death. Here, we describe a novel, endogenous pathway that prevents axonal degeneration. We show that in response to axonal injury, periaxonal Schwann cells release erythropoietin (EPO), which via EPO receptor binding on neurons, prevents axonal degeneration. We demonstrate that the relevant axonal injury signal that stimulates EPO production from surrounding glial cells is nitric oxide. In addition, we show that this endogenous pathway can be therapeutically exploited by administering exogenous EPO. In an animal model of distal axonopathy, systemic EPO administration prevents axonal degeneration, and this is associated with a reduction in limb weakness and neuropathic pain behavior. Our in vivo and in vitro data suggest that EPO prevents axonal degeneration and therefore may be therapeutically useful in a wide variety of human neurological diseases characterized by axonopathy.

The Protooncogene Ski Controls Schwann Cell Proliferation and Myelination

Schwann cell proliferation and subsequent differentiation to nonmyelinating and myelinating cells are closely linked processes. Elucidating the molecular mechanisms that control these events is key to the understanding of nerve development, regeneration, nerve-sheath tumors, and neuropathies. We define the protooncogene Ski, an inhibitor of TGF-beta signaling, as an essential component of the machinery that controls Schwann cell proliferation and myelination. Functional Ski overexpression inhibits TGF-beta-mediated proliferation and prevents growth-arrested Schwann cells from reentering the cell cycle. Consistent with these findings, myelinating Schwann cells upregulate Ski during development and remyelination after injury. Myelination is blocked in myelin-competent cultures derived from Ski-deficient animals, and genes encoding myelin components are downregulated in Ski-deficient nerves. Conversely, overexpression of Ski in Schwann cells causes an upregulation of myelin-related genes. The myelination-regulating transcription factor Oct6 is involved in a complex modulatory relationship with Ski. We conclude that Ski is a crucial signal in Schwann cell development and myelination.

Ascorbic acid treatment corrects the phenotype of a mouse model of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is the most common hereditary peripheral neuropathy, affecting 1 in 2,500 people. The only treatment currently available is rehabilitation or corrective surgery. The most frequent form of the disease, CMT-1A, involves abnormal myelination of the peripheral nerves. Here we used a mouse model of CMT-1A to test the ability of ascorbic acid, a known promoter of myelination, to correct the CMT-1A phenotype. Ascorbic acid treatment resulted in substantial amelioration of the CMT-1A phenotype, and reduced the expression of PMP22 to a level below what is necessary to induce the disease phenotype. As ascorbic acid has already been approved by the FDA for other clinical indications, it offers an immediate therapeutic possibility for patients with the disease.

Synthesis of Tissues and Organs

Symbolism that describes the synthetic processes for chemical compounds has been used to describe, in qualitative terms, the synthesis of tissues and organs at the correct anatomical site. The synthetic process is summarized in the reaction diagram, a shorthand representation of the reactants, reactor, and products. Analysis of a large number of independent protocols has led to identification of the simplest synthetic pathways for two organs that have been studied extensively: skin and peripheral nerves. These apparently irreducible reaction diagrams for the two organs are not only simple but surprisingly similar, a fact suggesting the existence of general rules for the synthesis of other organs as well. The only two reactants that are required are an active scaffold (a macromolecular network synthesized as a highly porous analogue of the extracellular matrix) and a seeding of epithelial cells of the organ being synthesized. Scaffolds possessed biological (regenerative) activity provided that they were capable of blocking the contraction process that leads to closure of the injured site. Such activity requires that the density of ligands for binding of contractile cells on the scaffold maintains a sufficiently high level over the period of synthesis.

Functional characterisation of the active ascorbic acid transport into cerebrospinal fluid using primary cultured choroid plexus cells

Crossing the blood-CSF barrier is an important pathway for certain nutrients to enter the CNS. Cultured choroid plexus epithelial cells are a potent model system to study active transport properties of this tissue in vitro. In the present study this in vitro model was used to analyse ascorbic acid transport across the blood-CSF barrier that is supposedly mediated by the Na(+)-dependent transporter SVCT2. The expression of SVCT2 in the cultured cells was proven by RT-PCR. Active transport across the cell monolayer resulted in ascorbic acid enrichment at the CSF mimicking side. Ascorbic acid transport and uptake were decreased to 13 and 27%, respectively, in the presence of 200 microM phloretin. Inhibition of both transepithelial substrate transport (to 7.5%) and cytoplasmatic uptake (to 20%) was observed in Na(+)-free medium indicating that a basolaterally located and Na(+)-dependent transporter mediates ascorbic acid uptake. Substituting Cl(-) by either iodide or D-gluconate increased ascorbic acid uptake by factors of 3.7 or 2.5, respectively. Similar observations were made when Na(+)-dependent myo-inositol transport was analysed. Additionally, in presence of 100 microM bumetanide, an inhibitor of Na(+)-Cl(-)-cotransport, indirectly increased ascorbic acid and myo-inositol transport rates were observed showing that ascorbic acid-Na(+)-cotransport might balance low intracellular Na(+) concentration.

Schwann cell chemokine receptors mediate HIV-1 gp120 toxicity to sensory neurons

Human immunodeficiency virus (HIV)-associated sensory neuropathy (HIV-SN) is the most common neurological complication of HIV infection. Currently, the pathogenesis of HIV-SN is unknown. Because there is no convincing evidence of neuronal infection, HIV neurotoxicity is likely to be effected either by secreted viral proteins such as the envelope glycoprotein gp120 or by neurotoxic cytokines released from infected/activated glial cells. We describe a model of gp120 toxicity to primary sensory neurons, in which gp120 induces neuritic degeneration and neuronal apoptosis. We show that Schwann cells, the cells that ensheath peripheral nerve axons, and which traditionally have been viewed as having a passive, supporting role, mediate this neurotoxicity. Ligation of the chemokine receptor CXCR4 on Schwann cells by gp120 resulted in the release of RANTES, which induced dorsal root ganglion neurons to produce tumor necrosis factor-alpha and subsequent TNFR1-mediated neurotoxicity in an autocrine fashion. This newly described Schwann cell-neuron interaction may be pathogenically relevant not only in HIV-SN but also in other peripheral neuropathies.

Inhibition of p38 mitogen-activated protein kinase interferes with cell shape changes and gene expression associated with Schwann cell myelination

In the present study we demonstrate that p38, a member of the mitogen-activated protein kinase (MAPK) family, is essential for ascorbate- and laminin-induced myelination in Schwann cell-dorsal root ganglion neuron cocultures. The inhibitory effect of the specific p38 blockers, PD 169316 and SB 203580, on ascorbate-induced myelination was exerted during the early stages (1-2 days) of ascorbate treatment. Inhibition of p38 was further shown to prevent the alignment of Schwann cells along axons in laminin-treated cocultures. The addition of laminin to Schwann cell-dorsal root ganglion neuron cocultures stimulated phosphorylation of p38, thereby demonstrating a link between laminin-induced myelination and p38 activation. Similarly, the small heat shock protein, Hsp27, which is phosphorylated by MAPKAPK2, a downstream substrate of p38, was phosphorylated in response to the addition of laminin to the cocultures. The p38 inhibitors did not affect the proliferation or survival of Schwann cells in the cocultures as assessed by BrdU incorporation and total cell counts. However, p38 inhibition interfered with an early stage in myelination, thereby preventing ascorbate-induced increases in the levels of mRNAs encoding MBP, MAG, and P(0) and reducing laminin deposition. These results indicate that activation of p38 by a signaling pathway(s) involving laminin and appropriate integrin receptor(s) is required for the alignment of Schwann cells with axons that precedes myelination.

Local ERM activation and dynamic growth cones at Schwann cell tips implicated in efficient formation of nodes of Ranvier

Nodes of Ranvier are specialized, highly polarized axonal domains crucial to the propagation of saltatory action potentials. In the peripheral nervous system, axo-glial cell contacts have been implicated in Schwann cell (SC) differentiation and formation of the nodes of Ranvier. SC microvilli establish axonal contact at mature nodes, and their components have been observed to localize early to sites of developing nodes. However, a role for these contacts in node formation remains controversial. Using a myelinating explant culture system, we have observed that SCs reorganize and polarize microvillar components, such as the ezrin-binding phosphoprotein 50 kD/regulatory cofactor of the sodium-hydrogen exchanger isoform 3 (NHERF-1), actin, and the activated ezrin, radixin, and moesin family proteins before myelination in response to inductive signals. These components are targeted to the SC distal tips where live cell imaging reveals novel, dynamic growth cone-like behavior. Furthermore, localized activation of the Rho signaling pathway at SC tips gives rise to these microvillar component-enriched "caps" and influences the efficiency of node formation.

Antioxidants are necessary for myelination of dorsal root ganglion neurons, in vitro

We have demonstrated that myelination of dorsal root ganglion (DRG) axons occurs in a fully defined, serum-free medium (B27). This implies that there may be components in B27 medium that support myelination. To determine which of the components in B27 were essential for myelination, we systematically removed components from B27 until myelination was lost. We added these components to a fully defined minimal medium (N2) that supports neuron survival but not myelination. When antioxidants were removed from B27, myelination was lost. However, the individual antioxidants did not induce myelination when added to N2 medium. Addition of ascorbic acid along with the B27 antioxidants was sufficient to induce myelination in N2 medium, which was enhanced by retinyl acetate. Removal of vitamin E from B27 caused a partial loss of myelination, and addition of vitamin E to N2 medium containing ascorbic acid induced partial myelination. Addition of serum to the B27 myelinating medium inhibited myelination completely. These results indicate that antioxidants are important for myelination, in vitro. Vitamin E may play an important role. Use of a serum-free medium may be beneficial for in vitro myelination studies because serum has unknown inhibitory effects.

Formation of a beta1 integrin signaling complex in Schwann cells is independent of rho

Schwann cell adhesion to basal lamina is essential for peripheral nerve development. beta(1) integrin receptors for extracellular matrix cooperate with other receptors to transmit signals that coordinate cell cycle progression and initiation of differentiation, including myelin-specific gene expression. In Schwann cell/sensory neuron cocultures, beta(1) integrins complex with focal adhesion kinase (FAK), fyn kinase, paxillin, and schwannomin in response to basal lamina adhesion. To study the assembly of this signaling complex in Schwann cells (SCs), we induced beta(1) integrin clustering on suspended cells using an immobilized antibody and recovered a complex containing beta(1) integrin, FAK, paxillin, and schwannomin. In adherent subconfluent cells, the proteins colocalized to filopodia, ruffling membranes and focal contacts. We assessed the role of rhoGTPase in the process of integrin complex assembly by introducing C3 transferase (C3T), a rho inhibitor, into the cells. Although C3T caused dose-dependent morphological abnormalities, FAK, paxillin, and schwannomin were able to coimmunoprecipitate with beta(1) integrin. Additionally, colocalization of FAK, paxillin, and schwannomin with beta(1) integrin in filopodia and small focal contacts remained unchanged. We conclude that SCs do not require active rho to recruit signaling and structural proteins to beta(1) integrins clustered at the plasma membrane. Rho is required to establish large focal adhesions and to spread and stabilize plasma membrane extensions.

FK506 is neuroprotective in a model of antiretroviral toxic neuropathy

Antiretroviral toxic neuropathy is the most common neurological complication of human immunodeficiency virus infection. This painful neuropathy not only affects the quality of life of human immunodeficiency virus-infected patients but also severely limits viral suppression strategies. We have developed an in vitro model of this toxic neuropathy to better understand the mechanism of neurotoxicity and to test potential neuroprotective compounds. We show that among the dideoxynucleosides, ddC appears to be the most neurotoxic, followed by ddI and then d4T. This reflects their potency in causing neuropathy. AZT, which does not cause a peripheral neuropathy in patients, does not cause significant neurotoxicity in our model. Furthermore, in this model, we show that the immunophilin ligand FK506 but not cyclosporin A prevents the development of neurotoxicity by ddC, as judged by amelioration of ddC-induced "neuritic pruning," neuronal mitochondrial depolarization, and neuronal necrotic death. This finding suggests a calcineurin-independent mechanism of neuroprotection. As calcineurin inhibition underlies the immunosuppressive properties of these clinically used immunophilin ligands, this holds promise for the neuroprotective efficacy of nonimmunosuppressive analogs of FK506 in the prevention or treatment of antiretroviral toxic neuropathy.

Paxillin binds schwannomin and regulates its density-dependent localization and effect on cell morphology

Neurofibromatosis type 2 is an autosomal dominant disorder characterized by tumors, predominantly schwannomas, in the nervous system. It is caused by mutations in the gene NF2, encoding the growth regulator schwannomin (also known as merlin). Mutations occur throughout the 17-exon gene, with most resulting in protein truncation and undetectable amounts of schwannomin protein. Pathogenic mutations that result in production of defective schwannomin include in-frame deletions of exon 2 and three independent missense mutations within this same exon. Mice with conditional deletion of exon 2 in Schwann cells develop schwannomas, which confirms the crucial nature of exon 2 for growth control. Here we report that the molecular adaptor paxillin binds directly to schwannomin at residues 50-70, which are encoded by exon 2. This interaction mediates the membrane localization of schwannomin to the plasma membrane, where it associates with beta 1 integrin and erbB2. It defines a pathogenic mechanism for the development of NF2 in humans with mutations in exon 2 of NF2.

Progesterone synthesis and myelin formation in peripheral nerves

Progesterone is synthesized in the nervous system by neurons and glial cells. Because of their simple structure, plasticity and capacity of regeneration, peripheral nerves are particularly well suited for studying the biosynthesis, mechanisms of action and effects of the hormone. Schwann cells, the myelinating glial cells in the peripheral nervous system, synthesize progesterone in response to a diffusible neuronal signal. In peripheral nerves, the local synthesis of progesterone plays an important role in the formation of myelin sheaths. This has been shown in vivo, after cryolesion of the mouse sciatic nerve, and in vitro, in cocultures of Schwann cells and sensory neurons. Schwann cells also express an intracellular receptor for progesterone, which thus functions as an autocrine signalling molecule. Progesterone may promote myelination by activating the expression of genes coding for transcription factors (Krox-20) and/or for myelin proteins (P0, PMP22). Recently, it has been proposed that progesterone may indirectly regulate myelin formation by influencing gene expression in neurons. Steroid hormones also influence the proliferation of Schwann cells: estradiol becomes a potent mitogen for Schwann cells when levels of cAMP are elevated and glucocorticosteroids have been shown to increase the mitogenic effects of peptide growth factors.

Schwann cell myelination occurred without basal lamina formation in laminin alpha2 chain-null mutant (dy3K/dy3K) mice

The laminin alpha2 chain is a major component of basal lamina in both skeletal muscle and the peripheral nervous system. Laminin alpha2 chain deficiency causes merosin-deficient congenital muscular dystrophy, which affects not only skeletal muscles, but also the peripheral and central nervous systems. It has been reported that the formation of basal lamina is required for myelination in the peripheral nervous system. In fact, the spinal root of dystrophic mice (dy/dy mice), whose laminin alpha2 chain expression is greatly reduced, shows lack of basal lamina and clusters of naked axons. To investigate the role of laminin alpha2 chain and basal lamina in vivo, we examined the peripheral nervous system of dy3K/dy3K mice, which are null mutants of laminin alpha2 chain. The results indicate the presence of myelination although Schwann cells lacked basal lamina in the spinal roots of dy3K/dy3K mice, suggesting that basal lamina is not an absolute requirement for myelination in vivo. Immunohistochemically, the expression of laminin alpha4 chain was increased and laminin alpha5 chain was preserved in the endoneurium of the spinal root. Laminin alpha4 and alpha5 chains may play the critical role in myelination instead of laminin alpha2 chain in dy3K/dy3K mice. In addition, the motor conduction velocity of the sciatic nerve was significantly reduced compared with that of wild-type littermate. This reduction in conduction velocity may be due to small axon diameter, thin myelin sheath and the patchy disruption of the basal lamina of the nodes of Ranvier in dy3K/dy3K mice.

High-affinity sodium-vitamin C co-transporters (SVCT) expression in embryonic mouse neurons

The sodium-vitamin C co-transporters SVCT1 and SVCT2 transport the reduced form of vitamin C, ascorbic acid. High expression of the SVCT2 has been demonstrated in adult neurons and choroid plexus cells by in situ hybridization. Additionally, embryonic mesencephalic dopaminergic neurons express the SVCT2 transporter. However, there have not been molecular and kinetic analyses addressing the expression of SVCTs in cortical embryonic neurons. In this work, we confirmed the expression of a SVCT2-like transporter in different regions of the fetal mouse brain and in primary cultures of neurons by RT-PCR. Kinetic analysis of the ascorbic acid uptake demonstrated the presence of two affinity constants, 103 microM and 8 microM. A K(m) of 103 microM corresponds to a similar affinity constant reported for SVCT2, while the K(m) of 8 microM might suggest the expression of a very high affinity transporter for ascorbic acid. Our uptake analyses also suggest that neurons take up dehydroascorbic acid, the oxidized form of vitamin C, through the glucose transporters. We consider that the early expression of SVCTs transporters in neurons is important in the uptake of vitamin C, an essential molecule for the fetal brain physiology. Vitamin C that is found at high concentration in fetal brain may function in preventing oxidative free radical damage, because antioxidant radical enzymes mature only late in the developing brain.

Subcellular localization of a glutathione-dependent dehydroascorbate reductase within specific rat brain regions

Recently, we described the occurrence of a dehydroascorbate reductase within the rat CNS. This enzyme regenerates ascorbate after it is oxidized during normal aerobic metabolism. In this work, we describe the neuronal compartmentalization of the enzyme, using transmission electron microscopy of those brain areas in which the enzyme was most densely present when observed under light microscopy. In parallel biochemical studies, we performed immunoblotting and measured the enzyme activity of the cytoplasm and different nuclear fractions. Given the abundance of ascorbate in the caudate-putamen, we focused mostly on the occurrence of dehydroascorbate reductase at the striatal subcellular level. We also studied cerebellar Purkinje cells, hippocampal CA3 pyramidal cells and giant neurons in the magnocellular part of the red nucleus. In addition to neurons, immunolabeling was found in striatal endothelial cells, in the basal membrane of blood vessels and in perivascular astrocytes. In neuronal cytosol, the enzyme was observed in a peri-nuclear position and on the nuclear membrane. In addition, in both the striatum and the cerebellum, we found the enzyme within myelin sheets. Dehydroascorbate reductase was also present in the nucleus of neurons, as further indicated by measuring enzyme activity and by immunoblotting selected nuclear fractions. Immunocytochemical labeling confirmed that the protein was present in isolated pure nuclear fractions. Given the great amount of free radicals which are constantly generated in the CNS, the discovery of a new enzyme with antioxidant properties which translocates into neuronal nuclei appears to be a potential starting point to develop alternative strategies in neuroprotection.

Role of the extracellular matrix in myelination of peripheral nerve

Assembly of the extracellular matrix (ECM) has been tightly linked to compact myelin formation in the peripheral nervous system. We recently demonstrated that myelination of dorsal root ganglion (DRG) axons by Schwann cells may occur in the absence of basal lamina. We have now determined whether laminin deposition occurs around myelinating SC, even though basal lamina has not been assembled. DRG/SC co-cultures were prepared from E15 rat embryos and incubated in fully defined medium (B27) with and without ascorbic acid for 21-24 days. Cultures were stained with a rabbit anti-laminin antibody and examined by laser confocal fluorescence microscopy. Myelination occurred in both groups. In the presence of ascorbic acid, there was dense even laminin staining around myelinating SC. In the absence of ascorbic acid, laminin staining was also present but was irregular and less dense. DRG and SC were co-cultured without ascorbic acid in the presence or absence of a function blocking anti-beta(1) integrin receptor antibody. The antibody completely inhibited myelination. Finally, DRG/SC co-cultures were prepared both with and without ascorbic acid and incubated under control conditions or in the presence of continual, gentle motion. Movement in the absence of ECM significantly inhibited myelination. This demonstrates that laminin deposition on the surface of SC but not ECM assembly is required for formation of compact myelin. ECM is required to provide mechanical stability during the process of myelination.

PMP22 carrying the trembler or trembler-J mutation is intracellularly retained in myelinating Schwann cells

Missense mutations in the murine peripheral myelin protein-22 gene (Pmp22) underly the neuropathies in the trembler (Tr) and trembler-J (Tr-J) mice and in some humans with Charcot-Marie-Tooth disease. We have generated replication-defective adenoviruses containing epitope-tagged, wild-type-, Tr-, or Tr-J-PMP22 bicistronic with the Lac-Z reporter gene. These viruses were microinjected into the sciatic nerves of 10-day-old Sprague-Dawley rats and, later, analyzed by immunohistochemistry to determine the distribution of mutant protein in infected myelinating Schwann cells. We found that epitope-tagged, wild-type PMP22 is successfully transported to compact myelin, whereas the Tr and the Tr-J mutant proteins are retained in cytoplasmic compartment, colocalizing with the endoplasmic reticulum. These results provide in vivo evidence that the pathogenesis of the Tr and Tr-J mutations are most likely a function of abnormal retention within the endoplasmic reticulum of myelinating Schwann cells.

Influence of laminin-2 on Schwann cell-axon interactions

The dy/dy mouse suffers from a form of muscular dystrophy caused by a substantial reduction in laminin alpha2-chain protein, a major component of both muscle and Schwann cell basal laminae. This article examines the effect of laminin alpha2 deficiency on Schwann cell-axon interactions both in vivo at varying intervals after nerve crush, and in vitro, in cocultures of neurons and Schwann cells. The morphological spectrum of aberrant Schwann cell-axon associations seen in uncrushed dy/dy sciatic nerves was recapitulated during regeneration: myelination of regenerating axons was delayed compared with the process in unaffected mice and the relatively few myelin sheaths which were formed in dy/dy distal nerve stumps were often uncompacted. In vitro, Schwann cells dissociated from adult dy/dy sciatic nerves predictably failed to express detectable laminin alpha2-chain and displayed an unusual multipolar morphology. Branching of neurites, in terms both of numbers of terminal branches and of complexity of branching, from dorsal root ganglia neurons grown on dy/dy Schwann cells, was significantly less extensive than that seen when neurons were cocultured with Schwann cells from unaffected littermates, but this effect was reversed by exogenous laminin-2. Our results lend strong support to the view that laminin-2 is essential for establishing and/or maintaining Schwann cell-axon interactions, in normal and in regenerating nerves.

Progesterone as a neuroactive neurosteroid, with special reference to the effect of progesterone on myelination

Some steroids are synthesized within the central and peripheral nervous system, mostly by glial cells. These are known as neurosteroids. In the brain, certain neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3alpha,5alpha-reduced metabolite 3alpha, 5alpha-tetrahydroprogesterone (allopregnanolone) activates the type A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.

Progesterone synthesized by Schwann cells during myelin formation regulates neuronal gene expression

Previously, progesterone was found to regulate the initiation and biosynthetic rate of myelin synthesis in Schwann cell/neuronal cocultures. The mRNA for cytochrome P450scc (converts cholesterol to pregnenolone), 3beta-hydroxysteroid dehydrogenase (3beta-HSD, converts pregnenolone to progesterone), and the progesterone receptor were found to be markedly induced during active myelin synthesis. However, the cells in the cocultures responsible for these changes were not identified. In this study, in situ hybridization was used to determine the localization of the enzymes responsible for steroid biosynthesis. The mRNA for cytochrome P450scc and 3beta-HSD were detected only in actively myelinating cocultures and were localized exclusively in the Schwann cells. Using immunocytochemistry, with minimal staining of the Schwann cells, we found the progesterone receptor in the dorsal root ganglia (DRG) neurons. The progesterone receptor in the neurons translocated into the nuclei of these cells when progesterone was added to neuronal cultures or during myelin synthesis in the cocultures. Additionally, a marked induction of the progesterone receptor was found in neuronal cultures after the addition of progesterone. The induction of various genes in the neurons was also investigated using mRNA differential display PCR in an attempt to elucidate the mechanism of steroid action on myelin synthesis. Two novel genes were induced in neuronal cultures by progesterone. These genes, along with the progesterone receptor, were also induced in cocultures during myelin synthesis, and their induction was blocked by RU-486 (a progesterone receptor antagonist). These genes were not induced in Schwann cells cultured alone after the addition of progesterone. These results suggest that progesterone is synthesized in Schwann cells and that it can indirectly regulate myelin formation by activating transcription via the classical steroid receptor in the DRG neurons.

Axonal regulation of Schwann cell proliferation and survival and the initial events of myelination requires PI 3-kinase activity

In this report, we have investigated the signaling pathways that are activated by, and mediate the effects of, the neuregulins and axonal contact in Schwann cells. Phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein kinase kinase (MAPK kinase) are strongly activated in Schwann cells by glial growth factor (GGF), a soluble neuregulin, and by contact with neurite membranes; both kinase activities are also detected in Schwann cell-DRG neuron cocultures. Inhibition of the PI 3-kinase, but not the MAP kinase, pathway reversibly inhibited Schwann cell proliferation induced by GGF and neurites. Cultured Schwann cells undergo apoptosis after serum deprivation and can be rescued by GGF or contact with neurites; these survival effects were also blocked by inhibition of PI 3-kinase. Finally, we have examined the role of these signaling pathways in Schwann cell differentiation in cocultures. At early stages of coculture, inhibition of PI 3-kinase, but not MAPK kinase, blocked Schwann cell elongation and subsequent myelination but did not affect laminin deposition. Later, after Schwann cells established a one-to-one relationship with axons, inhibition of PI 3-kinase did not block myelin formation, but the myelin sheaths that formed were shorter, and the rate of myelin protein accumulation was markedly decreased. PI 3-kinase inhibition had no observable effect on the maintenance of myelin sheaths in mature myelinated cocultures. These results indicate that activation of PI 3-kinase by axonal factors, including the neuregulins, promotes Schwann cell proliferation and survival and implicate PI 3-kinase in the early events of myelination.

Axonal regulation of Schwann cell proliferation and survival and the initial events of myelination requires PI 3-kinase activity

In this report, we have investigated the signaling pathways that are activated by, and mediate the effects of, the neuregulins and axonal contact in Schwann cells. Phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein kinase kinase (MAPK kinase) are strongly activated in Schwann cells by glial growth factor (GGF), a soluble neuregulin, and by contact with neurite membranes; both kinase activities are also detected in Schwann cell-DRG neuron cocultures. Inhibition of the PI 3-kinase, but not the MAP kinase, pathway reversibly inhibited Schwann cell proliferation induced by GGF and neurites. Cultured Schwann cells undergo apoptosis after serum deprivation and can be rescued by GGF or contact with neurites; these survival effects were also blocked by inhibition of PI 3-kinase. Finally, we have examined the role of these signaling pathways in Schwann cell differentiation in cocultures. At early stages of coculture, inhibition of PI 3-kinase, but not MAPK kinase, blocked Schwann cell elongation and subsequent myelination but did not affect laminin deposition. Later, after Schwann cells established a one-to-one relationship with axons, inhibition of PI 3-kinase did not block myelin formation, but the myelin sheaths that formed were shorter, and the rate of myelin protein accumulation was markedly decreased. PI 3-kinase inhibition had no observable effect on the maintenance of myelin sheaths in mature myelinated cocultures. These results indicate that activation of PI 3-kinase by axonal factors, including the neuregulins, promotes Schwann cell proliferation and survival and implicate PI 3-kinase in the early events of myelination.

Association of beta 1 integrin with focal adhesion kinase and paxillin in differentiating Schwann cells

Schwann cells (SCs) differentiate into a myelinating cell when simultaneously adhering to an axon destined for myelination and basal lamina. We are interested in defining the signaling pathway activated by basal lamina. Using SC/sensory neuron (N) cocultures, we identified beta1 integrin and F-actin as components of a pathway leading to myelin gene expression and myelination (Fernandez-Valle et al., 1994, 1997). Here, we show that focal adhesion kinase (FAK) and paxillin are constitutively expressed by SCs contacting axons in the absence of basal lamina. Tyrosine phosphorylation of FAK and paxillin increases as SCs form basal lamina and differentiate. FAK and paxillin specifically coimmunoprecipitate with beta1 integrin in differentiating SC/N cocultures but not SC-only cultures. Paxillin coimmunoprecipitates with FAK and fyn kinase in differentiating SC/N cocultures. A subset of tyrosine-phosphorylated beta1 integrin, FAK, and paxillin molecules reside in the insoluble, F-actin-rich fraction of differentiating cocultures. Cytochalasin D, an actin depolymerizing agent, decreases tyrosine phosphorylation of FAK and paxillin and their association with beta1 integrin and causes a dose-dependent increase in the abundance of insoluble FAK and paxillin complexes. Collectively, our work indicates that beta1 integrin, FAK, paxillin, and fyn kinase form an actin-associated complex in SCs adhering to basal lamina in the presence of axons. This complex may be important for initiating the process of SC differentiation into a myelinating cell.

Association of beta 1 integrin with focal adhesion kinase and paxillin in differentiating Schwann cells

Schwann cells (SCs) differentiate into a myelinating cell when simultaneously adhering to an axon destined for myelination and basal lamina. We are interested in defining the signaling pathway activated by basal lamina. Using SC/sensory neuron (N) cocultures, we identified beta1 integrin and F-actin as components of a pathway leading to myelin gene expression and myelination (Fernandez-Valle et al., 1994, 1997). Here, we show that focal adhesion kinase (FAK) and paxillin are constitutively expressed by SCs contacting axons in the absence of basal lamina. Tyrosine phosphorylation of FAK and paxillin increases as SCs form basal lamina and differentiate. FAK and paxillin specifically coimmunoprecipitate with beta1 integrin in differentiating SC/N cocultures but not SC-only cultures. Paxillin coimmunoprecipitates with FAK and fyn kinase in differentiating SC/N cocultures. A subset of tyrosine-phosphorylated beta1 integrin, FAK, and paxillin molecules reside in the insoluble, F-actin-rich fraction of differentiating cocultures. Cytochalasin D, an actin depolymerizing agent, decreases tyrosine phosphorylation of FAK and paxillin and their association with beta1 integrin and causes a dose-dependent increase in the abundance of insoluble FAK and paxillin complexes. Collectively, our work indicates that beta1 integrin, FAK, paxillin, and fyn kinase form an actin-associated complex in SCs adhering to basal lamina in the presence of axons. This complex may be important for initiating the process of SC differentiation into a myelinating cell.

Ascorbate regulation and its neuroprotective role in the brain

Ascorbic acid (vitamin C) occurs physiologically as the ascorbate anion: a water-soluble antioxidant that is found throughout the body. However, despite the high, homeostatically regulated levels of brain ascorbate, its specific functions in the CNS are only beginning to be elucidated. Certainly, it acts as part of the intracellular antioxidant network, and as such is normally neuroprotective. There is also evidence that it acts as a neuromodulator. A possibly unique role it might have is as an antioxidant in the brain extracellular microenvironment, where its concentration is modulated by glutamate-ascorbate heteroexchange at glutamate uptake sites. Ongoing studies of ascorbate and glutamate transporters should lead to rapid progress in understanding ascorbate regulation and function.

Astrocytes induce oligodendrocyte processes to align with and adhere to axons

In order to study the signals that control the onset of myelination, we cocultured highly purified postnatal retinal ganglion cells and optic nerve oligodendrocytes under serum-free conditions that promote their survival for at least a month and found that no myelination occurred. Although the addition of optic nerve astrocytes induced the oligodendrocyte processes to align with, and adhere to, axons, myelination still did not occur. The effect of astrocytes was mimicked by removal of polysialic acid from both cell types using neuroaminidase. These findings provide evidence for a novel role for astrocytes in controlling the onset of myelination by promoting adhesion of oligodendrocyte processes to axons. They also suggest that other, as yet unidentified, cell-cell interactions are necessary to induce the myelination process itself.

Heterophilic binding of L1 on unmyelinated sensory axons mediates Schwann cell adhesion and is required for axonal survival

This study investigated the function of the adhesion molecule L1 in unmyelinated fibers of the peripheral nervous system (PNS) by analysis of L1- deficient mice. We demonstrate that L1 is present on axons and Schwann cells of sensory unmyelinated fibers, but only on Schwann cells of sympathetic unmyelinated fibers. In L1-deficient sensory nerves, Schwann cells formed but failed to retain normal axonal ensheathment. L1-deficient mice had reduced sensory function and loss of unmyelinated axons, while sympathetic unmyelinated axons appeared normal. In nerve transplant studies, loss of axonal-L1, but not Schwann cell-L1, reproduced the L1-deficient phenotype. These data establish that heterophilic axonal-L1 interactions mediate adhesion between unmyelinated sensory axons and Schwann cells, stabilize the polarization of Schwann cell surface membranes, and mediate a trophic effect that assures axonal survival.

Myelin formation by Schwann cells in the absence of beta4 integrin

The interaction of the Schwann cell with its basal lamina has been hypothesized to be an important prerequisite for the formation of a myelin sheath in the peripheral nervous system. One possible player in this interaction is beta4 integrin; it is up-regulated during myelin formation and, in association with alpha6 integrin, can interact with particular components of the Schwann cell basal lamina. In order to characterize the functional roles of beta4 integrin during myelination, we investigated myelination in the absence of beta4 integrin, i.e., in peripheral nerve tissue from beta4 integrin-deficient mice. Because the mutants die within several hours after birth, we cultured dorsal root ganglia from neonatal mutants under conditions that promote myelination, quantified the myelin segments by immunofluorescence, and investigated the ultrastructure of the cultured myelin sheaths. In another approach, we quantified the few myelin sheaths that are detectable in femoral nerves of newborn animals. Based on both approaches, we conclude that myelination by Schwann cells can occur in the absence of beta4 integrin demonstrating that this Schwann cell component is dispensable for myelin formation in peripheral nerves.

Isolation and serum-free culture of primary Schwann cells from human fetal peripheral nerve

We have developed a method for isolating Schwann cells (SC) from human fetal peripheral nerve and maintaining these SC in vitro under serum-free conditions. This method yields essentially pure SC which have a bipolar, spindle-shaped morphology; align in fascicles; and express typical glial cell markers. Human fetal SC can be maintained for months under serum-free conditions with the neuregulin NDF beta. These human fetal SC can mimic axonal contact in vivo by retaining the functional capacity to strongly associate with neurites of cultured human fetal dorsal root ganglia. These isolation, culture, and coculture techniques provide a method for investigating SC-neuron interactions as well as development and function of human fetal SC.