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For a long time, neurogenic placodes and migratory neural crest cells were considered the immediate sources building neurons of peripheral nervous system. Recently, a number of discoveries revealed the existence of another progenitor type-a nerve-associated multipotent Schwann cell precursors (SCPs) building enteric and parasympathetic neurons as well as neuroendocrine chromaffin cells. SCPs are neural crest-derived and are similar to the crest cells by their markers and differentiation potential. Such similarities, but also considerable differences, raise many questions pertaining to the medical side, fundamental developmental biology and evolution. Here, we discuss the genesis of Schwann cell precursors, their role in building peripheral neural structures and ponder on their role in the origin in congenial diseases associated with peripheral nervous systems.

Mammals differ in their regeneration potential after traumatic injury, which might be caused by species-specific regeneration programs. Here, we compared murine and human Schwann cell (SC) response to injury and developed an ex vivo injury model employing surgery-derived human sural nerves. Transcriptomic and lipid metabolism analysis of murine SCs following injury of sural nerves revealed down-regulation of lipogenic genes and regulator of lipid metabolism, including Pparg (peroxisome proliferator-activated receptor gamma) and S1P (sphingosine-1-phosphate). Human SCs failed to induce similar adaptations following ex vivo nerve injury. Pharmacological PPARg and S1P stimulation in mice resulted in up-regulation of lipid gene expression, suggesting a role in SCs switching towards a myelinating state. Altogether, our results suggest that murine SC switching towards a repair state is accompanied by transcriptome and lipidome adaptations, which are reduced in humans.

Background: In the peripheral nervous system (PNS), specialized glial cells called Schwann cells produce myelin, a lipid-rich insulating sheath that surrounds axons and promotes rapid action potential propagation. During development, Schwann cells must undergo extensive cytoskeletal rearrangements in order to become mature, myelinating Schwann cells. The intracellular mechanisms that drive Schwann cell development, myelination, and accompanying cell shape changes are poorly understood., Methods: Through a forward genetic screen in zebrafish, we identified a mutation in the atypical guanine nucleotide exchange factor, dock1, that results in decreased myelination of peripheral axons. Rescue experiments and complementation tests with newly engineered alleles confirmed that mutations in dock1 cause defects in myelination of the PNS. Whole mount in situ hybridization, transmission electron microscopy, and live imaging were used to fully define mutant phenotypes., Results: We show that Schwann cells in dock1 mutants can appropriately migrate and are not decreased in number, but exhibit delayed radial sorting and decreased myelination during early stages of development., Conclusions: Together, our results demonstrate that mutations in dock1 result in defects in Schwann cell development and myelination. Specifically, loss of dock1 delays radial sorting and myelination of peripheral axons in zebrafish.

Schwann cells are the main glial cells of the peripheral nervous system (PNS) and play key roles in peripheral nerve development and function, including providing myelin that is essential for normal movement and sensation in the adult. Schwann cells can be readily destabilized by a wide variety of distinct conditions that range from nerve injury to immune assaults, metabolic disturbances, microbial infections, or genetic defects, leading to the breakdown of myelin (demyelination) and a subsequent switch in phenotypic states. This striking feature of Schwann cells forms the cornerstone of several debilitating and even fatal PNS neurological disorders that include the demyelinating neuropathies Guillain Barré syndrome (GBS) and Charcot-Marie-Tooth disease (CMT), and PNS cancers, including Neurofibromatosis.Primary Schwann cell cultures have proved a valuable tool to dissect key mechanisms that regulate proliferation, survival, differentiation, and myelination of these glial cell types. In this chapter, we describe the steps involved in the isolation and purification of Schwann cells from rodent peripheral nerves and the use of these cultures to model myelination in vitro.

Biomaterials can be utilized to assist in the transplantation of Schwann cells to the central and peripheral nervous system. The biomaterials can be natural or man-made, and can have preformed shapes or injectable formats. Biomaterials can play multiple roles in cellular transplantation; for example, they can assist with cellular integration and protect Schwann cells from cell death initiated by the lack of a substrate, an occurrence known as "anoikis." In addition, biomaterials can be engineered to increase cell proliferation and differentiation by the addition of ligands bound to the substrate. Here, we describe the incorporation of Schwann cells to both man-made and natural matrices for in vitro and in vivo measures relevant to Schwann cell transplantation strategies.

Peripheral glia includes predominantly myelinating and non-myelinating Schwann cells in addition to satellite, terminal and enteric glia as well as other unresolved subtypes with localized functions. Of these subtypes, all of them originate from neural crest-derived embryonic Schwann cell precursors (SCPs). Specific gene regulatory networks control neural crest specification and downstream events, including SCP differentiation and myelination. Embryonic SCPs are multipotent and generate neuroendocrine cells, parasympathetic and enteric neurons, melanocytes and other cell types. The evolutionary origin of peripheral Schwann cell lineage is not widely discussed in the literature, despite numerous similarities between central and peripheral glia. Here, we review the major features of the Schwann cell lineage and proceed to an evolutionary discussion around possible relations between central and peripheral glial cells., (Copyright © 2017 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Schwann cells are the myelinating glial cells of the peripheral nervous system and establish myelin sheaths on large caliber axons in order to accelerate their electrical signal propagation. Apart from this well described function, these cells revealed to exhibit a high degree of differentiation plasticity as they were shown to re- and dedifferentiate upon injury and disease as well as to actively participate in regenerative- and inflammatory processes. This review focuses on the crosstalk between glial- and immune cells observed in many peripheral nerve pathologies and summarizes functional evidences of molecules, regulators and factors involved in this process. We summarize data on Schwann cell's role presenting antigens, on interactions with the complement system, on Schwann cell surface molecules/receptors and on secreted factors involved in immune cell interactions or para-/autocrine signaling events, thus strengthening the view for a broader (patho) physiological role of this cell lineage.

Rhizomelic chondrodysplasia punctata (RCDP) is a developmental disorder characterized by hypotonia, cataracts, abnormal ossification, impaired motor development, and intellectual disability. The underlying etiology of RCDP is a deficiency in the biosynthesis of ether phospholipids, of which plasmalogens are the most abundant form in nervous tissue and myelin; however, the role of plasmalogens in the peripheral nervous system is poorly defined. Here, we used mouse models of RCDP and analyzed the consequence of plasmalogen deficiency in peripheral nerves. We determined that plasmalogens are crucial for Schwann cell development and differentiation and that plasmalogen defects impaired radial sorting, myelination, and myelin structure. Plasmalogen insufficiency resulted in defective protein kinase B (AKT) phosphorylation and subsequent signaling, causing overt activation of glycogen synthase kinase 3β (GSK3β) in nerves of mutant mice. Treatment with GSK3β inhibitors, lithium, or 4-benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione (TDZD-8) restored Schwann cell defects, effectively bypassing plasmalogen deficiency. Our results demonstrate the requirement of plasmalogens for the correct and timely differentiation of Schwann cells and for the process of myelination. In addition, these studies identify a mechanism by which the lack of a membrane phospholipid causes neuropathology, implicating plasmalogens as regulators of membrane and cell signaling.

The neuroglia of the peripheral nervous system (PNS) are derived from the neural crest and are a diverse family of cells. They consist of myelinating Schwann cells, non-myelinating Schwann cells, satellite cells, and perisynaptic Schwann cells. Due to their prominent role in the formation of myelin, myelinating Schwann cells are the best recognised of these cells. However, Schwann cells and the other neuroglia of the PNS have many functions that are independent of myelination and contribute significantly to the functioning of the peripheral nerve in both health and disease. Here we discuss the contribution of PNS neuroglial cells to clinical deficit in neurodegenerative disease, peripheral neuropathy, and pain., (© 2014 Peripheral Nerve Society.)

Iron, either in its chelated form or as holotransferrin (hTf), prevents the dedifferentiation of Schwann cells (SC), cells responsible for the myelination of the peripheral nervous system (PNS). This dedifferentiation is promoted by serum deprivation through cAMP release, PKA activation, and CREB phosphorylation. Since iron elicits its effect in a transferrin (Tf)-free environment, in this work we postulate that non-transferrin-bound iron (NTBI) uptake must be involved. Divalent metal transporter 1(DMT1) has been widely described in literature as a key player in iron metabolism, but never before in the PNS context. The presence of DMT1 was demonstrated in nerve homogenate, isolated adult-rat myelin, and cultured SC by Western Blot (WB) analysis and confirmed through its colocalization with S-100β (SC marker) by immunocytochemical and immunohistochemical analyses. Furthermore, the existence of its mRNA was verified in sciatic nerve homogenate by RT-PCR and throughout SC maturational stages. Finally, we describe DMT1's subcellular location in the plasma membrane by confocal microscopy of SC and WB of different subcellular fractions. These data allow us to suggest the participation of DMT1 as part of a Tf independent iron uptake mechanism in SC and lead us to postulate a crucial role for iron in SC maturation and, as a consequence, in PNS myelination., (Copyright © 2013 International Union of Biochemistry and Molecular Biology, Inc.)

Ascorbic acid has been shown to be an essential component for in vitro myelination and to improve the clinical and pathological phenotype of a mouse model of Charcot-Marie-tooth disease 1A. The mechanism of ascorbic acid uptake into peripheral nerves, however, has not been addressed so far. Hence, we studied the expression and activity of sodium-dependent vitamin C transporters 1 and 2 (SVCT1 and 2) in the peripheral nervous system. Using immunohistochemistry, immunoblotting, and reverse transcription PCR, we could show that SVCT1 and 2 were differentially expressed in myelinated peripheral nerve fibers and Schwann cell (SC) cultures. SVCT1 was expressed at very low levels confined to the axons, whereas SVCT2 was highly expressed both in the axons and in the SCs. SVCT2 was localized particularly in SC compartments of uncompacted myelin. Uptake assays using (14)C-labeled ascorbic acid showed transport of ascorbic acid into SC cultures. Ascorbic acid transport was dependent on the concentration of sodium, magnesium, and calcium in the extracellular medium. Treatment with the flavonoid phloretin, a known inhibitor of SVCT1 and 2, and specific RNA interference with SVCT2 caused significant reductions in ascorbic acid uptake into SCs. Phloretin-inhibited uptake of ascorbic acid was further shown in freshly dissected, cell-culture-naïve rat sciatic nerves. These results provide evidence for the first time that uptake of ascorbic acid in the peripheral nervous system is crucially dependent on the expression and activity of SVCT2., ((c) 2009 Wiley-Liss, Inc.)

The mechanisms controlling the differentiation of immature Schwann cells (SCs) into nonmyelinating SCs is not known. Laminins are extracellular matrix proteins critical for myelinating SC differentiation, but their roles in nonmyelinating SC development have not been established. Here, we show that the peripheral nerves of mutant mice with laminin-deficient SCs do not form Remak bundles, which consist of a single nonmyelinating SC interacting with multiple unmyelinated axons. These mutant nerves show aberrant L1 and neural cell adhesion molecule (N-CAM) expression pattern during development. The homophilic and heterophilic interactions of N-CAM are also impaired in the mutant nerves. Other molecular markers for nonmyelinating SCs, including Egr-1, glial fibrillary acidic protein, and AN2/NG2, are all absent in adult mutant nerves. Analysis of expression of SC lineage markers demonstrates that nonmyelinating SCs do not develop in mutant nerves. Additionally, mutant mice are insensitive to heat stimuli and show a decreased number of C-fiber sensory neurons, indicating reduced nociceptive sensory function. These results show that laminin participates in nonmyelinating SC development and Remak bundle formation and suggest a possible role for laminin deficiency in peripheral sensory neuropathies., ((c) 2008 Wiley-Liss, Inc.)

During development, Schwann cells (SCs) interpret different extracellular cues to regulate their migration, proliferation, and the remarkable morphological changes associated with the sorting, ensheathment, and myelination of axons. Although interactions between extracellular matrix proteins and integrins are critical to some of these processes, the downstream signaling pathways they control are still poorly understood. Integrin-linked kinase (ILK) is a focal adhesion protein that associates with multiple binding partners to link integrins to the actin cytoskeleton and is thought to participate in integrin and growth factor-mediated signaling. Using SC-specific gene ablation, we report essential functions for ILK in radial sorting of axon bundles and in remyelination in the peripheral nervous system. Our in vivo and in vitro experiments show that ILK negatively regulates Rho/Rho kinase signaling to promote SC process extension and to initiate radial sorting. ILK also facilitates axon remyelination, likely by promoting the activation of downstream molecules such as AKT/protein kinase B.

Myelinated axons are organized into a series of specialized domains with distinct molecular compositions and functions. These domains, which include the node of Ranvier, the flanking paranodal junctions, the juxtaparanodes, and the internode, form as the result of interactions with myelinating Schwann cells. This domain organization is essential for action potential propagation by saltatory conduction and for the overall function and integrity of the axon.

Interactions between neuronal and glial cells are crucial for establishing a functional nervous system. Many aspects of Schwann cell development and physiology are regulated by neuronal signals; possibly the most spectacular is the elaboration of the myelin sheath. An extensive line of research has revealed that one neuronal factor, termed "neuregulin", promotes Schwann cell growth and survival, migration along the extending axon, and myelination. The versatility of glial responses elicited by this factor is thus clearly astounding.

The myelinating and nonmyelinating Schwann cells in peripheral nerves are derived from the neural crest, which is a transient and multipotent embryonic structure that also generates the other main glial subtypes of the peripheral nervous system (PNS). Schwann cell development occurs through a series of transitional embryonic and postnatal phases, which are tightly regulated by a number of signals. During the early embryonic phases, neural crest cells are specified to give rise to Schwann cell precursors, which represent the first transitional stage in the Schwann cell lineage, and these then generate the immature Schwann cells. At birth, the immature Schwann cells differentiate into either the myelinating or nonmyelinating Schwann cells that populate the mature nerve trunks. In this review, we will discuss the biology of the transitional stages in embryonic and early postnatal Schwann cell development, including the phenotypic differences between them and the recently identified signaling pathways, which control their differentiation and maintenance. In addition, the role and importance of the microenvironment in which glial differentiation takes place will be discussed.

During late fetal life, Schwann cells in the peripheral nerves singled out by the larger axons will transit through a promyelinating stage before exiting the cell cycle and initiating myelin formation. A network of extra- and intracellular signaling pathways, regulating a transcriptional program of cell differentiation, governs this progression of cellular changes, culminating in a highly differentiated cell. In this review, we focus on the roles of a number of transcription factors not only in myelination, during normal development, but also in demyelination, following nerve trauma. These factors include specification factors involved in early development of Schwann cells from neural crest (Sox10) as well as factors specifically required for transitions into the promyelinating and myelinating stages (Oct6/Scip and Krox20/Egr2). From this description, we can glean the first, still very incomplete, contours of a gene regulatory network that governs myelination and demyelination during development and regeneration.

RhoGTPases are molecular switches that integrate extracellular signals to perform diverse cellular responses. This ability relies on the network of proteins regulating RhoGTPases activity and localization, and on the interaction of RhoGTPases with many different cellular effectors. Myelination is an ideal place for RhoGTPases regulation, as it is the result of fine orchestration of many stimuli from at least two cell types. Recent work has revealed that RhoGTPases are required for Schwann cells to sort, ensheath, and myelinate axons. Here, we will review these recent advances showing the critical roles for RhoGTPases in various aspects of Schwann development and myelination, including the recent discovery of their involvement in Charcot-Marie-Tooth disease. Comparison with potential roles of RhoGTPases in central nervous system myelination will be drawn.

Laminins and collagens are extracellular matrix proteins that play essential roles in peripheral nervous system development. Laminin signals regulate Schwann cell proliferation and survival as well as actin cytoskeleton dynamics, which are essential steps for radial sorting and myelination of peripheral axons by Schwann cells. Collagen and their receptors promote Schwann cell adhesion, spreading, and myelination as well as neurite outgrowth. In this article, we will review the recent advances in the studies of laminin and collagen function in Schwann cell development.

The CNS contains relatively few unmyelinated nerve fibers, and thus benefits from the advantages that are conferred by myelination, including faster conduction velocities, lower energy consumption for impulse transmission, and greater stability of point-to-point connectivity. In the PNS many fibers or regions of fibers the Schwann do not form myelin. Examples include C fibers nociceptors, postganglionic sympathetic fibers, and the Schwann cells associated with motor nerve terminals at neuromuscular junctions. These examples retain a degree of plasticity and a capacity to sprout collaterally that is unusual in myelinated fibers. Nonmyelin-forming Schwann cells, including those associated with uninjured fibers, have the capacity to act as the "first responders" to injury or disease in their neighborhoods.

Understanding the control of myelin formation by oligodendrocytes is essential for treating demyelinating diseases. Neuregulin-1 (NRG1) type III, an EGF-like growth factor, is essential for myelination in the PNS. It is thus thought that NRG1/ErbB signaling also regulates CNS myelination, a view suggested by in vitro studies and the overexpression of dominant-negative ErbB receptors. To directly test this hypothesis, we generated a series of conditional null mutants that completely lack NRG1 beginning at different stages of neural development. Unexpectedly, these mice assemble normal amounts of myelin. In addition, double mutants lacking oligodendroglial ErbB3 and ErbB4 become myelinated in the absence of any stimulation by neuregulins. In contrast, a significant hypermyelination is achieved by transgenic overexpression of NRG1 type I or NRG1 type III. Thus, NRG1/ErbB signaling is markedly different between Schwann cells and oligodendrocytes that have evolved an NRG/ErbB-independent mechanism of myelination control.

Based on their characteristics and function--migration, neural protection, proliferation, axonal guidance and trophic effects--glial cells may be regarded as probably the most versatile cells in our body. For many years, these cells were considered as simply support cells for neurons. Recently, it has been shown that they are more versatile than previously believed--as true stem cells in the nervous system--and are important players in neural function and development. There are several glial cell types in the nervous system: the two most abundant are oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Although both of these cells are responsible for myelination, their developmental origins are quite different. Oligodendrocytes originate from small niche populations from different regions of the central nervous system, while Schwann cells develop from a stem cell population (the neural crest) that gives rise to many cell derivatives besides glia and which is a highly migratory group of cells.

Schwann cells provide trophic support and in some cases, insulation to axons. After injury, Schwann cells undergo phenotypic modulation, acquiring the capacity to proliferate, migrate, and secrete soluble mediators that control Wallerian degeneration and regeneration. Amongst the soluble mediators are pro-inflammatory cytokines that function as chemoattractants but also may sensitize nociceptors. At the same time, Schwann cells produce factors that counterbalance the pro-inflammatory cytokines, including, for example, interleukin-10 and erythropoietin (Epo). Epo and its receptor, EpoR, are up-regulated in Schwann cells after peripheral nerve injury. EpoR-dependent cell signaling may limit production of TNF-alpha by Schwann cells within the first five days after injury. In addition, EpoR-dependent cell signaling may reduce axonal degeneration and facilitate recovery from chronic pain states. Other novel factors that regulate Schwann cell phenotype in nerve injury have been recently identified, including the low-density lipoprotein receptor related protein (LRP-1). Our recent studies indicate that LRP-1 may be essential for Schwann cell survival after peripheral nerve injury. To analyze the function of specific Schwann cell gene products in nerve injury and sensory function, conditional gene deletion and expression experiments in mice have been executed using promoters that are selectively activated in myelinating or non-myelinating Schwann cells. Blocking ErbB receptor-initiated cell-signaling in either myelinating or non-myelinating Schwann cells results in unique sensory dysfunctions. Data obtained in gene-targeted animals suggest that sensory alterations can result from changes in Schwann cell physiology without profound myelin degeneration or axonopathy. Aberrations in Schwann cell biology may lie at the foundation of neuropathic pain and represent an exciting target for therapeutic intervention.

Schwann cell (SC) differentiation to the myelinating phenotype is characterized by the elaboration of a lipid-rich membrane and the expression of myelin-specific proteins. Insulin-like growth factor-1 (IGF-1) has been identified as a growth factor that stimulates the early events of myelination in SCs that signals via the PI3K/Akt pathway. Given the role of IGF-1 in promoting myelination, we performed studies to determine if the fatty acid biosynthetic pathway was a target of IGF-1 signaling in the formation of myelin membrane in dorsal root ganglion neuron/Schwann cell (DRG/SC) cocultures. We report that the fatty acid profile of lipid extracts of cocultures treated with IGF-1 match that reported for native myelin membrane by electrospray mass spectroscopy analysis. We also demonstrate de novo fatty acid biosynthesis in response to IGF-1 treatment in DRG/SC cocultures metabolically labeled with (13)C-acetate as a carbon source for fatty acid synthesis. Consistent with this finding, Western blot analysis of lysates from both cocultures and purified SCs reveal that IGF-1 stimulates two key fatty acid synthesizing enzymes. Additionally, we show that stimulation of fatty acid synthesizing enzymes is mediated by the PI3K/Akt signaling pathway. We also show that the fatty acid synthesizing enzymes and associated signaling pathways are elevated during the period of myelin membrane formation in sciatic nerve. Collectively, these findings demonstrate that IGF-1 plays an important regulatory function during myelin membrane formation.

Connexins are transmembrane proteins forming gap junction channels for direct intercellular and, for example in myelinating glia cells, intracellular communication. In mature myelin-forming Schwann cells, expression of multiple connexins, i.e. connexin (Cx) 43, Cx29, Cx32, and Cx46 (after nerve injury) has been detected. However, little is known about connexin protein expression during Schwann cell development. Here we use histochemical methods on wildtype and Cx29lacZ transgenic mice to investigate the developmental expression of connexins in the Schwann cell lineage. Our data demonstrate that in the mouse Cx43, Cx29, and Cx32 protein expression is activated in a developmental sequence that is clearly correlated with major developmental steps in the lineage. Only Cx43 was expressed from neural crest cells onwards. Cx29 protein expression was absent from neural crest cells but appeared as neural crest cells generated precursors (embryonic day 12) both in vivo and in vitro. This identifies Cx29 as a novel marker for cells of the defined Schwann cell lineage. The only exception to this were dorsal roots, where the expression of Cx29 was delayed four days relative to ventral roots and spinal nerves. Expression of Cx32 commenced postnatally, coinciding with the onset of myelination. Thus, the coordinated expression of connexin proteins in cells of the embryonic and postnatal Schwann cell lineage might point to a potential role in peripheral nerve development and maturation., (Copyright 2006 Wiley-Liss, Inc.)

Glial cells are widely dispersed in the central nervous system (CNS) as well as in the peripheral nervous system (PNS). In the PNS, perisynaptic Schwann cells (PSCs) are the glial cells associated with the pre- and postsynaptic elements of the neuromuscular junction (NMJ). They, as other glial cells of the CNS, respond to high-frequency motor nerve stimulation with an increase in intracellular Ca(2+). In addition to detecting and responding to neurotransmission, PSCs are involved in short-term plasticity events where they depress neurotransmission through G-protein-dependent mechanisms and potentiate synaptic activity via Ca(2+)-dependent mechanisms. In this review, we will discuss evidence that outlines the role of PSCs in short- and long-term modulation of synaptic activity. We will also emphasize present functional similarities and differences in PSC activation at different NMJs. The importance of glial-neural interactions along myelinating axons will also be discussed.

High-throughput gene expression analyses of murine models of the peripheral nervous system (PNS), and its cellular components, have yielded enormous amounts of expression data of the PNS in various conditions. These data provided clues for future research directions to further decipher this complex organ in relation to acquired and inherited PNS diseases. Various studies addressing the validity of mouse models for human conditions in other tissues and cell types have indicated that in many cases the mouse model only poorly represents the human situation. To determine how well the mouse can serve as model to study the biological processes occurring in the PNS, we compared the gene expression profiles that we generated for mouse and human sciatic nerve and cultured Schwann cells derived thereof. A two-way analysis based on the differentially expressed genes between the sciatic nerve and the cultured Schwann cell, and which takes into account the differential expression between mouse and man, indicates that the human PNS is well represented by that of the mouse in terms of the "biological processes" ontology.

There continues to be interest in Schwann cells (SC) as a possible source of myelinating cells for transplantation into the central nervous system (CNS) of patients with multiple sclerosis (MS) and spinal cord injury. It has been suggested that CNS glial cells interfere with SC migration, survival, maturation, and clinically significant remyelination in the CNS. To investigate the effects of CNS glial cells on SC, we examined the effects of serum-free supernatants obtained from rat mixed CNS glial cultures on rat neonatal SC cultures. Supernatants from 1-, 3-, and 5-day CNS glial cultures induced proliferation of SC assayed at 5 days in vitro but did not induce SC differentiation as measured by induction of surface expression of galactolipids (GalL). High concentrations of cAMP simulate many of the effects of axolemma on SC; CNS glial cell supernatants did not inhibit cAMP induction of SC differentiation. CNS glial cell supernatants had no apparent effect on SC viability at 48 hr as measured by trypan blue exclusion. We have previously demonstrated that incubation of SC with transforming growth factor-beta1 (TGF-beta1) + tumor necrosis factor-alpha (TNF-alpha) induces SC death via apoptosis. We now show that CNS glial supernatants inhibits TGF-beta1/TNF-alpha-induced SC death. Our data show that soluble products of CNS glial cells do not induce or inhibit SC differentiation or increase cell death but have the potential to increase proliferation of SC and their resistance to cytokine-mediated death, and thus may affect the outcome of SC transplantation into the CNS., ((c) 2006 Wiley-Liss, Inc.)

The aquaporins (AQPs) are a family of water channel proteins with at least 13 mammalian members (AQPs 0-12) expressed in diverse fluid transporting tissues. AQP1, AQP4, and AQP9 have been identified in the central nervous system and demonstrated or proposed to play important roles in brain water homeostasis. Aquaporin expression in the peripheral nervous system is poorly studied. Here we report that the AQP1 water channel is specifically localized to glial cells of the peripheral nervous system by immunohistochemistry, RT-PCR, and immunoblotting. Paraffin-embedded biopsies of human pancreas, esophagus, and sciatic nerves were accessed by immunoperoxidase staining using affinity-purified AQP1, AQP4, and AQP9 antibodies. Strong AQP1 expression was identified in pancreatic nerve plexuses and in the submucosal and myenteric nerve plexuses in the esophagus. AQP1 was localized to the same cell population expressing glial fibrillary acidic protein (GFAP), but not to the neurons in the plexuses, indicating glial cell-specific expression. RT-PCR and immunoblot analysis of microdissected pancreatic ganglia confirmed the expression of AQP1 transcript and protein. Pancreatic and sciatic nerve bundles, which contain nonmyelinating and myelinating Schwann cells, respectively, were also selectively labeled by AQP1 antibody. AQP4 and AQP9, which are broadly expressed in astroglial cells in brain and spinal cord, were not localized in glial cells in the peripheral nerve plexuses. These results suggest that AQPs are differentially expressed in the peripheral versus central nervous system and that channel-mediated water transport mechanisms may be involved in peripheral neuronal activity by regulating water homeostasis in nerve plexuses and bundles., (Copyright 2006 Wiley-Liss, Inc.)

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.

In the dorsal root entry zone (DREZ) peripheral sensory axons fail to regenerate past the peripheral nervous system/central nervous system (PNS/CNS) interface. Additionally, in the spinal cord, central fibers that regenerate into Schwann cell (SC) bridges can enter but do not exit at the distal Schwann cell/astrocyte (AC) boundary. At both interfaces where limited mixing of the two cell types occurs, one can observe an up-regulation of inhibitory chondroitin sulfate proteoglycans (CSPGs). We treated confrontation Schwann cell/astrocyte cultures with the following: (1) a deoxyribonucleic acid (DNA) enzyme against the glycosaminoglycan (GAG)-chain-initiating enzyme, xylosyltransferase-1 (XT-1), (2) a control DNA enzyme, and (3) chondroitinase ABC (Ch'ase ABC) to degrade the GAG chains. Both techniques for reducing CSPGs allowed Schwann cells to penetrate deeply into the territory of the astrocytes. After adding sensory neurons to the assay, the axons showed different growth behaviors depending upon the glial cell type that they first encountered during regeneration. Our results help to explain why regeneration fails at PNS/CNS glial boundaries.

D-serine appears to be a natural agonist at the "glycine site" of the NMDA receptor and is created by conversion from L-serine by serine racemase. This racemase has been localized to protoplasmic astrocytes that ensheath synapses and modulate neuronal activity in the CNS, but serine racemase expression in the PNS has not been reported. Immunofluorescence indicated that Schwann cells and other endoneurial components of rat spinal nerve contain serine racemase, and western blot analysis detected the enzyme in lysates of sciatic nerve. Cultures from sciatic nerve contained Schwann cells and fibroblasts, and both cell types showed serine racemase expression by immunofluorescence and Western blot; the quantities per unit of total protein appeared slightly lower than that expressed in cultured astrocytes. Cultures enriched for each cell type were subjected to reverse transcriptase polymerase chain reaction, further confirming serine racemase mRNA in Schwann cells and fibroblasts. Finally, immunodetection of D-serine itself was observed in cultured Schwann cells and fibroblasts. These expression patterns of serine racemase may indicate roles for D-serine in peripheral nerve transduction.

Studies of ganglionic glia turnover in the sensory nervous system have implications for understanding nervous system maintenance and repair. These glial cells of the sensory ganglia in the peripheral nervous system (PNS) comprise satellite cells (SCs) and, to a lesser extent, Schwann cells. SCs proliferate in response to trauma such as axotomy; however, the half-life of these glial cells under normal circumstances has not been estimated. To estimate the half-life of sensory ganglionic glial cells, we employed the DNA precursor analog 5-bromo-2'-deoxyuridine (BrdU) to measure the rate of turnover of these cells. BrdU was administered to inbred C57BL6 and outbred Swiss white mice via their drinking water. BrdU incorporation into ganglionic glia in the PNS was estimated by immunofluorescent staining of nervous tissue sections, and the fraction of ganglionic glial cells that acquired BrdU label was measured as a function of time. Mathematical modeling of the rate of uptake of BrdU into murine ganglionic glia enables calculation of the half-life of these cells. The kinetics of BrdU uptake is linear, consistent with ganglionic glia being a homogenous population. The value of the proliferation rate (p) plus death rate (d) derived from the slope of BrdU uptake as a function of time is approximately 2.4 x 10(-3) cells per day. Assuming that p = d (because ganglionic glial numbers are in equilibrium and they are assumed to neither emigrate from, or immigrate into, sensory ganglia), then the daily death rate is d = 1.2 x 10(-3) cells/day, which implies a half-life for ganglionic glia of about 600 days. Thus murine ganglionic glia in the untraumatized state appear to behave as a homogenous, slowly replicating population.

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.

Schwann cells require laminin-2 throughout nerve development, because mutations in the alpha2 chain in dystrophic mice interfere with sorting of axons before birth and formation of myelin internodes after birth. Mature Schwann cells express several laminin receptors, but their expression and roles in development are poorly understood. Therefore, we correlated the onset of myelination in nerve and synchronized myelinating cultures to the appearance of integrins and dystroglycan in Schwann cells. Only alpha6beta1 integrin is expressed before birth, whereas dystroglycan and alpha6beta4 integrin appear perinatally, just before myelination. Although dystroglycan is immediately polarized to the outer surface of Schwann cells, alpha6beta4 appears polarized only after myelination. We showed previously that Schwann cells lacking beta1 integrin do not relate properly to axons before birth. Here we show that the absence of beta1 before birth is not compensated by other laminin receptors, whereas coexpression of both dystroglycan and beta4 integrin is likely required for beta1-null Schwann cells to myelinate after birth. Finally, both beta1-null and dystrophic nerves contain bundles of unsorted axons, but they are predominant in different regions: in spinal roots in dystrophic mice and in nerves in beta1-null mice. We show that differential compensation by laminin-1, but not laminin receptors may partially explain this. These data suggest that the action of laminin is mediated by beta1 integrins during axonal sorting and by dystroglycan, alpha6beta1, and alpha6beta4 integrins during myelination.

Inducible transgenesis provides a valuable technique for the analysis of gene function in vivo. We report the generation and characterization of mouse lines carrying glia lineage-specific transgenes expressing an improved variant of the tamoxifen-inducible Cre recombinase, CreERT2, where the recombinase is fused to a mutated ligand binding domain of the human estrogen receptor. Using a PLP-CreERT2 transgene, we have generated mice that show specific inducible Cre function, as analyzed by cross-breeding experiments into the Rosa26 Cre-LacZ reporter line, in developing and adult Schwann cells, in mature myelinating oligodendrocytes, and in undifferentiated NG2-positive oligodendrocyte precursors in the adult. Using a P0Cx-CreERT2 transgene, we have also established mouse lines with inducible Cre function specifically in the Schwann cell lineage. These tamoxifen-inducible CreERT2 lines will allow detailed spatiotemporally controlled analysis of gene functions in loxP-based conditional mutant mice in both developing and adult Schwann cells and in the oligodendrocyte lineage.

In contrast to the peripheral nervous system (PNS), little structural and functional regeneration of the central nervous system (CNS) occurs spontaneously following injury in adult mammals. The inability of the CNS to regenerate is mainly attributed to its own inhibitorial environment such as glial scar formation and the myelin sheath of oligodendrocytes. Therefore, one of the strategies to promote axonal regeneration of the CNS is to experimentally modify the environment to be similar to that of the PNS. Schwann cells are the myelinating glial cells in the PNS, and are known to play a key role in Wallerian degeneration and subsequent regeneration. Central nervous system regeneration can be elicited by Schwann cell transplantation, which provides a suitable environment for regeneration. The underlying cellular mechanism of regeneration is based upon the cooperative interactions between axons and Schwann cells involving the production of neurotrophic factors and other related molecules. Furthermore, tight and gap junctional contact between the axon and Schwann cell also mediates the molecular interaction and linking. In this review, the role of the Schwann cell during the regeneration of the sciatic (representing the PNS) and optic (representing the CNS) nerves is explained. In addition, the possibility of optic nerve reconstruction by an artificial graft of Schwann cells is also described. Finally, the application of cells not of neuronal lineage, such as bone marrow stromal cells (MSCs), in nerve regeneration is proposed. Marrow stromal cells are known as multipotential stem cells that, under specific conditions, differentiate into several kinds of cells. The strategy to transdifferentiate MSCs into the cells with a Schwann cell phenotype and the induction of sciatic and optic nerve regeneration are described.

Some years ago we showed that in the adult rat and mouse neuropeptide Y (NPY) is expressed by olfactory ensheathing cells, a special type of glial cell involved in guiding of the continuously renewing olfactory axons. In the present study, using immunohistochemistry combined with confocal laser scanning microscopy, and mRNA in situ hybridization, we have analyzed whether NPY is also expressed in other axon-related glial cell systems during prenatal development. NPY was found to be expressed in the dorsal and ventral rootlets along the spinal cord from E13 and onward and in the rootlets of the cranial nerves and in the sensory ganglia from E14 and onward. In some cases, NPY-immunoreactivity (IR) was also found along peripheral nerves. NPY-IR was expressed in a dot-like fashion, similar to the NPY expression observed in olfactory ensheathing cells. At E18 the NPY-immunoreactive dots had disappeared from almost all ganglia and rootlets, and only in the most central part of the rootlets some weak dot-like NPY-IR was observed. At E20 it had disappeared completely from all rootlets and nerves, except the olfactory nerve. Most of the dot-like NPY-IR did not co-localize with the neuronal marker PGP 9.5. Based on its spatiotemporal expression, it is concluded that NPY is expressed by Schwann cell precursors. NPY expressed by Schwann cell precursors might have a role in axonal growth or axonal guidance, or both., (Copyright 2000 Wiley-Liss, Inc.)

To better understand the mechanism by which glucocorticosteroids (GLUC) could enhance myelination in the PNS, cultured rat Schwann cells were transiently transfected with reporter constructs in which luciferase expression was controlled by the promoter region of either the peripheral myelin protein-22 (PMP22) or the protein zero (P(0)) genes. GLUC stimulated the activity of the P(0) promoter and the PMP22 promoters 1 and 2. The effect of GLUC was specific as estradiol and testosterone did not activate the promoters. The antagonist RU486 did not abolish the effect of GLUC, but instead stimulated promoter activities by itself. In the mammary carcinoma cell line 34i, which expresses GLUC receptors, GLUC did not stimulate the P(0) and PMP22 promoters while the promoter of the mouse mammary tumor virus was strongly activated. Thus, the activation by GLUC of the promoter activities of two peripheral myelin protein genes is Schwann cell-specific.

We prepared a monoclonal antibody that recognizes oligodendrocytes and Schwann cells in zebrafish. On immunoblots, the antibody mainly recognized three protein bands of 34 kDa in a membrane fraction from adult zebrafish brain. Medaka fish (Oryzias latipes) also possessed the same protein bands in a membrane fraction. The antibody did not stain neurons, but stained cells in fiber tracts and cranial and spinal nerves. In order to determine the nature of these cells, the staining pattern of the monoclonal antibody was compared with that of a myelin basic protein antiserum. Both antibodies stained oligodendrocytes and Schwann cells in fixed sections from the adult zebrafish. Both antigens were also co-localized in cultured glial cells. Taken together, these results indicate that the new monoclonal antibody recognizes myelinating glial cells in zebrafish and will be useful for the analysis of piscine glia.

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.

Schwann cells of the peripheral nervous system are distinguished by morphological and functional criteria in myelinating and non-myelinating subtypes. We and others have previously reported that Schwann cells in isolated peripheral human and rat nerve respond to extracellular application of ATP with a rise in the intracellular free calcium concentration [Ca2+]i. In the present study, the receptors mediating these Ca2+ transients have been investigated in myelinating and non-myelinating Schwann cells of intact fascicles of isolated human sural nerves, rat ventral roots, and rat vagus nerves. Microfluorometry and confocal laser scanning was used on preparations stained with the Ca2+-sensitive dyes Calcium Green-1 and Fura Red. In myelinating Schwann cells of human and rat nerves, the ATP-induced rise of [Ca2+]i resulted from the activation of a P2Y2 purinoceptor subtype (rank order of potency: UTP > or = ATP >> 2-MeSATP = ADP). In contrast, in non-myelinating Schwann cells, Ca2+ transients were produced by activation of a P2Y1 purinoceptor subtype (rank order of potency: 2-MeSATP > ATP > ADP >> UTP). The P1 agonist adenosine and alpha,alpha-meATP did not evoke Ca2+ signals. Ca2+ transients in both types of Schwann cells were found to be due to Ca2+ release from cyclopiazonic acid-sensitive intracellular stores. However, inhibition by suramin was only found in non-myelinating Schwann cells. These findings indicate that mammalian Schwann cells express phenotype-specific P2Y receptor subtypes.

Recent data show that during development protein zero, P0, the most abundant peripheral nerve myelin protein, is detecable long before myelination. In particular, the expression of P0 in a fraction of migrating neural crest reveals progenitor cell heterogeneity in the developing PNS. Here we review the regulation and potential function of P0 during peripheral gliogenesis.

Schwann cells are one of the principal components of the peripheral nervous system. They play a crucial role in nerve regeneration and can be used clinically in the repair of injured nerves. We have established serum-free, defined culture conditions that rapidly expand adult human Schwann cells without fibroblast growth. We find that Gas6, a ligand for the Axl and Rse/Tyro3 receptor protein tyrosine kinase family, stimulates human Schwann cell growth, increasing both cell number and thymidine incorporation. Gas6 has synergistic effects with the other known human Schwann cell mitogens, heregulin/glial growth factor and forskolin. Addition of Gas6 causes phosphorylation of Axl and Rse/Tyro3 simultaneously and results in ERK-2 activation. A combination of Gas6 with heregulin and forskolin, on a defined background, supports maximal Schwann cell proliferation, while preserving the typical Schwann cell morphology and expression of the Schwann cell markers S-100, glial fibrillary acidic protein, and low-affinity nerve growth factor receptor. Gas6 mRNA is present in both spinal motor neurons and large neurons of the dorsal root ganglia, and neural injury has been reported to upregulate Rse/Axl in the schwann cell. This is the first demonstration of a potentially important biological role for the human Gas6/Rse-Axl system.

During the past few years important advances have been realized in the understanding of the molecular and cellular mechanisms operating in the differentiation of Schwann cells from neural crest cells. In particular the development of in vitro culture systems has allowed the analysis of commitment to the Schwann cell lineage, identification of an intermediate between the neural crest cell and the Schwann cell, and initial analysis of the factors controlling determination and differentiation processes. These studies point in particular to major roles for neuregulins which appear to be involved in the control of cell survival, proliferation, and differentiation, as well as of gene expression at different levels of the pathway.