TGF-betas upregulate NCAM and L1 expression in cultured Schwann cells, suppress cyclic AMP-induced expression of O4 and galactocerebroside, and are widely expressed in cells of the Schwann cell lineage in vivo

The Role of Schwann Cells in Cancer

Schwann cells (SCs) are the most abundant cell type in the nerves in the peripheral nervous system and compose a family of subtypes that are endowed with a variety of different functions. SCs facilitate the transmission of neural impulses, provide nutrients and protection for neurons, guide axons in nerve repair, and regulate immune functions. In the context of cancer, recent studies have revealed an active role of SCs in promoting cancer cell invasion, modulating immune responses, and transmitting pain sensation.

Conditioned Medium From the Stem Cells of Human Exfoliated Deciduous Teeth Ameliorates Neuropathic Pain in a Partial Sciatic Nerve Ligation Model

In neuropathic pain (NP), injury or diseases of the somatosensory system often result in highly debilitating chronic pain. Currently, there is no effective drug for the complete and definitive treatment of NP. We investigated the therapeutic potential of conditioned medium (CM) derived from stem cells from human exfoliated deciduous teeth (SHED-CM) against NP using a mouse partial sciatic nerve ligation (PSL) model. Abnormal pain sensation, such as tactile allodynia and hyperalgesia, can be caused by PSL. In the behavioral test, intravenous administration of SHED-CM greatly improved the PSL-induced hypersensitivity. We found that treatment with SHED-CM resulted in the recruitment of M2 macrophages in the injured sciatic nerve and ipsilateral L4/L5 dorsal root ganglion and suppressed microglial activation in the spinal cord. Notably, specific depletion of the anti-inflammatory M2 macrophages by mannosylated-Clodrosome markedly reduced the antinociceptive effect of SHED-CM. Intravenous administration of CM from M2 induced by SHED-CM (M2-CM) ameliorated the PSL-induced hypersensitivity. We found that M2-CM directly suppressed the expression of nociceptive receptors as well as proinflammatory mediators in Schwann cells. Taken together, our data suggest that SHED-CM ameliorates NP through the induction of the analgesic anti-inflammatory M2 macrophages. Thus, SHED-CM may be a novel therapeutic candidate for NP.

The Role of c-Jun and Autocrine Signaling Loops in the Control of Repair Schwann Cells and Regeneration

After nerve injury, both Schwann cells and neurons switch to pro-regenerative states. For Schwann cells, this involves reprogramming of myelin and Remak cells to repair Schwann cells that provide the signals and mechanisms needed for the survival of injured neurons, myelin clearance, axonal regeneration and target reinnervation. Because functional repair cells are essential for regeneration, it is unfortunate that their phenotype is not robust. Repair cell activation falters as animals get older and the repair phenotype fades during chronic denervation. These malfunctions are important reasons for the poor outcomes after nerve damage in humans. This review will discuss injury-induced Schwann cell reprogramming and the concept of the repair Schwann cell, and consider the molecular control of these cells with emphasis on c-Jun. This transcription factor is required for the generation of functional repair cells, and failure of c-Jun expression is implicated in repair cell failures in older animals and during chronic denervation. Elevating c-Jun expression in repair cells promotes regeneration, showing in principle that targeting repair cells is an effective way of improving nerve repair. In this context, we will outline the emerging evidence that repair cells are sustained by autocrine signaling loops, attractive targets for interventions aimed at promoting regeneration.

Magnetic separation of peripheral nerve-resident cells underscores key molecular features of human Schwann cells and fibroblasts: an immunochemical and transcriptomics approach

Nerve-derived human Schwann cell (SC) cultures are irreplaceable models for basic and translational research but their use can be limited due to the risk of fibroblast overgrowth. Fibroblasts are an ill-defined population consisting of highly proliferative cells that, contrary to human SCs, do not undergo senescence in culture. We initiated this study by performing an exhaustive immunological and functional characterization of adult nerve-derived human SCs and fibroblasts to reveal their properties and optimize a protocol of magnetic-activated cell sorting (MACS) to separate them effectively both as viable and biologically competent cells. We next used immunofluorescence microscopy imaging, flow cytometry analysis and next generation RNA sequencing (RNA-seq) to unambiguously characterize the post-MACS cell products. High resolution transcriptome profiling revealed the identity of key lineage-specific transcripts and the clearly distinct neural crest and mesenchymal origin of human SCs and fibroblasts, respectively. Our analysis underscored a progenitor- or stem cell-like molecular phenotype in SCs and fibroblasts and the heterogeneity of the fibroblast populations. In addition, pathway analysis of RNA-seq data highlighted putative bidirectional networks of fibroblast-to-SC signaling that predict a complementary, yet seemingly independent contribution of SCs and fibroblasts to nerve regeneration. In sum, combining MACS with immunochemical and transcriptomics approaches provides an ideal workflow to exhaustively assess the identity, the stage of differentiation and functional features of highly purified cells from human peripheral nerve tissues.

Schwann Cell Cultures: Biology, Technology and Therapeutics

Schwann cell (SC) cultures from experimental animals and human donors can be prepared using nearly any type of nerve at any stage of maturation to render stage- and patient-specific populations. Methods to isolate, purify, expand in number, and differentiate SCs from adult, postnatal and embryonic sources are efficient and reproducible as these have resulted from accumulated refinements introduced over many decades of work. Albeit some exceptions, SCs can be passaged extensively while maintaining their normal proliferation and differentiation controls. Due to their lineage commitment and strong resistance to tumorigenic transformation, SCs are safe for use in therapeutic approaches in the peripheral and central nervous systems. This review summarizes the evolution of work that led to the robust technologies used today in SC culturing along with the main features of the primary and expanded SCs that make them irreplaceable models to understand SC biology in health and disease. Traditional and emerging approaches in SC culture are discussed in light of their prospective applications. Lastly, some basic assumptions in vitro SC models are identified in an attempt to uncover the combined value of old and new trends in culture protocols and the cellular products that are derived.

Axo-glial interdependence in peripheral nerve development

During the development of the peripheral nervous system, axons and myelinating Schwann cells form a unique symbiotic unit, which is realized by a finely tuned network of molecular signals and reciprocal interactions. The importance of this complex interplay becomes evident after injury or in diseases in which aspects of axo-glial interaction are perturbed. This Review focuses on the specific interdependence of axons and Schwann cells in peripheral nerve development that enables axonal outgrowth, Schwann cell lineage progression, radial sorting and, finally, formation and maintenance of the myelin sheath.

Repair Schwann cell update: Adaptive reprogramming, EMT, and stemness in regenerating nerves

Schwann cells respond to nerve injury by cellular reprogramming that generates cells specialized for promoting regeneration and repair. These repair cells clear redundant myelin, attract macrophages, support survival of damaged neurons, encourage axonal growth, and guide axons back to their targets. There are interesting parallels between this response and that found in other tissues. At the cellular level, many other tissues also react to injury by cellular reprogramming, generating cells specialized to promote tissue homeostasis and repair. And at the molecular level, a common feature possessed by Schwann cells and many other cells is the injury-induced activation of genes associated with epithelial-mesenchymal transitions and stemness, differentiation states that are linked to cellular plasticity and that help injury-induced tissue remodeling. The number of signaling systems regulating Schwann cell plasticity is rapidly increasing. Importantly, this includes mechanisms that are crucial for the generation of functional repair Schwann cells and nerve regeneration, although they have no or a minor role elsewhere in the Schwann cell lineage. This encourages the view that selective tools can be developed to control these particular cells, amplify their repair supportive functions and prevent their deterioration. In this review, we discuss the emerging similarities between the injury response seen in nerves and in other tissues and survey the transcription factors, epigenetic mechanisms, and signaling cascades that control repair Schwann cells, with emphasis on systems that selectively regulate the Schwann cell injury response.

The Success and Failure of the Schwann Cell Response to Nerve Injury

The remarkable plasticity of Schwann cells allows them to adopt the Remak (non-myelin) and myelin phenotypes, which are specialized to meet the needs of small and large diameter axons, and differ markedly from each other. It also enables Schwann cells initially to mount a strikingly adaptive response to nerve injury and to promote regeneration by converting to a repair-promoting phenotype. These repair cells activate a sequence of supportive functions that engineer myelin clearance, prevent neuronal death, and help axon growth and guidance. Eventually, this response runs out of steam, however, because in the long run the phenotype of repair cells is unstable and their survival is compromised. The re-programming of Remak and myelin cells to repair cells, together with the injury-induced switch of peripheral neurons to a growth mode, gives peripheral nerves their strong regenerative potential. But it remains a challenge to harness this potential and devise effective treatments that maintain the initial repair capacity of peripheral nerves for the extended periods typically required for nerve repair in humans.

Direct conversion of adult human skin fibroblasts into functional Schwann cells that achieve robust recovery of the severed peripheral nerve in rats

Direct conversion is considered a promising approach to obtain tissue-specific cells for cell therapies; however, this strategy depends on exogenous gene expression that may cause undesired adverse effects such as tumorigenesis. By optimizing the Schwann cell induction system, which was originally developed for trans-differentiation of bone marrow mesenchymal stem cells into Schwann cells, we established a system to directly convert adult human skin fibroblasts into cells comparable to authentic human Schwann cells without gene introduction. Serial treatments with beta-mercaptoethanol, retinoic acid, and finally a cocktail of basic fibroblast growth factor, forskolin, platelet-derived growth factor-AA, and heregulin-β1 (EGF domain) converted fibroblasts into cells expressing authentic Schwann cell markers at an efficiency of approximately 75%. Genome-wide gene expression analysis suggested the conversion of fibroblasts into the Schwann cell-lineage. Transplantation of induced Schwann cells into severed peripheral nerve of rats facilitated axonal regeneration and robust functional recovery in sciatic function index comparable to those of authentic human Schwann cells. The contributions of induced Schwann cells to myelination of regenerated axons and re-formation of neuromuscular junctions were also demonstrated. Our data clearly demonstrated that cells comparable to functional Schwann cells feasible for the treatment of neural disease can be induced from adult human skin fibroblasts without gene introduction. This direct conversion system will be beneficial for clinical applications to peripheral and central nervous system injuries and demyelinating diseases.

NTE/PNPLA6 is expressed in mature Schwann cells and is required for glial ensheathment of Remak fibers

Neuropathy target esterase (NTE) or patatin-like phospholipase domain containing 6 (PNPLA6) was first linked with a neuropathy occurring after organophosphate poisoning and was later also found to cause complex syndromes when mutated, which can include mental retardation, spastic paraplegia, ataxia, and blindness. NTE/PNPLA6 is widely expressed in neurons but experiments with its Drosophila orthologue Swiss-cheese (SWS) suggested that it may also have glial functions. Investigating whether NTE/PNPLA6 is expressed in glia, we found that NTE/PNPLA6 is expressed by Schwann cells in the sciatic nerve of adult mice with the most prominent expression in nonmyelinating Schwann cells. Within Schwann cells, NTE/PNPLA6 is enriched at the Schmidt-Lanterman incisures and around the nucleus. When analyzing postnatal expression patterns, we did not detect NTE/PNPLA6 in promyelinating Schwann cells, while weak expression was detectable at postnatal day 5 in Schwann cells and increased with their maturation. Interestingly, NTE/PNPLA6 levels were upregulated after nerve crush and localized to ovoids forming along the nerve fibers. Using a GFAP-based knock-out of NTE/PNPLA6, we detected an incomplete ensheathment of Remak fibers whereas myelination did not appear to be affected. These results suggest that NTE/PNPLA6 is involved in the maturation of nonmyelinating Schwann cells during development and de-/remyelination after neuronal injury. Since Schwann cells play an important role in maintaining axonal viability and function, it is therefore likely that changes in Schwann cells contribute to the locomotory deficits and neuropathy observed in patients carrying mutations in NTE.

Cell Transplantation and Neuroengineering Approach for Spinal Cord Injury Treatment: A Summary of Current Laboratory Findings and Review of Literature

Spinal cord injury (SCI) can cause severe traumatic injury to the central nervous system (CNS). Current therapeutic effects achieved for SCI in clinical medicine show that there is still a long way to go to reach the desired goal of full or significant functional recovery. In basic medical research, however, cell transplantation, gene therapy, application of cytokines, and biomaterial scaffolds have been widely used and investigated as treatments for SCI. All of these strategies when used separately would help rebuild, to some extent, the neural circuits in the lesion area of the spinal cord. In light of this, it is generally accepted that a combined treatment may be a more effective strategy. This review focuses primarily on our recent series of work on transplantation of Schwann cells and adult stem cells, and transplantation of stem cell-derived neural network scaffolds with functional synapses. Arising from this, an artificial neural network (an exogenous neuronal relay) has been designed and fabricated by us—a biomaterial scaffold implanted with Schwann cells modified by the neurotrophin-3 (NT-3) gene and adult stem cells modified with the TrkC (receptor of NT-3) gene. More importantly, experimental evidence suggests that the novel artificial network can integrate with the host tissue and serve as an exogenous neuronal relay for signal transfer and functional improvement of SCI.

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Transcriptional control of neural crest specification into peripheral glia

The neural crest is a transient migratory multipotent cell population that originates from the neural plate border and is formed at the end of gastrulation and during neurulation in vertebrate embryos. These cells give rise to many different cell types of the body such as chondrocytes, smooth muscle cells, endocrine cells, melanocytes, and cells of the peripheral nervous system including different subtypes of neurons and peripheral glia. Acquisition of lineage-specific markers occurs before or during migration and/or at final destination. What are the mechanisms that direct specification of neural crest cells into a specific lineage and how do neural crest cells decide on a specific migration route? Those are fascinating and complex questions that have existed for decades and are still in the research focus of developmental biologists. This review discusses transcriptional events and regulations occurring in neural crest cells and derived lineages, which control specification of peripheral glia, namely Schwann cell precursors that interact with peripheral axons and further differentiate into myelinating or nonmyelinating Schwann cells, satellite cells that remain tightly associated with neuronal cell bodies in sensory and autonomous ganglia, and olfactory ensheathing cells that wrap olfactory axons, both at the periphery in the olfactory mucosa and in the central nervous system in the olfactory bulb. Markers of the different peripheral glia lineages including intermediate multipotent cells such as boundary cap cells, as well as the functions of these specific markers, are also reviewed. Enteric ganglia, another type of peripheral glia, will not be discussed in this review. GLIA 2015;63:1883-1896.

Effect of local application of transforming growth factor-β at the nerve repair site following chronic axotomy and denervation on the expression of regeneration-associated genes. Laboratory investigation

OBJECTIVES

Although peripheral nerves can regenerate after traumatic injury, functional recovery is often suboptimal, especially after injuries to large nerve trunks such as the sciatic nerve or brachial plexus. Current research with animal models suggests that the lack of functional recovery resides in the lack of sufficient mature axons reaching their targets due to the loss of neurotrophic support by Schwann cells in the distal stump of injured nerves. This study was designed to investigate the effect of one-time application of transforming growth factor-β (TGF-β) at the repair site of chronically injured nerve.

METHODS

The authors used the rat tibial nerve injury and repair model to investigate the effects of application of physiological concentrations of TGF-β plus forskolin or forskolin alone in vivo at the repair site on gene and protein expression and axon regeneration at 6 weeks after nerve repair. They used gene expression profiling and immunohistochemical analysis of indicative activated proteins in Schwann cells to evaluate the effects of treatments on the delayed repair. They also quantified the regenerated axons distal to the repair site by microscopy of paraffin sections.

RESULTS

Both treatment with forskolin only and treatment with TGF-β plus forskolin resulted in increased numbers of axons regenerated compared with saline-only control. There was robust activation and proliferation of both Schwann cells and macrophages reminiscent of the processes during Wallerian degeneration. The treatment also induced upregulation of genes implicated in cellular activation and growth as detected by gene array.

CONCLUSIONS

Addition of TGF-β plus forskolin to the repair after chronic nerve injury improved axonal regeneration, probably via upregulation of required genes, expression of growth-associated protein, and reactivation of Schwann cells and macrophages. Further studies are required to better understand the mechanism of the positive effect of TGF-β treatment on old nerve injuries.

Novel distribution of cluster of differentiation 200 adhesion molecule in glial cells of the peripheral nervous system of rats and its modulation after nerve injury

This study examined CD200 expression in different peripheral nerves and ganglia. Intense CD200 immunoreactivity was consistently localized in unmyelinated nerve fibers as opposed to a faint immunostaining in the myelinated nerve fibers. By light microscopy, structures resembling the node of Ranvier and Schmidt-Lanterman incisures in the myelinated nerve fibers displayed CD200 immunoreactivity. Ultrastructural study revealed CD200 expression on the neurilemma of Schwann cells whose microvilli and paranodal loops at the node of Ranvier were immunoreactive. The CD200 immunoexpression was also localized in the satellite glial cells of sensory and autonomic ganglia and in the enteric glial cells. Double labeling of CD200 with specific antigens of satellite glia or Schwann cells in the primary cultures of dorsal root ganglia had shown a differential expression of CD200 in the peripheral glial cells. The existence of CD200 in glial cells in the peripheral nervous system (PNS) was corroborated by the expression of CD200 mRNA and protein in a rat Schwann cell line RSC96. Using the model of crush or transected sciatic nerve, it was found that CD200 expression was attenuated or diminished at the site of lesion. A remarkable feature, however, was an increase in incidence of CD200-labelled Schmidt-Lanterman incisures proximal to the injured site at 7 days postlesion. Because CD200 has been reported to impart immunosuppressive signal, we suggest that its localization in PNS glial cells may play a novel inhibitory role in immune homeostasis in both normal and pathological conditions.

Sphingolipidomics: methods for the comprehensive analysis of sphingolipids

Sphingolipids comprise a highly diverse and complex class of molecules that serve as both structural components of cellular membranes and signaling molecules capable of eliciting apoptosis, differentiation, chemotaxis, and other responses in mammalian cells. Comprehensive or "sphingolipidomic" analyses (structure specific, quantitative analyses of all sphingolipids, or at least all members of a critical subset) are required in order to elucidate the role(s) of sphingolipids in a given biological context because so many of the sphingolipids in a biological system are inter-converted structurally and metabolically. Despite the experimental challenges posed by the diversity of sphingolipid-regulated cellular responses, the detection and quantitation of multiple sphingolipids in a single sample has been made possible by combining classical analytical separation techniques such as high-performance liquid chromatography (HPLC) with state-of-the-art tandem mass spectrometry (MS/MS) techniques. As part of the Lipid MAPS consortium an internal standard cocktail was developed that comprises the signaling metabolites (i.e. sphingoid bases, sphingoid base-1-phosphates, ceramides, and ceramide-1-phosphates) as well as more complex species such as mono- and di-hexosylceramides and sphingomyelin. Additionally, the number of species that can be analyzed is growing rapidly with the addition of fatty acyl Co-As, sulfatides, and other complex sphingolipids as more internal standards are becoming available. The resulting LC-MS/MS analyses are one of the most analytically rigorous technologies that can provide the necessary sensitivity, structural specificity, and quantitative precision with high-throughput for "sphingolipidomic" analyses in small sample quantities. This review summarizes historical and state-of-the-art analytical techniques used for the identification, structure determination, and quantitation of sphingolipids from free sphingoid bases through more complex sphingolipids such as sphingomyelins, lactosylceramides, and sulfatides including those intermediates currently considered sphingolipid "second messengers". Also discussed are some emerging techniques and other issues remaining to be resolved for the analysis of the full sphingolipidome.

Level and localization of polysialic acid is critical for early peripheral nerve regeneration

PolySia, the most striking post-translational modification of the neural cell adhesion molecule, is down-regulated during postnatal development. After peripheral nerve lesion, polySia is located on neuronal and glial cells normally not synthesizing polySia. However, structural consequences of reduced polySia content for peripheral nerve regeneration have not yet been clear. Furthermore, the contribution of sialyltransferases ST8SiaII and ST8SiaIV for the up-regulation of polySia has not been studied so far. In order to investigate the impact of polySia on regeneration processes of myelinated axons, we examined mouse mutants retaining only one functional sialyltransferase allele. In the absence of ST8SiaII, quantification of myelinated axons revealed a significant decrease in number and size of regenerated fibers without impairment of remyelination. In contrast, St8SiaIV deficiency resulted in increased fiber outgrowth and axonal maturation. Western blot analysis demonstrated that both ST8SiaII and St8SiaIV direct up-regulation of polySia. Cell-specific induction of polySia in myelinating Schwann cells and on regenerated axons in the presence of ST8SiaIV, but not ST8SiaII, indicates that not only the amount of polySia but also its cellular localization has a high impact on the regeneration progress of peripheral nerves.

Transforming growth factor-beta (TGF-beta) and brain tumours

Since its discovery in the late 1970s considerable research has linked transforming growth factor-beta (TGF-beta) to several human diseases such as fibrosis, auto-immunity and cancer. TGF-beta acts initially as a growth inhibitory factor in early stages of tumour development. In contrast, as tumours evolve, they develop mechanisms to evade the growth-regulatory effects of TGF-beta, resulting in greater tumour invasiveness, increased metastatic potential and inhibition of surrounding immune responses. However, although extensively studied, the molecular mechanisms that trigger tumour cells to "switch" from TGF-beta-inhibited to TGF-beta-promoted are still not fully understood. Contradictory studies that demonstrate opposite cellular effects mediated by TGF-beta are abundant throughout the literature. This review summarizes the current molecular mechanisms involved in the tumour suppressive and tumour progressive characteristics of TGF-beta in brain tumours. Potential therapeutic agents that target TGF-beta and related proteins being evaluated against brain tumours is also discussed.

The immunocompetence of Schwann cells

Schwann cells are the myelinating glial cells of the peripheral nervous system that support and ensheath axons with myelin to enable rapid saltatory signal propagation in the axon. Immunocompetence, however, has only recently been recognized as an important feature of Schwann cells. An autoimmune response against components of the peripheral nervous system triggers disabling inflammatory neuropathies in patients and corresponding animal models. The immune system participates in nerve damage and disease manifestation even in non-inflammatory hereditary neuropathies. A growing body of evidence suggests that Schwann cells may modulate local immune responses by recognizing and presenting antigens and may also influence and terminate nerve inflammation by secreting cytokines. This review summarizes current knowledge on the interaction of Schwann cells with the immune system, which is involved in diseases of the peripheral nervous system.

Immune circuitry in the peripheral nervous system

PURPOSE OF REVIEW

The aim of this review is to describe the local immune circuitry in the peripheral nervous system and its dialogue with systemic immunity under pathological conditions. Specifically, interactions of the immune system with cellular and extracellular components within peripheral nerve and immune functions of tissue-resident endoneurial macrophages and Schwann cells will be discussed.

RECENT FINDINGS

New insights into the elements involved in the pathogenesis of immune-mediated disorders of the peripheral nervous system provide a better understanding of the complex interplay of these cellular and molecular components in the immunology of the peripheral nervous system.

SUMMARY

The application of innovative and cutting-edge technologies to the study of immunoinflammatory disorders of the peripheral nervous system provides a better understanding of underlying principles of the organization of the immune network present in the peripheral nerve and its dialogue with the systemic immune system. This may foster the development of specific and highly effective therapies for immune-mediated disorders of the peripheral nerve.

TGFbeta type II receptor signaling controls Schwann cell death and proliferation in developing nerves

During development, Schwann cell numbers are precisely adjusted to match the number of axons. It is essentially unknown which growth factors or receptors carry out this important control in vivo. Here, we tested whether the type II transforming growth factor (TGF) beta receptor has a role in this process. We generated a conditional knock-out mouse in which the type II TGFbeta receptor is specifically ablated only in Schwann cells. Inactivation of the receptor, evident at least from embryonic day 18, resulted in suppressed Schwann cell death in normally developing and injured nerves. Notably, the mutants also showed a strong reduction in Schwann cell proliferation. Consequently, Schwann cell numbers in wild-type and mutant nerves remained similar. Lack of TGFbeta signaling did not appear to affect other processes in which TGFbeta had been implicated previously, including myelination and response of adult nerves to injury. This is the first in vivo evidence for a growth factor receptor involved in promoting Schwann cell division during development and the first genetic evidence for a receptor that controls normal developmental Schwann cell death.

CITED1 homozygous null mice display aberrant pubertal mammary ductal morphogenesis

Expression microarray analysis identified CITED1 among a group of genes specifically upregulated in the pubertal mouse mammary gland. At puberty, CITED1 localizes to the luminal epithelial cell population of the mammary ducts and the body cells of the terminal end buds. Generation of CITED1 gene knockout mice showed that homozygous null mutants exhibit retarded mammary ductal growth at puberty and, in addition, dilated ductal structures with a lack of spatial restriction of the subtending branches. Analysis of CITED1 homozygous null and heterozygous null mammary gland gene expression using microarrays suggested that the mammary-specific phenotype seen in the homozygous null females is due to a disturbance in the transcription of a number of key mediators of pubertal ductal morphogenesis. These include estrogen and TGFbeta responsive genes, such as the EGFR/ErbB2 ligand, amphiregulin, whose transcription we suggest is directly or indirectly regulated by CITED1.

Schwann cell behavior in three-dimensional collagen gels: Evidence for differential mechano-transduction and the influence of TGF-beta 1 in morphological polarization and differentiation

Schwann cells (SCs) cultured on and within magnetically aligned collagen gels were examined for their abilities to spread and exhibit contact guidance, two functions that are relevant to their potential enhancement of neurite migration and regeneration in entubulation repair of transection-type nerve injuries. Cells seeded at or near the surfaces of gels abandoned their initially spherical shapes, adopting spread morphologies rapidly compared to cells within the gels. Those few cells within the gels that did spread exhibited marked contact guidance responses, aligning strongly with the aligned collagen fibrils. Spreading of cells in gels could not be induced by varied cell concentration, collagen density, mitogen presence, inclusion of soluble laminin, or use of fibrin gel in lieu of collagen. However, cells that settled at the interface between collagen gel layers during gellation of the top layer above a preformed bottom layer were highly spread. This suggests that a differential mechanical interaction across the cell at an interface, where at least one surface presents constituents of the basal lamina, permits the Schwann cell to rapidly revert to a spread, differentiated phenotype. Unlike other reagents, TGF-beta1 was able to induce significant SC spreading as early as 4 h post-seeding. Consistent with the differential-mechanical cue mechanism, TGF-beta1 appears to facilitate this response, at least in part, by upregulating beta1 integrin expression, thereby enabling the SC to more acutely detect these local cues in the mechanical environment.

Transforming growth factor beta 1 may regulate the stability of mature myelin sheaths

The molecular mechanisms underlying peripheral neuropathies have only been partially elucidated. In particular, the regulatory factors that control the stability and turnover of mature myelin are largely unknown. Transforming growth factor beta 1 (TGF-beta1), and its associated receptors, are expressed by mature Schwann cells. On this basis, we postulated that TGF-beta1 may be an autocrine regulator of mature myelin. This hypothesis was tested by examining the ultrastructure of myelin in adult mice that have a null mutation of their TGF-beta1 gene. We report here that the myelin of these mice is grossly abnormal. At the nodes of Ranvier, the cytoplasmic collars of the Schwann cells were expanded and the myelin had a honeycomb appearance. Focal (tomacula-like) hypermyelin structures were observed in the internodal regions of a significant number of axons in mutant nerve, and were not observed in littermate controls. Axon diameters were within the normal range and no axonal pathology was evident in mutant nerve and macrophages were absent. Results imply that lack of TGF-beta1 may have a direct effect on Schwann cells. We suggest that TGF-beta1 may stabilise compact myelin via an autocrine mechanism.

TGFbeta1 modulates the phenotype of Schwann cells at the transcriptional level

We have examined the effects of transforming growth factor beta1 (TGFbeta1) on gene expression in cultured rat Schwann cells (SCs). TGFbeta1 decreased the steady-state mRNA levels of several genes that are expressed by myelinating SCs but had varied effects on the mRNA levels of NCAM, L1, GAP-43, and p75-genes that are expressed by denervated and nonmyelinating SCs. TGFbeta1 antagonized the effects of forskolin on the mRNA levels of the transcription factors Oct-6/tst-1/SCIP and Krox20. Transcriptional run-off analysis demonstrated that the effects of TGFbeta1 on gene expression occur at least in part at the level of transcription. Thus, TGFbeta1 suppresses the expression of genes that characterize the different phenotypes of SCs, and these changes occur at least in part at a transcriptional level.

Transforming growth factor-beta and forskolin attenuate the adverse effects of long-term Schwann cell denervation on peripheral nerve regeneration in vivo

Transforming growth factor-beta (TGF-beta) plays a central role in the regulation of Schwann cell (SC) proliferation and differentiation and is essential for the neurotrophic effects of several neurotrophic factors (reviewed by Unsicker and Krieglstein, 2000; Unsicker and Strelau, 2000). However, its role in peripheral nerve regeneration in vivo is not yet understood. Our studies were carried out to characterize (1) the effects of duration of regeneration, and chronic SC denervation on the number of tibial (TIB) motor neurons that regenerated axons over a fixed distance (25 mm into distal common peroneal [CP] nerve stumps), and (2) the effect of in vitro incubation of 6-month chronically denervated sciatic nerve explants with TGF-beta and forskolin on their capacity to support axonal regeneration in vivo. TIB--CP cross-suture in Silastic tubing was used, and regeneration into 0-24-week chronically denervated CP stumps was allowed for either 1.5 or 3 months. Chronically denervated rat sciatic nerve explants (3 x 3 mm(2)) were incubated in vitro with either DMEM and 15% fetal calf serum (D-15) plus TGF-beta/forskolin or D-15 alone for 48 h and placed into a 10-mm Silastic tube that bridged the proximal and distal nerve stumps of a freshly cut TIB nerve. The number of tibial motor neurons that regenerated axons through the explants and 25 mm into the distal nerve stump after 6 months, and TIB regeneration into the CP nerve stumps, were assessed using retrograde tracers, fluorogold, or fluororuby. We found that all tibial motor neurons regenerate their axons 25 mm into 0-4-week denervated CP nerve stumps after a regeneration period of 3 months. Reducing regeneration time to 1.5 months and chronic denervation, reduced the number of motor neurons that regenerated axons over 25 mm. Exposure of 6-month denervated nerve explants to TGF-beta/forskolin increased the number of motor neurons that regenerated through them from 258 +/-13; mean +/- SE to 442 +/- 22. Hence, acute treatment of atrophic SC with TGF-beta can reactivate the growth-permissive SC phenotype to support axonal regeneration.

Neurons modulate oxytocin receptor expression in rat cultured astrocytes: involvement of TGF-beta and membrane components

We examined the effect of neurons on oxytocin (OT) receptors (OTR) and OTR gene expression in cultured astrocytes. The addition of neuron-conditioned medium induced an increase of both OTR binding and OTR mRNA level. This effect was enhanced after the medium was boiled or acidified. As it is known that transforming growth factor-beta (TGF-beta) can be released from carrier proteins by acid or heat, TGF-beta1 and 2 were tested and found to induce an increase of OTR binding. Furthermore, TGF-beta antibody abolished the stimulatory effect of normal or acidified neuron-conditioned medium. Neurons added to cultured astrocytes without contact mimicked the stimulatory effect of the conditioned medium. In contrast, neurons added with contact, induced a decrease in OTR binding and an increase of mRNA level, whereas neuronal membranes induced a decrease of both OTR binding and mRNA levels. In conclusion, the present data demonstrate that in vitro, neurons are able to modulate astrocytic OTR expression at the level of both protein and mRNA. They stimulate this expression through their release of TGF-beta and inhibit it by the action of unknown membrane components.

Transforming growth factor beta (TGFbeta) mediates Schwann cell death in vitro and in vivo: examination of c-Jun activation, interactions with survival signals, and the relationship of TGFbeta-mediated death to Schwann cell differentiation

In some situations, cell death in the nervous system is controlled by an interplay between survival factors and negative survival signals that actively induce apoptosis. The present work indicates that the survival of Schwann cells is regulated by such a dual mechanism involving the negative survival signal transforming growth factor beta (TGFbeta), a family of growth factors that is present in the Schwann cells themselves. We analyze the interactions between this putative autocrine death signal and previously defined paracrine and autocrine survival signals and show that expression of a dominant negative c-Jun inhibits TGFbeta-induced apoptosis. This and other findings pinpoint activation of c-Jun as a key downstream event in TGFbeta-induced Schwann cell death. The ability of TGFbeta to kill Schwann cells, like normal Schwann cell death in vivo, is under a strong developmental regulation, and we show that the decreasing ability of TGFbeta to kill older cells is attributable to a decreasing ability of TGFbeta to phosphorylate c-Jun in more differentiated cells.

Interferon-gamma, tumor necrosis factor-alpha, and transforming growth factor-beta inhibit cyclic AMP-induced Schwann cell differentiation

Schwann cells differentiate in vivo in response to contact with axons, and cAMP simulates some of these aspects of differentiation in vitro, particularly morphologic changes and expression of certain phenotypic molecules. Unfractionated inflammatory cytokines inhibit cAMP-induced Schwann cell expression of galactolipids (Gal). We sought to identify which cytokines were responsible for this inhibition and to determine whether other phenotypic indicators of Schwann cell differentiation were also affected. Neonatal rat Schwann cells were incubated in vitro with 1 mM 8 Bromo cAMP (8 Br cAMP) with or without the addition of interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-2, IL-6, tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), or transforming growth factor-beta (TGF-beta). Cells were then examined for morphologic changes and for expression of surface Gal and low-affinity nerve growth factor receptor (NGFRp75), employing indirect immunofluorescence. 8 Br cAMP induced Schwann cell upregulation of Gal, downregulation of NGFRp75, and the cells became enlarged and somewhat amorphous and irregular in appearance. Cells treated with IFN-gamma or TNF-alpha alone were more bipolar and more evenly distributed on coverslips than were control cells, whereas TGF-beta alone induced elongated cells often in a swirling pattern. None of the cytokines alone induced upregulation of Gal or downregulation of NGFRp75. TNF-alpha, IFN-gamma, and TGF-beta inhibited the 8 Br cAMP-induced morphologic changes, as well as the upregulation of Gal and downregulation of NGFRp75. The other cytokines had no effects on Gal or NGFRp75 expression. Thus, these three cytokines, which are present in inflammatory lesions in the peripheral nervous system, are capable of inhibiting Schwann cell differentiation.

Proliferative and morphological effects of endothelins in Schwann cells: roles of p38 mitogen-activated protein kinase and Ca(2+)-independent phospholipase A2

The regulation of Schwann cell (SC) proliferation and morphology is critical to nerve homeostasis. We have previously reported that endothelins (ETs) regulate the activity of different effectors in SC including adenylyl cyclase, phospholipases C and A2 and mitogen-activated protein kinases (MAPKs). These effects imply a possible participation of ETs in the regulation of SC phenotype. We have now investigated the effects of endothelins on the proliferation and morphology of SC, and compared them with the responses to platelet-derived growth factor (PDGF), a known mitogen in these cells. Both endothelin-1 (ET-1) and PDGF increased the incorporation of [3H]thymidine and the proportion of SC in S and G2/M, with a concomitant decrease in the G0/G1 stage cells. Treatment with ET-1 produced rapid changes in the morphology of the SC, characterized by the appearance of cell spreading with shorter processes. The response to ET-1 was considered to represent a proliferative phenotype, in contrast to the effects of forskolin, which decreased [3H]thymidine incorporation in immortalized SC (iSC) and lead to a differentiated morphology with longer extensions. While both ET-1 and PDGF displayed a proliferative effect on SC, treatment with PDGF did not affect the morphology of these cells to a significant extent. A role for p38 MAPK and Ca(2+)-independent phospholipase A2 in the changes in morphology and proliferation of iSC driven by ET-1 was suggested by the effects of selective inhibitors of these pathways [SB202190 and HELSS, respectively]. The unique pattern of signaling pathways recruited by ET-1 and its combined effects on regulation of phenotype and proliferation of SC suggest an important role for this peptide during nerve degeneration/regeneration.

Distribution of TGF-beta, the TGF-beta type I receptor and the R-II receptor in peripheral nerves and mechanoreceptors; observations on changes after traumatic injury

The mechanisms governing the regeneration of denervated peripheral mechanoreceptors are similar to those of peripheral nerves. The ability to regenerate depends partly on changes of the Schwann cell phenotype. The transforming growth factor beta (TGF-beta) family have been implicated in induction of Schwann cell proliferation, production of extracellular matrix and neurotrophin synthesis as well as synthesis or repression of cell adhesion molecules. Hence, they may prove to be of importance for regenerative mechanisms in peripheral mechanoreceptors. The distribution of TGF-beta, the receptors I and II and intra-cellular second messengers, Smad 2/3 and 4 was assessed in sensory neurones, peripheral nerves and mechanoreceptors by immuno-histochemistry, immuno-electron microscopy and in situ hybridisation. TGF-beta2 mRNA and TGF-beta2-like immunoreactivity (IR) were expressed in injured small and medium sized rat sensory neurones of dorsal root ganglia. TGF-beta and receptor II mRNA and immunoreactivities (IR) were present in satellite cells. Intact and injured sensory neurones expressed receptor I mRNA and Smad 2 mRNA. TGF-beta2 mRNA was found in transected nerve stumps and in sensory mechanoreceptors. TGF-beta1, 2 and Smad 4 were also observed in inner core lamellar cells of intact and denervated cat Pacinian corpuscles. Lamellar cells of intact and denervated Meissner corpuscles were TGF-beta immunoreactive. Merkel cells were receptors I and II immunoreactive. In conclusion, cutaneous and subcutaneous mechanoreceptors differ with regard to the expression of TGF-beta isoforms and receptors.

Soluble forms of NCAM and F3 neuronal cell adhesion molecules promote Schwann cell migration: identification of protein tyrosine phosphatases zeta/beta as the putative F3 receptors on Schwann cells

Neural cell adhesion molecule (NCAM) and F3 are both axonal adhesion molecules which display homophilic (NCAM) or heterophilic (NCAM, F3) binding activities and participate in bidirectional exchange of information between neurones and glial cells. Engineered Fc chimeric molecules are fusion proteins that contain the extracellular part of NCAM or F3 and the Fc region of human IgG1. Here, we investigated the effect of NCAM-Fc and F3-Fc chimeras on Schwann cell (SC) migration. Binding sites were identified at the surface of cultured SCs by chimera coated fluorospheres. The functional effect of NCAM-Fc and F3-Fc binding was studied in two different SC migration models. In the first, migration is monitored at specific time intervals inside a 1-mm gap produced in a monolayer culture of SCs. In the second, SCs from a dorsal root ganglion explant migrate on a sciatic nerve cryosection. In both systems addition of the chimeras significantly increased the extent of SC migration and this effect could be prevented by the corresponding anti-NCAM or anti-F3 blocking antibodies. Furthermore, antiproteoglycan-type protein tyrosine phosphatase zeta/beta (RPTPzeta/beta) antibodies identified the presence of RPTPzeta/beta on SCs and prevented the enhancing effect of soluble F3 on SC motility by 95%. The F3-Fc coated Sepharose beads precipitated RPTPzeta/beta from SC lysates. Altogether these data point to RPTPzeta/beta is the putative F3 receptor on SCs. These results identify F3 and NCAM receptors on SC as potential mediators of signalling occurring between axons and glial cells during peripheral nerve development and regeneration.

Functions of transforming growth factor-beta isoforms in the nervous system. Cues based on localization and experimental in vitro and in vivo evidence

This review briefly describes the cellular distribution and documented roles of the transforming growth factor (TGF)-beta isoforms TGF-beta2 and -beta3 in the central and peripheral nervous system. TGF-beta2 and -beta3 are coexpressed in developing radial glial and mature astroglial and Schwann cells, as well as in subpopulations of differentiated neurons, most prominently in cortical, hippocampal, and brainstem/spinal cord motor neurons. In vitro studies have suggested a number of potential, physiologically relevant functions for TGF-betas including regulation of astroglial cell proliferation, expression of adhesion molecules, survival promoting roles for neurons in combination with established neurotrophic factors, and differentiative actions on neurons.

Role of Schwann cells in retinal ganglion cell axon regeneration

It is a well known fact that the injured PNS can successfully regenerate, on the other hand, the CNS such as retinal ganglion cell (RGC) axons of adult mammals is incapable of regeneration. After injury, RGC axons rapidly degenerate and most cell bodies go through the process of cell death, while glial cells at the site of injury undergo a series of responses which underlie the so-called glial scar formation. However, it has become apparent that RGCs do have an intrinsic capacity to regenerate which can be elicited by experimental replacement of the inhibitory glial environment with a permissive peripheral nerve milieu. Schwann cells are a major component of the PNS and play a role in regeneration, by producing various kinds of functional substances such as diffusible neurotrophic factors, extracellular matrix and cell adhesion molecules. RGC regeneration can be induced by cooperation of these substances. The contact of RGC axons to Schwann cells based upon the structural and molecular linkages seems to be indispensable for the stable and successful regeneration. In addition to cell adhesion molecules such as NCAM and L1, data from our laboratory show that Schwann cells utilize short focal tight junctions to provide morphological stabilization of the contact with the elongating axon, as well as a small scale of gap junctions to facilitate traffic of substances between them. Moreover, our results show that modifications of functional properties in neighboring glial cells of optic nerve are induced by transplantation of Schwann cells. Astrocytes usually considered to form a glial scar guide the regenerating axons in cooperation with Schwann cells. A decrease of the oligodendrocyte marker O4 and migration of ED-1 positive macrophages is observed within the optic nerve stump. Accordingly, RGC regeneration is not a simple phenomenon of axonal elongation on the Schwann cell membrane, but is based on direct and dynamic communication between the axon and the Schwann cell, and is also accompanied by changes and responses among the glial cell populations, which may be partly associated with the mechanisms of optic nerve regeneration.

Nerve growth factor-hypersecreting Schwann cell grafts augment and guide spinal cord axonal growth and remyelinate central nervous system axons in a phenotypically appropriate manner that correlates with expression of L1

Schwann cells contribute to efficient axonal regeneration after peripheral nerve injury and, when grafted to the central nervous system (CNS), also support a modest degree of central axonal regeneration. This study examined (1) whether Schwann cells grafted to the CNS exhibit normal patterns of differentiation and association with spinal axons and what signals putatively modulate these interactions, and (2) whether Schwann cells overexpressing neurotrophic factors enhance axonal regeneration. Thus, primary Schwann cells were transduced to hypersecrete human nerve growth factor (NGF) and were grafted to spinal cord injury sites in adult rats. Comparisons were made to nontransfected Schwann cells. From 3 days to 6 months later, grafted Schwann cells exhibited a phenotypic and temporal course of differentiation that matched patterns normally observed after peripheral nerve injury. Schwann cells spontaneously aligned into regular spatial arrays within the cord, appropriately remyelinated coerulospinal axons that regenerated into grafts, and appropriately ensheathed but did not myelinate sensory axons extending into grafts. Coordinate expression of the cell adhesion molecule L1 on Schwann cells and axons correlated with establishment of appropriate patterns of axon-Schwann cell ensheathment. Transduction of Schwann cells to overexpress NGF robustly increased axonal growth but did not otherwise alter the nature of interactions with growing axons. These findings suggest that signals expressed on Schwann cells that modulate peripheral axonal regeneration and myelination are also recognized in the CNS and that the modification of Schwann cells to overexpress growth factors significantly augments their capacity to support extensive axonal growth in models of CNS injury.

Transforming growth factor beta1-regulated cell proliferation and expression of neural cell adhesion molecule in B104 neuroblastoma cells: differential effects of ethanol

The expression and activity of factors influencing early neuronal development are altered by ethanol. Such factors include growth factors, for example, platelet-derived growth factor and basic fibroblast growth factor (for cell proliferation), and cell adhesion molecules (for neuronal migration). One agent, transforming growth factor beta1 (TGFbeta1), may affect both events. We tested the hypothesis that ethanol alters myriad TGFbeta1-mediated activities [i.e., cell proliferation and neural cell adhesion molecule (N-CAM) expression] using B104 neuroblastoma cells. TGFbeta1 inhibited the proliferation of B104 cells as evidenced by decreases in cell number and [3H]thymidine ([3H]dT) incorporation. TGFbeta1 induced sustained activation of extracellular signal-regulated kinases (ERKs), which are part of the family of mitogen-activated protein kinases (MAPKs). Treatment with PD98059 (a MAPK kinase blocker) abolished TGFbeta1-regulated inhibition of [3H]dT incorporation. TGFbeta1-mediated growth inhibition was potentiated by ethanol exposure. Ethanol also produced prolonged activation of ERK, an effect that was partially eliminated by treatment with PD98059. On the other hand, TGFbeta1 up-regulated N-CAM expression, and this up-regulation was not affected by treatment with PD98059. Ethanol inhibited the TGFbeta1-induced up-regulation of N-CAM expression in a concentration-dependent manner. Thus, TGFbeta1 affects ERK-dependent cell proliferation and ERK-independent N-CAM expression in B104 cells. Both activities are sensitive to ethanol and may underlie the ethanol-induced alterations in the proliferation and migration of CNS neurons.

TGF-beta1-dependent differential expression of a rat homolog for latent TGF-beta binding protein in astrocytes and C6 glioma cells

Transforming growth factor-beta1 (TGF-beta1) is widely recognized for its multiple roles in development, cellular maintenance, and protection against injury. In the brain, TGF-beta1 upregulation in microglia/macrophages is a predominant response to lesion and during pathology. However, the precise functions of TGF-beta1 in this context are still enigmatic. The present study investigates changes in astroglial gene expression as a major target of TGF-beta1 signaling in the brain. Differential display reverse transcription-polymerase chain reaction (DDRT-PCR) was used to identify several gene fragments differentially regulated by TGF-beta1 in rat astrocytes and C6 glioma cells. Among the cDNAs regulated by TGF-beta1 in C6 cells two cDNAs showed homology to alpha-tropomyosin and glycerol-3-phosphate dehydrogenase, respectively. Cloning of a full length cDNA corresponding to a differentially regulated gene fragment revealed close homology to latent TGF-beta binding protein (LTBP)-2. Data using antisense LTBP-2 oligonucleotides to decrease LTBP-2 expression suggest that LTBP-2 functions to activate TGF-beta. Therefore, it is likely that upregulation of the rat LTBP-2 homolog mRNA in C6 cells and cortical astrocytes by TGF-1 might lead to self-activation and exaggeration of TGF-beta signaling. These data will extend our current understanding of TGF-beta1 functioning on lesion-related features of glial cells.

TGF-beta plays a key role in morphogenesis of the pancreatic islets of Langerhans by controlling the activity of the matrix metalloproteinase MMP-2

Islets of Langerhans are microorgans scattered throughout the pancreas, and are responsible for synthesizing and secreting pancreatic hormones. While progress has recently been made concerning cell differentiation of the islets of Langerhans, the mechanism controlling islet morphogenesis is not known. It is thought that these islets are formed by mature cell association, first differentiating in the primitive pancreatic epithelium, then migrating in the extracellular matrix, and finally associating into islets of Langerhans. This mechanism suggests that the extracellular matrix has to be degraded for proper islet morphogenesis. We demonstrated in the present study that during rat pancreatic development, matrix metalloproteinase 2 (MMP-2) is activated in vivo between E17 and E19 when islet morphogenesis occurs. We next demonstrated that when E12.5 pancreatic epithelia develop in vitro, MMP-2 is activated in an in vitro model that recapitulates endocrine pancreas development (Miralles, F., P. Czernichow, and R. Scharfmann. 1998. Development. 125: 1017-1024). On the other hand, islet morphogenesis was impaired when MMP-2 activity was inhibited. We next demonstrated that exogenous TGF-beta1 positively controls both islet morphogenesis and MMP-2 activity. Finally, we demonstrated that both islet morphogenesis and MMP-2 activation were abolished in the presence of a pan-specific TGF-beta neutralizing antibody. Taken together, these observations demonstrate that in vitro, TGF-beta is a key activator of pancreatic MMP-2, and that MMP-2 activity is necessary for islet morphogenesis.

TNF-alpha and TGF-beta act synergistically to kill Schwann cells

Interactions between cytokines and Schwann cells (SC) are important in development, repair, and disorders of the peripheral nervous system (PNS). Tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) are two prominent cytokines which may be involved in these processes and their gene products are upregulated in some experimental neuropathies. This study focuses on the in vitro effects of these cytokines, both singly and in combination, on cultured SC. Expression of both Type I and Type II TNF-alpha receptors was demonstrated on the SC surface by immunocytochemistry. Treatment of SC with a combination of TNF-alpha plus TGF-beta causes significant detachment and cell death while treatment with each cytokine alone is not significantly cytotoxic. When compared with control cultures, SC treated with the combination of cytokines exhibit an increase in the number of cells with condensed nuclei and evidence of DNA fragmentation, characteristics consistent with cells undergoing programmed cell death. Thus, TNF-alpha plus TGF-beta induce SC loss of adhesion which is predominantly due to cell death. Apoptotic mechanisms are likely to contribute to some extent to this cell death. These findings provide in vitro evidence to support the hypothesis that cytokines can directly damage SC in PNS disorders.

Nerve injury and inflammatory cytokines modulate gap junctions in the peripheral nervous system

In the peripheral nervous system (PNS), myelinating Schwann cells express the gap junction protein connexin32 (Cx32) and lower levels of connexin43 (Cx43). Although the function of Cx43 in Schwann cells is not yet known, in adult mammals Cx32 is thought to form reflexive contacts within individual myelinating glial cells and provide direct pathways for intracellular ionic and metabolic exchange from the cell body to the innermost periaxonal cytoplasmic regions. In response to nerve injury, Schwann cells in the degenerating region down-regulate expression of Cx32 and there is increased expression of connexin46 (Cx46) mRNA and protein. The cascade of Schwann cell responses seen after the injury-induced decrease in Cx32, and the observation that dividing Schwann cells express Cx46, and possibly other connexins, and are coupled through gap junction channels, raise the intriguing possibility that there are coordinated changes in Schwann cell proliferation and connexin expression. Moreover, intercellular junctional coupling among cells in general may be important during injury responses. Consistent with this hypothesis, dividing Schwann cells are preferentially coupled through junctional channels as compared to non-dividing cells, which are generally uncoupled. Moreover, the strength of junctional coupling among cultured Schwann cells is modulated by a number of cytokines to which Schwann cells are exposed to in vivo after nerve injury, and Cx46 mRNA and protein levels correlate with the degree of coupling. Other injury-induced cellular changes in connexin expression that may be functionally important during injury responses include a transient increase in Cx43 in endoneurial fibroblasts. This paper reviews what is known about connexin expression and function in the adult mammalian PNS, and focuses on some of the changes that occur following nerve injury. Moreover, evidence that inflammatory cytokines released after injury modulate connexin expression and junctional coupling strength is presented.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.