Tumor necrosis factor alpha and human Schwann cells: signalling and phenotype modulation without cell death

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Pathology of Initial Axon Segments in Chronic Inflammatory Demyelinating Polyradiculoneuropathy and Related Disorders

The diagnosis of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is based on a combination of clinical, electrodiagnostic and laboratory features. The different entities of the disease include chronic immune sensory polyradiculopathy (CISP) and autoimmune nodopathies. It is debatable whether CIDP occurring in the course of other conditions, i.e., monoclonal IgG or IgA gammopathy, should be treated as a separate disease entity from idiopathic CIDP. This study aims to evaluate the molecular differences of the nodes of Ranvier and the initial axon segment (AIS) and juxtaparanode region (JXP) as the potential cause of phenotypic variation of CIDP while also seeking new pathomechanisms since JXP is sequestered behind the paranode and autoantibodies may not access the site easily. The authors initially present the structure of the different parts of the neuron and its functional significance, then discuss the problem of whether damage to the juxtaparanodal region, Schwann cells and axons could cause CIDP or if these damages should be separated as separate disease entities. In particular, AIS's importance for modulating neural excitability and carrying out transport along the axon is highlighted. The disclosure of specific pathomechanisms, including novel target antigens, in the heterogeneous CIDP syndrome is important for diagnosing and treating these patients.

Chronic TNFα Exposure Induces Robust Proliferation of Olfactory Ensheathing Cells, but not Schwann Cells

TNFα is persistently elevated in many injury and disease conditions. Previous reports of cytotoxicity of TNFα for oligodendrocytes and their progenitors suggest that the poor endogenous remyelination in patients with traumatic injury or multiple sclerosis may be due in part to persistent inflammation. Understanding the effects of inflammatory cytokines on potential cell therapy candidates is therefore important for evaluating the feasibility of their use. In this study, we assessed the effects of long term exposure to TNFα on viability, proliferation, migration and TNFα receptor expression of cultured rat olfactory ensheathing cells (OECs) and Schwann cells (SCs). Although OECs and SCs transplanted into the CNS produce similar myelinating phenotypes, and might be expected to have similar therapeutic uses, we report that they have very different sensitivities to TNFα. OECs exhibited positive proliferative responses to TNFα over a much broader range of concentrations than SCs. Low TNFα concentrations increased proliferation and migration of both OECs and SCs, but SC number declined in the presence of 100 ng/ml or higher concentrations of TNFα. In contrast, OECs exhibited enhanced proliferation even at high TNFα concentrations (up to 1 µg/ml) and showed no evidence of TNF cytotoxicity even at 4 weeks post-treatment. Furthermore, while both OECs and SCs expressed TNFαR1 and TNFαR2, TNFα receptor levels were downregulated in OECs after exposure to100 ng/ml TNFα for 5-7 days, but were either elevated or unchanged in SCs. These results imply that OECs may be a more suitable cell therapy candidate if transplanted into areas with persistent inflammation.

Dysregulated expression of death, stress and mitochondrion related genes in the sciatic nerve of presymptomatic SOD1(G93A) mouse model of Amyotrophic Lateral Sclerosis

Schwann cells are the main source of paracrine support to motor neurons. Oxidative stress and mitochondrial dysfunction have been correlated to motor neuron death in Amyotrophic Lateral Sclerosis (ALS). Despite the involvement of Schwann cells in early neuromuscular disruption in ALS, detailed molecular events of a dying-back triggering are unknown. Sciatic nerves of presymptomatic (60-day-old) SOD1(G93A) mice were submitted to a high-density oligonucleotide microarray analysis. DAVID demonstrated the deregulated genes related to death, stress and mitochondrion, which allowed the identification of Cell cycle, ErbB signaling, Tryptophan metabolism and Rig-I-like receptor signaling as the most representative KEGG pathways. The protein-protein interaction networks based upon deregulated genes have identified the top hubs (TRAF2, H2AFX, E2F1, FOXO3, MSH2, NGFR, TGFBR1) and bottlenecks (TRAF2, E2F1, CDKN1B, TWIST1, FOXO3). Schwann cells were enriched from the sciatic nerve of presymptomatic mice using flow cytometry cell sorting. qPCR showed the up regulated (Ngfr, Cdnkn1b, E2f1, Traf2 and Erbb3, H2afx, Cdkn1a, Hspa1, Prdx, Mapk10) and down-regulated (Foxo3, Mtor) genes in the enriched Schwann cells. In conclusion, molecular analyses in the presymptomatic sciatic nerve demonstrated the involvement of death, oxidative stress, and mitochondrial pathways in the Schwann cell non-autonomous mechanisms in the early stages of ALS.

Molecules involved in the crosstalk between immune- and peripheral nerve Schwann cells

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.

Gene network revealed involvements of Birc2, Birc3 and Tnfrsf1a in anti-apoptosis of injured peripheral nerves

Crush injury or axotomy of peripheral nerves results in the rapid production of the inflammatory cytokines, which were confirmed in various models, to some extent, to be noxious to the myelin sheath or Schwann cells (SCs). TNF-α is one of the primary initiators of the inflammatory cascade and exerts pleiotropic functions in the physiological conditions by binding to its receptors, type I (TNFRI) and type II (TNFRII). The pathway molecules TNFRI, Birc2 and Birc3 play key roles during the activation of the signaling. Injured peripheral nerves, preventing them from TNF-α-mediated destruction and proceeding to successful regeneration, might initiate an anti-apoptotic mechanism. To identity the exact functions of TNFRI, Birc2 and Birc3, as well as its involved pathways in the cellular events, we inferred a dynamic gene regulatory network from short time-series measurements of the proximal nerve segment cDNA microarray following rat sciatic nerve transection. TNFRI family member Tnfrsf1a, Birc2 and Birc3 were mined out integrating as master regulators to mediate inflammatory responses. Experiments revealed that Tnfrsf1a, Birc2 and Birc3 proteins colocalized with S100 in the rat peripheral nerve tissues, and the expression levels increased with the time extension. Knockdown of the proteins induced the apoptotic formation of primary cultured SCs by upregulation of caspase 3 and caspase 6. Our systematic analysis indicated that Tnfrsf1a, Birc2 and Birc3 of SCs, not originally regarded as XIAP, were mainly responsible for the inflammation-mediated anti-apoptosis of peripheral nerves. Birc2 and Birc3 might be the most potential targets for anti-apoptotic protection mediated by inflammatory cytokines.

Transient lysosomal activation is essential for p75 nerve growth factor receptor expression in myelinated Schwann cells during Wallerian degeneration

Myelinated Schwann cells in the peripheral nervous system express the p75 nerve growth factor receptor (p75NGFR) as a consequence of Schwann cell dedifferentiation during Wallerian degeneration. p75NGFR has been implicated in the remyelination of regenerating nerves. Although many studies have shown various mechanisms underlying Schwann cell dedifferentiation, the molecular mechanism contributing to the re-expression of p75NGFR in differentiated Schwann cells is largely unknown. In the present study, we found that lysosomes were transiently activated in Schwann cells after nerve injury and that the inhibition of lysosomal activation by chloroquine or lysosomal acidification inhibitors prevented p75NGFR expression at the mRNA transcriptional level in an ex vivo Wallerian degeneration model. Lysosomal acidification inhibitors suppressed demyelination, but not axonal degeneration, thereby suggesting that demyelination mediated by lysosomes may be an important signal for inducing p75NGFR expression. Tumor necrosis factor-α (TNF-α) has been suggested to be involved in regulating p75NGFR expression in Schwann cells. In this study, we found that removing TNF-α in vivo did not significantly suppress the induction of both lysosomes and p75NGFR. Thus, these findings suggest that lysosomal activation is tightly correlated with the induction of p75NGFR in demyelinating Schwann cells during Wallerian degeneration.

Frataxin deficiency induces Schwann cell inflammation and death

Mutations in the frataxin gene cause dorsal root ganglion demyelination and neurodegeneration, which leads to Friedreich's ataxia. However the consequences of frataxin depletion have not been measured in dorsal root ganglia or Schwann cells. We knocked down frataxin in several neural cell lines, including two dorsal root ganglia neural lines, 2 neuronal lines, a human oligodendroglial line (HOG) and multiple Schwann cell lines and measured cell death and proliferation. Only Schwann cells demonstrated a significant decrease in viability. In addition to the death of Schwann cells, frataxin decreased proliferation in Schwann, oligodendroglia, and slightly in one neural cell line. Thus the most severe effects of frataxin deficiency were on Schwann cells, which enwrap dorsal root ganglia neurons. Microarray of frataxin-deficient Schwann cells demonstrated strong activations of inflammatory and cell death genes including interleukin-6 and Tumor Necrosis Factor which were confirmed at the mRNA and protein levels. Frataxin knockdown in Schwann cells also specifically induced inflammatory arachidonate metabolites. Anti-inflammatory and anti-apoptotic drugs significantly rescued frataxin-dependent Schwann cell toxicity. Thus, frataxin deficiency triggers inflammatory changes and death of Schwann cells that is inhibitable by inflammatory and anti-apoptotic drugs.

Receptor for advanced glycation end products (RAGEs) and experimental diabetic neuropathy

OBJECTIVE

Heightened expression of the receptor for advanced glycation end products (RAGE) contributes to development of systemic diabetic complications, but its contribution to diabetic neuropathy is uncertain. We studied experimental diabetic neuropathy and its relationship with RAGE expression using streptozotocin-induced diabetic mice including a RAGE(-/-) cohort exposed to long-term diabetes compared with littermates without diabetes.

RESEARCH DESIGN AND METHODS

Structural indexes of neuropathy were addressed with serial (1, 3, 5, and 9 months of experimental diabetes) electrophysiological and quantitative morphometric analysis of dorsal root ganglia (DRG), peripheral nerve, and epidermal innervation. RAGE protein and mRNA levels in DRG, peripheral nerve, and epidermal terminals were assessed in WT and RAGE(-/-) mice, with and without diabetes. The correlation of RAGE activation with nuclear factor (NF)-kappaB and protein kinase C beta II (PKC beta II) protein and mRNA expression was also determined.

RESULTS

Diabetic peripheral epidermal axons, sural axons, Schwann cells, and sensory neurons within ganglia developed dramatic and cumulative rises in RAGE mRNA and protein along with progressive electrophysiological and structural abnormalities. RAGE(-/-) mice had attenuated structural features of neuropathy after 5 months of diabetes. RAGE-mediated signaling pathway activation for NF-kappaB and PKC beta II pathways was most evident among Schwann cells in the DRG and peripheral nerve.

CONCLUSIONS

In a long-term model of experimental diabetes resembling human diabetic peripheral neuropathy, RAGE expression in the peripheral nervous system rises cumulatively and relates to progressive pathological changes. Mice lacking RAGE have attenuated features of neuropathy and limited activation of potentially detrimental signaling pathways.

Inflammatory mediators in diabetic and non-diabetic lumbosacral radiculoplexus neuropathy

Nerve microvasculitis and ischemic injury appear to be the primary and important pathogenic alterations in lumbosacral radiculoplexus neuropathy of patients with (DLRPN) and without (LRPN) diabetes mellitus (DM). Here, we examine the involvement of inflammatory mediators in DLRPN and LRPN. Paraffin sections of sural nerves from 19 patients with DLRPN, 13 patients with LRPN, and 20 disease control patients were immunostained for intercellular adhesion molecule-1 (ICAM-1), tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and nuclear factor kappaB (NF-kappaB). The findings were correlated with histopathology. The pathologic and immunohistochemical alterations of DLRPN and LRPN nerves were indistinguishable. The nerves of both types of LRPN had a significantly greater number of ICAM-1 positive vessels than did the controls (P < 0.01). TNF-alpha expression was seen in Schwann cells and some macrophages of DLRPN and LRPN nerves, whereas IL-6 expression was minimal. There was greater NF-kappaB immunoreactivity in vessels and endoneurial cells of DLRPN and LRPN nerves than of the controls (P < 0.001). NF-kappaB expression correlated with the number of empty nerve strands (P < 0.01) and the frequency of axonal degeneration (P < 0.05), whereas TNF-alpha expression correlated inversely with the number of empty nerve strands of teased fibers (P < 0.05). Our findings suggest that up-regulation of inflammatory mediators target different cells at different disease stages and that these mediators may be sequentially involved in an immune-mediated inflammatory process that is shared by both DLRPN and LRPN. Up-regulated inflammatory mediators may be immunotherapeutic targets in these two conditions.

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.

Orchestration of the inflammatory response in ischemia-reperfusion injury

Ischemia to nerve can cause fiber degeneration and reperfusion following ischemia [ischemia-reperfusion (IR)] adds the additional insult of an inflammatory response and oxidative injury. Limited information is available on the molecular mediators and their endoneurial targets. In this study, using a highly reproducible animal model of IR injury to nerve and selective immunolabeling methods [for nuclear factor kappa B (NF-kappaB), intercellular adhesion molecule-1 (ICAM-1), cytokines, and inflammatory cells] over an expanded time frame, we evaluated the temporal pattern and localization of mediators of the inflammatory response. Sixty rats were used. Nine groups (N=6 each) underwent complete hind limb ischemia for 4 h, followed by reperfusion durations of 0, 3, 12, 24, and 48 h, and 7, 14, 28, and 42 days. One group underwent sham operation (N=6). The earliest change was ICAM-1 expression in the microvessel (endothelial cell) followed almost immediately by NF-kappaB activation with axonal expression (24 and 48 h), followed by endoneurial edema and ischemic fiber degeneration (7 and 14 days). Granulocytic infiltration was followed by endoneurial infiltration of mononuclear phagocytes (14 days), expression of interleukin 6 (IL-6) (microvessels), and subsequent Schwann cell NF-kappaB expression. Granulocytes, tumor necrosis factor alpha, and IL-6-positive cells were observed primarily within the epineurium. IR results in changes in a number of interacting networks of targets and inflammatory mediators. NF-kappaB activation has a central orchestrating role involving both the axon and the Schwann cell in effecting the inflammatory response.

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.

Induction of neural-like differentiation in human mesenchymal stem cells derived from bone marrow, fat, spleen and thymus

Mesenchymal stem cells (MSCs) from bone marrow (BM) and sub-cutaneous fat are known to differentiate into neural cells under appropriate stimuli. We describe here the neural-like differentiation of human MSCs obtained from spleen and thymus, induced either with chemical factors or with co-culture with human Schwann cells (Sc). Under the effect of neural differentiation medium, most MSCs from BM, fat, spleen and thymus acquired morphological changes suggestive of cells of astrocytic/neuronal and oligodendroglial lineages with general up-regulation of neural molecules not correlated with morphological changes. The process was transient and reversible, as MSCs recovered basal morphology and phenotype, as well as their multilineage differentiation potential. Thus, we hypothesized that chemical factors may prime MSCs for neural differentiation, by inducing initial and poorly specific changes. By contrast, co-cultures of MSCs of different origin with Sc induced long-lasting and Sc differentiation, i.e., the expression of Sc myelin proteins for up to 12 days. Our results show that a MSC reservoir is present in tissues other than BM and fat, and that MSCs of different origin have similar neural differentiation potential. This evidence provides new insights into BM-like tissue plasticity and may have important implications for future therapeutic interventions in chronic neuropathies.

The role of neuroinflammation in neuropathic pain: mechanisms and therapeutic targets

Neuroinflammation is a proinflammatory cytokine-mediated process that can be provoked by systemic tissue injury but it is most often associated with direct injury to the nervous system. It involves neural-immune interactions that activate immune cells, glial cells and neurons and can lead to the debilitating pain state known as neuropathic pain. It occurs most commonly with injury to peripheral nerves and involves axonal injury with Wallerian degeneration mediated by hematogenous macrophages. Therapy is problematic but new trials with anti-cytokine agents, cytokine receptor antibodies, cytokine-signaling inhibitors, and glial and neuron stabilizers provide hope for future success in treating neuropathic pain.

TNFalpha mediates Schwann cell death by upregulating p75NTR expression without sustained activation of NFkappaB

Administration of tumour necrosis factor alpha (TNFalpha) to axotomised mouse neonatal sciatic nerves increased Schwann cell apoptosis in the distal nerve segments, 5-fold greater than axotomy alone. TNFalpha upregulated the low affinity neurotrophin receptor, p75NTR, indicative of phenotype reversion in Schwann cells. Furthermore, re-expression of p75NTR and downregulation of the pro-myelinating transcription factor, Oct 6, in Schwann cells occurred by treatment with TNFalpha, even after the maturation of these cells with brain derived neurotrophic factor (BDNF). TNFalpha treatment of Schwann cells produced only a transient activation of NFkappaB. More importantly, in NFkappaB (p65) mutant mice, axotomy increased Schwann cell apoptosis further than that seen in mice expressing NFkappaB (p65), implicating a survival role for NFkappaB. Collectively, these data suggest that TNFalpha can potentiate Schwann cell death through the modulation of their phenotype. Immature Schwann cells express a high level of p75NTR and as a consequence are susceptible to extracellular death stimuli because of the lack of sustained NFkappaB translocation.

Cytokines and Mycobacterium leprae induce apoptosis in human Schwann cells

The development of deformities during the course of leprosy disease is a major public health concern worldwide. It is possible that cytokine production and apoptosis of Schwann cells (SCs) directly affect nerve degeneration and regeneration leading to injury of the myelin sheath and axon. In the present study, the expression of TNFalpha, TGFbeta, and their receptors, in addition to cell death triggered by cytokines or whole Mycobacterium leprae were investigated in a human SC line. The results showed the presence of TNF-Rs and TGF-RII on the SC membrane and the shedding of TNF-Rs during the culture period. Evaluation of cell death was performed through TUNEL and flow cytometry techniques. TNFalpha/TGFbeta combination as well as M. leprae infection triggered an increase in the apoptosis rate in the cultured SC. Moreover, reverse transcriptase-polymerase chain reaction assay revealed that M. leprae upregulated the expression of such cytokines and their receptors on the SC line. Despite the detection of TNFalpha mRNA, no protein was found in the culture supernatants. The data indicate that induction of SC death after cell interaction with M. leprae may, in fact, be implicated in the pathogenesis of nerve damage, which can most likely be modulated by in vivo cytokine production.

Tumor necrosis factor receptor 1 and 2 proteins are differentially regulated during Wallerian degeneration of mouse sciatic nerve

The pro-inflammatory cytokine tumor necrosis factor-alpha (TNF) is involved in injury-induced peripheral nerve pathology and in the generation of neuropathic pain. Here, we investigated local protein levels of the two known TNF receptors, TNF receptor 1 and 2 (TNFR1, TNFR2), on days 0, 1, 3, 7, 14, and 28 after unilateral crush or chronic constriction injury (CCI) of mouse sciatic nerves using enzyme-linked immunoassay. Both receptors were detectable at a low level in nerve homogenates from naive mice. After crush or CCI, TNFR1 increased by 2-fold on days 3 and day 7. Unlike TNFR1, TNFR2 was markedly upregulated already on day 1 after crush or CCI. TNFR2 increased by 7-fold on days 3 and 7, and remained elevated at a lower level until day 28 after both CCI and crush injury. These data indicate that endoneurial TNFR1 and TNFR2 proteins are differentially regulated during Wallerian degeneration.

Molecular mechanisms in Schwann cell survival and death during peripheral nerve development, injury and disease

The mechanisms determining the fate of Schwann cells during disease and injury of the adult mammalian peripheral nervous system (PNS) are becoming defined by current advances in molecular neurobiology. It is now apparent that the molecular pathways which regulate the production of the mature myelinating Schwann cell during development may also apply to degenerative and regenerative mechanisms following PNS disease. This review outlines neurobiological responses of Schwann cells during development, injury and disease in order to define the molecular pathways which regulate these crucial events. These mechanisms have implications for our attempts to intervene pharmacologically during pathologies of the PNS.

Gangliosides act as onconeural antigens in paraneoplastic neuropathies

We describe two patients with progressive neuropathy and lung cancer in whom gangliosides (GS) may represent the oncoantigens. Patient 1 had motor neuropathy, high titers of IgG1 and IgG3 to GD1a and GM1, and expansion of circulating gamma-delta T lymphocytes, a T-cell subset responding to glycolipids. Patient 2 presented with Miller-Fisher-like syndrome and IgG3 activity to disialo-GS. In both cases, decreased autoimmune responses and stabilization of neuropathy were accomplished by tumor treatment. By immunohistochemistry, patient 1's IgG bound to his own tumor and to structures of normal nervous system expressing GD1a or GM1. Infiltration of IgG in the same neural structures was found at his autopsy. Regarding cellular immunity, the proportion of gamma-delta T lymphocytes infiltrating carcinoma from patient 1 was significantly higher than in neoplastic controls. These results indicate that GS may represent onconeural antigens in paraneoplastic neuropathy (PNN); their expression on neoplastic tissue may elicit autoimmune responses, which also target neural structures.

Cyclic AMP and tumor necrosis factor-alpha regulate CXCR4 gene expression in Schwann cells

Rat peripheral nerve Schwann cells have been shown to express the alpha-chemokine receptor CXCR4 as well as the corresponding ligand stromal cell-derived factor-1 (SDF-1). We have investigated gene regulatory mechanisms acting on the expression of CXCR4 in cultured rat Schwann cells and found that receptor expression at transcript- and protein levels is directly dependent on intracellular cyclic AMP. Such increased levels of CXCR4 expression were found to be efficiently reversed by the action of tumor necrosis factor-alpha (TNFalpha). We also provide evidence that the POU box transcription factor Oct-6/SCIP is involved in the control of CXCR4 transcription. Finally, we could demonstrate that CXCR4 activation by SDF-1alpha increases the number of dying Schwann cells, indicating that this receptor/ligand interaction is modulating cell survival. Our data, therefore, suggest that in the Schwann cell lineage signal transduction cascades controlled by the activation of TNF- and CXCR4 receptors are functionally coupled.

Inhibition of p38 MAP kinase activity enhances axonal regeneration

Tumor necrosis factor alpha (TNF)-induced cellular signaling through the p38 mitogen-activated protein kinase (p38 MAPK) pathway plays a critical role in Wallerian degeneration and subsequent regeneration, processes that depend on Schwann cell (SC) activity. TNF dose-dependently induces Schwann cell and macrophage activation in vivo and apoptosis in primary SC cultures in vitro, while inhibition of p38 MAPK is thought to block these cellular processes. We show with Western blots that after sciatic nerve crush injury, phosphorylated p38 (p-p38) MAPK is significantly increased (P < 0.01) in distal nerve segments. In tissue sections, p38 co-localized immunohistochemically with activated Schwann cells (GFAP) and to a lesser degree with macrophages (ED-1). In other experiments, animals were gavaged with Scios SD-169 (10 or 30 mg/kg) or excipient (PEG300) 1 day before and daily after crush injury to the sciatic nerve. SD-169 is a proprietary oral inhibitor of p38 MAPK activity. The rate of axonal regeneration was determined by the functional pinch test and was significantly increased in treated animals 8 days after crush injury (P < 0.05; 30 mg/kg dose). In SD-169-treated animals with nerve transection, nerve fibers regenerating through a silicone chamber were morphologically more mature than untreated nerves when observed 28 days after transection. TNF immunofluorescence of distal nerve segments after crush injury suggested that SD-169 reduced SC TNF protein. In support of these findings, SD-169 significantly reduced (P < 0.05) TNF-mediated primary SC death in culture experiments. We conclude that inhibition of p38 activity promotes axonal regeneration through interactions with SC signaling and TNF activity.

T-cell cytotoxicity of human Schwann cells: TNFalpha promotes fasL-mediated apoptosis and IFN gamma perforin-mediated lysis

The ability of resident cells to induce apoptosis of invading immune cells is a major regulatory factor operating in immune-privileged tissues, including the nervous system. We investigated the cellular and molecular factors participating in modulation of immune response in peripheral nerves, focusing on two cytotoxic pathways: fas ligand (fasL) and perforin. fasL and perforin expression was found by immunochemistry on Schwann cells (Sc) in nerve biopsies from patients with chronic inflammatory demyelinating polyneuritis and on human Sc cultures. Treatment of Sc with tumor necrosis factor (TNF) alpha and interferon (IFN) gamma upregulated the expression of both molecules. In a coculture model, Sc exposed to TNFalpha or IFN gamma were able to induce both apoptotic and lytic injury of T-lymphocytes. Inactivation of fasL with the neutralizing antibody NOK-2 abolished T-cell apoptosis induced by Sc treated with TNFalpha, but not by Sc treated with IFN gamma. Conversely, T-cell lysis was significantly decreased when IFN gamma-activated Sc were treated with concanamycin A, which inhibited perforin release. At variance with T-lymphocytes, B-cells were less sensitive to cytokine-treated Sc toxicity. Thus, Sc exposed to inflammatory cytokines have the capacity of inducing selective damage of T-lymphocytes and have the potential of regulating the immune response in the peripheral nervous system.

Transcription factors c-Jun/activator protein-1 and nuclear factor-kappa B in oxidative stress response in mitochondrial diseases

Mitochondrial dysfunction leads to oxygen free radical (ROS) generation with consequent oxidative stress and cellular damage. Recently, activation of the cellular antioxidant system and apoptosis were demonstrated in skeletal muscle fibres from patients with mitochondrial diseases, but the underlying mechanisms remain unknown. Hydrogen peroxide, a by-product of ROS generation, is a chemical inducer of gene expression able to activate apoptosis and to promote the antioxidant response through the activation of nuclear factor-kappa B (NF-kappaB) and activator protein-1 (AP-1) transcription factor. Using immunohistochemistry and confocal microscopy, we evaluated the expression of NF-kappaB and AP-1 in muscle biopsies from patients with mitochondrial disease. In addition, we examined the expression of factors involved in their activation, such as NF-kappaB inducing kinase (NIK) and phosphorylated Jun-N-terminal kinase (p-JNK). Most fibres with respiratory chain dysfunction displayed nuclear staining for activated c-Jun/AP-1, but not for NF-kappaB. The same fibres reacted for p-JNK. Only some ragged red fibres immunoreacted for NIK. These data suggest that AP-1 is involved in the oxidative stress response in muscle fibres from patients with mitochondrial disease.

Enhanced expression of NGF receptors in multiple sclerosis lesions

The receptor for nerve growth factor (NGF) comprises a 75-kDa (NGFRp75) and a tyrosine kinase A (TrkA) subunit. In view of conflicting opinions on the identity of glial targets of NGF in human central nervous system (CNS), we examined the cellular distribution of both NGF receptor subunits in normal CNS and in chronic multiple sclerosis (MS) lesions. For this, we compared the pattern of recognition of 2 monoclonal antibodies (mAbs) and a polyclonal antiserum to NGFRp75. Only the 2 mAbs specifically recognized NGFRp75, while the polyclonal antiserum showed widespread reactivity. In normal CNS and silent MS lesions, immunohistochemistry with anti-NGFRp75 mAbs and for TrkA revealed perivascular cell reactivity. At the edge of chronic active MS lesions, selective NGFRp75 staining was prominent on reactive astrocytes, while throughout the lesion, NGFRp75 was expressed on microglia/macrophages. The vast majority of mature or precursor oligodendrocytes did not express NGFRp75. Both NGF receptors were co-expressed on a subset of inflammatory cells. Immunoreactivity for NGFRp75 on glial and immune cells did not correlate with the distribution of apoptotic figures, as detected by TUNEL. Thus, expression of NGF receptors in active MS lesions suggests a role for NGF in regulating the autoimmune response at both immune and glial cell levels.

Glial cells as targets for cytotoxic immune mediators

Oligodendrocytes and Schwann cells are the glia principally responsible for the synthesis and maintenance of myelin. Damage may occur to these cells in a number of conditions, but perhaps the most studied are the idiopathic inflammatory demyelinating diseases, multiple sclerosis in the CNS, and Guillain-Barré syndrome and its variants in the peripheral nervous system (PNS). This article explores the effects on these cells of cytotoxic immunological and inflammatory mediators: similarities are revealed, of which perhaps the most important is the sensitivity of both Schwann cells and oligodendrocytes to many such agents. This area of research is, however, characterised and complicated by numerous and often very substantial inter-observer discrepancies. Marked variability in cell culture techniques, and in assays of cell damage and death, provide artifactual explanations for some of this variability; true inter-species differences also contribute. Not the least important conclusion centres on the limited capacity of in vitro studies to reveal disease mechanisms: cell culture findings merely illustrate possibilities which must then be tested ex vivo using human tissue samples affected by the relevant disease.

Low-affinity nerve growth factor receptor (p75) in dermatofibrosarcoma protuberans and other nonneural tumors: a study of 1,150 tumors and fetal and adult normal tissues

Low-affinity nerve growth factor receptor (p75) is a member of the tumor necrosis factor receptor family. It may modulate the binding of nerve growth factor (NGF) to the functional high-affinity receptor tyrosine kinase (trk) A. NGF is thought to be responsible for growth, apoptosis, and function of the nervous system. The presence of this receptor (p75) was determined in a large group of neural and nonneural tumors and fetal and adult tissues. One thousand one hundred fifty tumors were analyzed with monoclonal antibody for p75, along with selected normal fetal and adult tissues. Immunoreactivity for p75 was present in adult pericytes, perivascular fibroblasts, basal cells of several types of epithelia, perineurial cells, and dendritic reticulum cells. Additionally, a wide zone of subepithelial mesenchyme and skeletal muscle were positive in the first-trimester fetus, but were diminished or negative in the adult. Consistently positive nonneural mesenchymal tumors included dermatofibrosarcoma protuberans (DFSP), embryonal and alveolar rhabdomyosarcoma, synovial sarcoma, and spindle cell hemangio(endotheli)oma. Schwann cell tumors, ganglioneuroma, granular cell tumor, and malignant peripheral nerve sheath tumor (MPNST) were also p75 positive. Mesenchymal nonneural tumors that were variably positive (32% to 69%) for p75 included fibrosarcoma variants, solitary fibrous tumor, hemangiopericytoma, spindle cell lipoma, Ewing's sarcoma, mesenchymal chondrosarcoma, and malignant melanoma. Nervous system tumors such as paragangliomas, neuroblastoma, meningioma, and perineurioma and nonneural mesenchymal tumors, including extraskeletal osteosarcoma, benign fibrous histiocytomas, fibromas, alveolar soft part sarcoma, epithelioid sarcoma, smooth muscle and gastrointestinal stromal tumors, and angiosarcomas, were almost always negative for p75. Epithelial tumors that were consistently positive included mixed tumor and adenoid cystic carcinoma, whereas mesothelioma, adenocarcinomas, and most squamous cell carcinomas were negative. p75 is not a specific marker for nerve sheath tumors. It is present in a variety of other mesenchymal tumors including synovial sarcoma and in CD34-positive tumors such as DFSP, spindle cell lipoma, and hemangiopericytoma. The presence of p75 in nonneural tumors such as DFSP and rhabdomyosarcoma mimic its presence in early fetal mesenchyme and skeletal muscle, suggesting oncofetal expression in these tumors. p75 may be useful to distinguish DFSP from benign fibrous histiocytoma.