Dibutyryl cyclic AMP and inflammatory cytokines mediate C3 expression in schwann cells

Immune-Mediated Neuropathies: Pathophysiology and Management

Dysfunction of the immune system can result in damage of the peripheral nervous system. The immunological mechanisms, which include macrophage infiltration, inflammation and proliferation of Schwann cells, result in variable degrees of demyelination and axonal degeneration. Aetiology is diverse and, in some cases, may be precipitated by infection. Various animal models have contributed and helped to elucidate the pathophysiological mechanisms in acute and chronic inflammatory polyradiculoneuropathies (Guillain-Barre Syndrome and chronic inflammatory demyelinating polyradiculoneuropathy, respectively). The presence of specific anti-glycoconjugate antibodies indicates an underlying process of molecular mimicry and sometimes assists in the classification of these disorders, which often merely supports the clinical diagnosis. Now, the electrophysiological presence of conduction blocks is another important factor in characterizing another subgroup of treatable motor neuropathies (multifocal motor neuropathy with conduction block), which is distinct from Lewis-Sumner syndrome (multifocal acquired demyelinating sensory and motor neuropathy) in its response to treatment modalities as well as electrophysiological features. Furthermore, paraneoplastic neuropathies are also immune-mediated and are the result of an immune reaction to tumour cells that express onconeural antigens and mimic molecules expressed on the surface of neurons. The detection of specific paraneoplastic antibodies often assists the clinician in the investigation of an underlying, sometimes specific, malignancy. This review aims to discuss the immunological and pathophysiological mechanisms that are thought to be crucial in the aetiology of dysimmune neuropathies as well as their individual electrophysiological characteristics, their laboratory features and existing treatment options. Here, we aim to present a balance of discussion from these diverse angles that may be helpful in categorizing disease and establishing prognosis.

Phagocytosis by Peripheral Glia: Importance for Nervous System Functions and Implications in Injury and Disease

The central nervous system (CNS) has very limited capacity to regenerate after traumatic injury or disease. In contrast, the peripheral nervous system (PNS) has far greater capacity for regeneration. This difference can be partly attributed to variances in glial-mediated functions, such as axon guidance, structural support, secretion of growth factors and phagocytic activity. Due to their growth-promoting characteristic, transplantation of PNS glia has been trialed for neural repair. After peripheral nerve injuries, Schwann cells (SCs, the main PNS glia) phagocytose myelin debris and attract macrophages to the injury site to aid in debris clearance. One peripheral nerve, the olfactory nerve, is unique in that it continuously regenerates throughout life. The olfactory nerve glia, olfactory ensheathing cells (OECs), are the primary phagocytes within this nerve, continuously clearing axonal debris arising from the normal regeneration of the nerve and after injury. In contrast to SCs, OECs do not appear to attract macrophages. SCs and OECs also respond to and phagocytose bacteria, a function likely critical for tackling microbial invasion of the CNS via peripheral nerves. However, phagocytosis is not always effective; inflammation, aging and/or genetic factors may contribute to compromised phagocytic activity. Here, we highlight the diverse roles of SCs and OECs with the focus on their phagocytic activity under physiological and pathological conditions. We also explore why understanding the contribution of peripheral glia phagocytosis may provide us with translational strategies for achieving axonal regeneration of the injured nervous system and potentially for the treatment of certain neurological diseases.

The role of primary infection of Schwann cells in the aetiology of infective inflammatory neuropathies

Numerous different pathogens are responsible for infective peripheral neuropathies and this is generally the result of the indirect effects of pathogen infection, namely anti pathogen antibodies cross reacting with epitopes on peripheral nerve, auto reactive T cells attacking myelin, circulating immune complexes and complement fixation. Primary infection of Schwann cells (SC) associated with peripheral nerve inflammation is rare requiring pathogens to cross the Blood Peripheral Nerve Barrier (BPNB) evade anti-pathogen innate immune pathways and invade the SC. Spirochetes Borrelia bourgdorferi and Trepomema pallidum are highly invasive, express surface lipo proteins, but despite this SC are rarely infected. However, Trypanosoma cruzi (Chaga's disease) and Mycobacterium leprae. Leprosy are two important causes of peripheral nerve infection and both demonstrate primary infection of SC. This is due to two novel strategies; T. cruzi express a trans-silalidase that mimics host neurotrophic factors and infects SC via tyrosine kinase receptors. M. leprae demonstrates multi receptor SC tropism and subsequent infection promotes nuclear reprogramming and dedifferentiation of host SC into progenitor stem like cells (pSLC) that are vulnerable to M. leprae infection. These two novel pathogen evasion strategies, involving stem cells and receptor mimicry, provide potential therapeutic targets relevant to the prevention of peripheral nerve inflammation by inhibiting primary SC infection.

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.

Innate immunity in the nervous system

The complement (C) system plays a central role in innate immunity and bridges innate and adaptive immune responses. A fine balance of C activation and regulation mediates the elimination of invading pathogens and the protection of the host from excessive C deposition on healthy tissues. If this delicate balance is disrupted, the C system may cause injury and contribute to the pathogenesis of various diseases, including neurodegenerative disorders and neuropathies. Here we review evidence indicating that C factors and regulators are locally synthesized in the nervous system and we discuss the evidence supporting the protective or detrimental role of C activation in health, injury, and disease of the nerve.

Mechanisms of immune regulation in the peripheral nervous system

The peripheral nervous system (PNS) is a target for heterogenous immune attacks mediated by different components of the systemic immune compartment. T cells, B cells, and macrophages can interact with endogenous, partially immune-competent glial cells and contribute to local inflammation. Cellular and humoral immune functions of Schwann cells have been well characterized in vitro. In addition, the interaction of the humoral and cellular immune system with the cellular and extracellular components in the PNS may determine the extent of tissue inflammation and repair processes such as remyelination and neuronal outgrowth. The animal model experimental autoimmune neuritis (EAN) allows direct monitoring of these immune responses in vivo. In EAN contributions to regulate autoimmunity in the PNS are made by adhesion molecules and by cytokines that orchestrate cellular interactions. The PNS has a significant potential to eliminate T cell inflammation via apoptosis, which is almost lacking in other tissues such as muscle and skin. In vitro experiments suggest different scenarios how specific cellular and humoral elements in the PNS may sensitize autoreactive T cells for apoptosis in vivo. Interestingly several conventional and novel immunotherapeutic approaches like glucocorticosteroids and high-dose antigen therapy induce T cell apoptosis in situ in EAN. A better understanding of immune regulation and its failure in the PNS may help to develop improved, more specific immunotherapies.

JNK1 activation mediates C5b-9-induced P0 mRNA instability and P0 gene expression in Schwann cells

The protein zero (P0) glycoprotein is an important component of compact peripheral nerve myelin produced by the glial cells of the mammalian peripheral nervous system. P0 mRNA expression is reduced following exposure of Schwann cells to sublytic C5b-9, the terminal activation complex of the complement cascade. Sublytic complement treatment decreased P0 mRNA by 81% within 6 h and required C5b-9 assembly. C5b-9 induced a threefold increase in both JNK1 activity and c-jun mRNA within 20 and 30 min, respectively, compared with cells treated with either human serum depleted of complement component C7 (C7dHS) or medium alone. Sublytic C5b-9 stimulation, in the presence of the transcription inhibitor Actinomycin D, decreased P0 mRNA expression by 52%, indicating that mRNA was selectively destabilized. This effect was prevented by pretreatment with L-JNK inhibitor 1 (L-JNKI1). To study a potential inhibition of P0 gene transcription, we transfected Schwann cells with a P0 promoter-firefly luciferase construct. Sublytic C5b-9 stimulation of the transfected cells decreased luciferase activity by 82% at 6 h, and this effect was prevented by pretreatment with L-JNKI1 inhibitor. Our results indicate that the ability of C5b-9 in vitro to affect P0 gene expression is mediated via JNK1 activation that leads to enhanced mRNA decay and transcriptional repression of P0.

Distinct disease mechanisms in peripheral neuropathies due to altered peripheral myelin protein 22 gene dosage or a Pmp22 point mutation

Point mutations affecting PMP22 can cause hereditary demyelinating and dysmyelinating peripheral neuropathies. In addition, duplication and deletion of PMP22 are associated with Charcot-Marie-Tooth disease Type 1A (CMT1A) and Hereditary Neuropathy with Liability to Pressure Palsy (HNPP), respectively. This study was designed to elucidate disease processes caused by misexpression of Pmp22 and, at the same time, to gain further information on the controversial molecular function of PMP22. To this end, we took advantage of the unique resource of a set of various Pmp22 mutant mice to carry out comparative expression profiling of mutant and wild-type sciatic nerves. Tissues derived from Pmp22-/- ("knockout"), Pmp22tg (increased Pmp22 copy number), and Trembler (Tr; point mutation in Pmp22) mutant mice were analyzed at two developmental stages: (i) at postnatal day (P)4, when normal myelination has just started and primary causative defects of the mutations are expected to be apparent, and (ii) at P60, with the goal of obtaining information on secondary disease effects. Interestingly, the three Pmp22 mutants exhibited distinct profiles of gene expression, suggesting different disease mechanisms. Increased expression of genes involved in cell cycle regulation and DNA replication is characteristic and specific for the early stage in Pmp22-/- mice, supporting a primary function of PMP22 in the regulation of Schwann cell proliferation. In the Tr mutant, a distinguishing feature is the high expression of stress response genes. Both Tr and Pmp22tg mice show strongly reduced expression of genes important for cholesterol synthesis at P4, a characteristic that is common to all three mutants at P60. Finally, we have identified a number of candidate genes that may play important roles in the disease process or in myelination per se.

Complement synthesis and activation in the brain of SIV-infected monkeys

Complement is one of the most critical defence tools against cerebral infections, but uncontrolled complement biosynthesis and activation can induce profound brain tissue damage. To clarify the role of complement in the pathogenesis of AIDS-associated neurological disorders, we analysed the synthesis of complement in the brains of SIV-infected rhesus macaques. Using immunohistochemical staining we could show that the cerebral synthesis of complement factors C1q and C3 was strongly upregulated in SIV-infected monkeys compared to the spontaneous synthesis in uninfected control monkeys. Astrocytes, neurons, microglia, infiltrating macrophages and multinuclear giant cells all contribute to the high amounts of C1q and C3 in the brain. Secreted C1q and C3 are also deposited on the membrane of neurons, a prerequisite for formation of the membrane-driven lytic membrane attack complex. The membrane deposition thus might suggest complement-induced lysis of bystander neurons as a potential mechanism for cell damage during viral infection of the brain.

Complement factors in adult peripheral nerve: a potential role in energy metabolism

Complement cascade factors are known to play a critical role in myelin clearance after peripheral nerve injury. Here we show that components of both the classical (C1qa, C1qb, C1qc, C2 and C4) and alternative (C3, B and adipsin) pathways are expressed by uninjured peripheral nerve as well. mRNAs of components of the alternative pathway were predominantly found in the peri/epineurium, although factor C3 and factor B were also detected in the endoneurial compartment of adult nerve. Interestingly, adipsin mRNA was detected only in peri/epineurium, while adipsin protein was present in both peri/epineurium and endoneurium. This suggests that adipsin is transported to the endoneurium via the circulation from the peri/epineurium or outside of the nerve. Factor 5 and factor 9, necessary for the formation of the membrane-attack complex, were not detected in any part of the healthy peripheral nerve, which together with the observed presence of negative regulators of complement activation, is likely to prevent damage to the healthy nerve caused by complement activation. By analogy with the known role of complement factors in fat, we propose that local expression of these factors plays a role in the regulation of fatty acid homeostasis in the nerve and, thereby, in energy metabolism cross-talk between different compartments of the peripheral nerve.

Mechanism of human immunodeficiency virus-induced complement expression in astrocytes and neurons

The cerebral complement system is hypothesized to contribute to neurodegeneration in the pathogenesis of AIDS-associated neurological disorders. Our former results have shown that the human immunodeficiency virus (HIV) strongly induces the synthesis of complement factor C3 in astrocytes. This upregulation explains in vivo data showing elevated complement levels in the cerebrospinal fluid of patients with AIDS-associated neurological symptoms. Since inhibition of complement synthesis and activation in the brain may represent a putative therapeutic goal to prevent virus-induced damage, we analyzed in detail the mechanisms of HIV-induced modulation of C3 expression. HIV-1 increased the C3 levels in astrocyte culture supernatants from 30 to up to 400 ng/ml; signal transduction studies revealed that adenylate cyclase activation with upregulation of cyclic AMP is the central signaling pathway to mediate that increase. Furthermore, activity of protein kinase C is necessary for HIV induction of C3, since inhibition of protein kinase C by prolonged exposure to the phorbol ester tetradecanoyl phorbol acetate partly abolished the HIV effect. The cytokines tumor necrosis factor alpha and gamma interferon were not involved in mediating the HIV-induced C3 upregulation, since neutralizing antibodies had no effect. Besides whole HIV virions, the purified viral proteins Nef and gp41 are biologically active in upregulating C3, whereas Tat, gp120, and gp160 were not able to modulate C3 synthesis. Further experiments revealed that neurons were also able to respond on incubation with HIV with increased C3 synthesis, although the precise pattern was slightly different from that in astrocytes. This strengthens the hypothesis that HIV-induced complement synthesis represents an important mechanism for the pathogenesis of AIDS in the brain.

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.

Sublytic C5b-9-stimulated Schwann cell survival through PI 3-kinase-mediated phosphorylation of BAD

Sublytic C5b-9 induces cell cycle activation, proliferation, and rescue from apoptosis in Schwann cells. The signaling pathways for C5b-9-mediated rescue were investigated. Following serum withdrawal, DNA fragmentation, detected by TUNEL and FACS analysis, was 56.7% +/- 7.3 and 91.9% +/- 2.4 in cultured sciatic nerve Schwann cells from 6-day-old rats after 18 h and 24 h, respectively. Apoptosis was confirmed by inhibition of DNA fragmentation in a dose-dependent manner by DMQD-CHO, a caspase-3 inhibitor. Treatment with sublytic C5b-9 generated with purified components (C5*9) or Ab+C7-depleted serum (C7dHS)+C7 rescued 89% and 86% of Schwann cells, respectively, as compared with cells treated with C5*6, C8, C9, or Ab+C7dHS. Sublytic C5b-9 increased Schwann cell PI-3 kinase and Akt activity maximally at 5 min 3.14 +/- 0.5-fold and 3.56 +/- 0.4-fold, respectively, over controls. ERK-1 activity was maximally stimulated 2.98-fold at 15 min. Inhibition of PI-3 kinase by LY294002 abrogated the C5b-9-mediated Schwann cell rescue from apoptosis, while inhibition of ERK-1 with PD098,059 did not. PI-3 kinase-Akt pathway activation by C5b-9 induced, within 15 min, a 6.34 +/- 1.2-fold increase in BAD phosphorylation at Ser 136, but not at Ser 112. Downstream Bcl-x(L) protein was increased 2.61-fold +/- 0.34-fold by 18 h and 3.9-fold +/- 0.84-fold by 24 h over controls. LY294002 prevented both BAD phosphorylation at Ser 136 and Bcl-x(L) protein induction, while PD098,059 did not. Our data indicated that sublytic C5b-9 rescued Schwann cell from apoptosis via activation of PI-3 kinase-Akt, BAD phosphorylation on Ser 136 and increased expression of Bcl-x(L). Sublytic C5b-9 detected on Schwann cell in vivo during inflammatory neuropathy may facilitate survival of Schwann cell capable of remyelination.

Complement activation in acquired and hereditary amyloid neuropathy

The pathogenesis of the axonal degeneration in acquired or hereditary amyloidosis is unknown. In this immunohistochemistry study, we examined 20 sural nerve biopsies from individuals with amyloid neuropathy (14 acquired and 6 hereditary) for evidence of complement activation. Complement activation products were detected on and around amyloid deposits within peripheral nerves. We found no difference in the extent, location or pattern of complement activation products between the 2 forms of amyloidosis. The presence of early classical pathway activation markers in the absence of antibody in hereditary cases suggests an antibody-independent activation of the classical pathway through binding of C1q. The lack of Factor Bb-suggested alternative pathway activation was not significant in these cases. The detection of C5b-9 neoantigen on amyloid deposits demonstrated that the full complement cascade was activated. Complement activation on amyloid deposits and the generation of C5b-9 in vivo may contribute to bystander injury of axons in the vicinity of amyloid deposits.

Free radicals upregulate complement expression in rabbit isolated heart

Both free radicals and complement activation can injure tissue. Our study determined whether free radicals alter complement production by the myocardium. Isolated hearts from New Zealand White rabbits were perfused on a Langendorff apparatus and exposed to xanthine (X; 100 microM) plus xanthine oxidase (XO; 8 mU/ml) (X/XO). The free radical-generating system significantly (P < 0.05) increased C1q and also increased C1r, C3, C8, and C9 transcription compared with controls. Immunohistological examination revealed augmented membrane attack complex deposition on X/XO-treated tissue. X/XO-treated hearts also exhibited significant (P < 0.05) increases in coronary perfusion pressure and left ventricular end-diastolic pressure and a decrease in left-ventricular developed pressure. N-(2-mercaptopropionyl)-glycine (3 mM), in conjunction with the superoxide dismutase mimetic SC-52608 (100 microM), significantly (P < 0.05) reduced the upregulation of C1q, C1r, C3, C8, and C9 mRNA expression elicited by X/XO. The antioxidants also ameliorated the deterioration in function caused by X/XO. Local complement activation may represent a mechanism by which free radicals mediate tissue injury.

Preconditioning reduces tissue complement gene expression in the rabbit isolated heart

Both preconditioning and inhibition of complement activation have been shown to ameliorate myocardial ischemia-reperfusion injury. The recent demonstration that myocardial tissue expresses complement components led us to investigate whether preconditioning affects complement expression in the isolated heart. Hearts from New Zealand White rabbits were exposed to either two rounds of 5 min global ischemia followed by 10 min reperfusion (ischemic preconditioning) or 10 microM of the ATP-dependent K+ (KATP) channel opener pinacidil for 30 min (chemical preconditioning) before induction of 30 min global ischemia followed by 60 min of reperfusion. Both ischemic and chemical preconditioning significantly (P < 0.05) reduced myocardial C1q, C1r, C3, C8, and C9 mRNA levels. Western blot and immunohistochemistry demonstrated a similar reduction in C3 and membrane attack complex protein expression. The K(ATP) channel blocker glyburide (10 microM) reversed the depression of C1q, C1r, C3, C8, and C9 mRNA expression observed in the pinacidil-treated hearts. The results suggest that reduction of local tissue complement production may be one means by which preconditioning protects the ischemic myocardium.