J Neural Transm DOI 10.1007/s00702-014-1188-0

NEUROLOGY AND PRECLINICAL NEUROLOGICAL STUDIES - REVIEW ARTICLE

The role of glutamate and its receptors in multiple sclerosis Ivana R. Stojanovic • Milos Kostic Srdjan Ljubisavljevic

•

Received: 31 October 2013 / Accepted: 27 February 2014 Ó Springer-Verlag Wien 2014

Abstract Glutamate is an excitatory neurotransmitter of the central nervous system, which has a central role in a complex communication network established between neurons, astrocytes, oligodendrocytes, and microglia. Multiple abnormal triggers such as energy deficiency, oxidative stress, mitochondrial dysfunction, and calcium overload can lead to abnormalities in glutamate signaling. Thus, the disturbance of glutamate homeostasis could affect practically all physiological functions and interactions of brain cells, leading to excitotoxicity. Excitotoxicity is the pathological process by which nerve cells are damaged or killed by excessive stimulation by glutamate. Although neuron degeneration and death are the ultimate consequences of multiple sclerosis (MS), it is now widely accepted that alterations in the function of surrounding glial cells are key features in the progression of the disease. The present knowledge raise the possibility that the modulation of glutamate release and transport, as well as receptors blockade or glutamate metabolism modulation, might be relevant targets for the development of future therapeutic interventions in MS. Keywords Multiple sclerosis
Glutamate
Excitotoxicity
Neurodegeneration Introduction Multiple sclerosis (MS) is an inflammatory, demyelinating, and neurodegenerative disease of the central nervous system (CNS) (van Horssen et al. 2012; Gray et al. 2013; I. R. Stojanovic (&)
M. Kostic
S. Ljubisavljevic Faculty of Medicine, University of Nis, Nis, Serbia and Montenegro e-mail: [email protected]

Sinnecker et al. 2012). In the early phase of the disease, inflammatory lymphocyte, macrophage, and activated microglia infiltrates, followed by an excessive inflammatory mediators’ production, lead to demyelination and axonal conduction block. The hallmarks of the advanced stage of the disease are diffuse degeneration and damage of neurons. The disease affects young individuals, more female than male. The general characteristics of the disease include immunoregulatory deficit, blood–brain barrier damage, and the consequent CNS parenchyma inflammatory cell invasion, neurological disabilities, demyelination, and MS plaque formation. Lesions, pathognomonic for multiple sclerosis, are demyelination plaques in which infiltration of immune cells, demyelination, oligodendrocyte death, and axonal degeneration have been observed. A variety in MS lesions suggests multiple mechanisms in the pathogenesis of this disease (Lucchinetti et al. 2000). The inflammatory cytokines and glutamate neurotoxicity have been proposed as major determinants accompanying the demyelination and axonal degeneration observed during the course of MS. Glutamate excitotoxicity has been emerged as a potential mechanism involved in the pathogenesis of MS. In fact, glutamate levels increase in the cerebrospinal fluid (CSF) (Sarchielli et al. 2003) and in the brains of MS patients (Cianfoni et al. 2007) and a-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), and kainate receptors are upregulated (Newcombe et al. 2008). Using the animal model of MS, known as experimental autoimmune encephalomyelitis (EAE), Sulkowski et al. (2013) demonstrated that pharmacological inhibition of ionotropic NMDA glutamate receptors (GluRs) by their antagonists (amantadine and memantine) suppressed neurological symptoms of the disease in EAE rats and reduced the expression of pro-inflammatory

123

I. R. Stojanovic et al.

cytokines in the brain. Conversely, antagonists of group I metabotropic glutamate receptors, mGluRs (LY 367385 and MPEP), did not affect the inflammatory process and the neurological condition of EAE rats. The beneficial effects in EAE and in MS are not limited only to oligodendrocytes, but also to neuronal damage (Centonze et al. 2009). Although excessive activation of GluRs triggers uncontrolled intracellular signaling cascades involved in neuronal toxicity, a family of astrocyte high-affinity glutamate transporters efficiently controls glutamate concentration in the synaptic cleft. Thus, excitotoxicity is also associated with a significant impairment of glutamate uptake. There is evidence of altered expression of glutamate transporters in MS (Vallejo-Illarramendi et al. 2006) and in animal models of the disease (Ohgoh et al. 2002). Alterations of the mechanisms of glutamate reuptake and the loss of glutamate transporters are found in MS lesions in the presence of activated microglia and synaptic damage suggestive of excitotoxicity (Vercellino et al. 2007). Glutamate Glutamate (Glu), a nonessential amino acid, is the major excitatory neurotransmitter in the central and peripheral nervous systems (CNS and PNS). As an excitatory neurotransmitter, a product of glutamine deamination, a critical step in nitrogen metabolism, and energy source, this amino acid has been well studied for its role in cellular homeostasis. The large body of evidence demonstrated the importance of glutamatergic signaling in long-term potentiation known to be fundamental for the processes, such as neuronal plasticity, learning, and memory (Alix and Domingues 2011). Compared to all other neurotransmitters, the levels of glutamate are extremely high about 1,000-fold higher than those of many other important neurotransmitters. Yet, under pathologic conditions, the glutamate concentration levels in the brain interstitial space can increase 55-fold (Bogaert et al. 2000). The extracellular concentrations of glutamate and other endogenous excitatory amino acids need to be kept low to limit tonic activation of receptors and to ensure that the depolarization-evoked release of glutamate is accompanied by a sufficient increase in GluR activation and subsequent signaling. If enough ionotropic Glu receptors are stimulated simultaneously, high concentrations of cation influx will result in an action potential—the fastest type of excitatory synaptic transmission throughout the CNS and PNS. After the signal is received by the target cell, excitatory amino acid transporters (EAATs, also known as glutamate transporters) expressed on postsynaptic and supporting glial cells are tasked with emptying the synaptic cleft of Glu to effectively turn off the signal and reset the system for generation and propagation of new action potentials. In addition, it has long

123

been recognized that excessive activation of GluRs can kill the cells that express these receptors. Cerebral oxygen and glucose deprivation may result in the excessive release of stored synaptic glutamate due to the loss of ATP stores, leading to potassium efflux and membrane depolarization and the opening of voltage-dependent sodium channels. As a result, glutamate is released by synaptic exocytosis and trapped in the interstitium due to the reversal of the glutamate transporters. This leads to the overstimulation of NMDA receptors, known as ‘‘glutamate excitotoxicity’’ (Dempsey et al. 2000; Palmer 2001). Increased extracellular glutamate may also contribute to ‘‘vasogenic edema’’ and the increase in microvascular permeability (Abbott 2000). Upon overstimulation of its ionotropic receptors, the consequent calcium influx leads to the production of reactive oxygen species, causing a further release of intracellular glutamate (Love 1999), representing a vicious cycle in neurons. Moreover, it has been shown that, in cerebral circulation, polymorphonuclear leukocytes, upon stimulation by inflammatory stimuli, are capable of releasing glutamate, which, through the stimulation of mGluR, causes a breakdown of the endothelial barrier (Collard et al. 2002; Sharp et al. 2003). In neuroinflammation, the most probable sources of Glu are activated microglial cells. Glial cells have the molecular machinery to communicate with neurons (and among themselves) using neurotransmitters (Agulhon et al. 2010; Perea and Araque 2010), thus participating in synaptic transmission via neuronal–glial networking in a manner that contributes to brain function (Verkhratsky 2010). Microglia and macrophage accumulation is a common pathological feature of active MS lesions. Upon lipopolysaccharide (LPS) influence, which mimics an inflammatory environment, they release a large amount of glutamate through unpaired hemichannels that are openly exposed to extracellular space (Yawata et al. 2008), or by the xc glutamate/cystine antiporter system (Piani and Fontana 1994). In these cells, glutamate is produced by the action of glutaminase that converts glutamine to glutamate and ammonia (Newsholme and Calder 1997). Excessive signalling by excitatory neurotransmitters, like glutamate and ATP, can be deleterious to neurons and oligodendroglia. Sustained activation of AMPA, kainate, and NMDA receptors damages oligodendrocytes (Matute 2006). Thus, overproduction of glutamate, as well as inhibition of glutamate uptake by activated microglia, can compromise glutamate homeostasis and induce oligodendrocyte excitotoxicity and myelin destruction. Glutamate receptors Glutamate initiates signaling cascades upon binding to its receptors—GluRs, divided into two major classes:

The role of glutamate

ionotropic and metabotropic (Traynelis et al. 2010; JulioPieper et al. 2011). Glutamate ionotropic receptors (iGluRs) are classified into AMPA, kainate, and NMDA subtypes according to their preferred agonist. Molecular cloning has revealed that each receptors subtype is composed of several subunits with high homology within each receptor class. Overactivation of these receptors leads to excitotoxicity, especially in brain regions that are developmentally and regionally vulnerable to this kind of injury. The two receptor families differ in their mechanisms of activation and downstream effectors: iGluRs are voltagegated ion channels that initiate Ca2? and/or K? influx and downstream signaling while mGluRs are atypical G-proteincoupled receptors (GPCRs), which activate second messenger pathways, such as phospholipase C (PLC), phosphoinositide 3 kinase/retrovirus AK thymoma/mTOR (PI3K/AKT/ mTOR), and mitogen-activated protein kinase (MAPK) signaling. Metabotropic GluRs contain the classic seven-transmembrane domain structure and initiate signaling cascades or cation influx upon Glu binding (Ribeiro et al. 2010). Both receptor families are further classified into subgroups based on amino acid sequence homology, pharmacology, second messenger associations, and other signaling characteristics. mGluRs are further categorized into the group I, II, and III subfamilies (Willard and Koochekpour 2013). The literature data about beneficial effects of NMDA, AMPA, and kainate receptor antagonists in EAE (Bolton and Paul 2006) point out the considerable involvement of glutamate in the pathology of multiple sclerosis. Increased activation of ionotropic receptors, associated with aberrant glutamate transporter mechanisms in resident cells of the CNS, contribute to excess glutamate levels the disturbance of normal homeostasis and nerve function. Figure 1

summarizes the involvement of glutamate, its receptors, and cell membrane transporters in neuroinflammation. The release of glutamate as a consequence of inflammatory mediators action, as well as direct discharge from resident or infiltrating cell, lead to an increase of glutamate concentrations in CNS. Activated microglia are present in multiple sclerosis lesions. Incubation of primary cultured rat microglia with rat-brain derived myelin for 24-h-induced microglial activation, inducing the consequent expression of neuronal caspases and neuronal death in cultured cerebellar granule cell neurons induced by microglial-derived soluble toxins. Co-incubation of microglia with agonists or antagonists of different metabotropic glutamate receptor (mGluR) subtypes ameliorated microglial neurotoxicity by inhibiting soluble neurotoxin production. Activation of microglial mGluR2 exacerbated myelin-evoked neurotoxicity while activation of mGluR3 was protective as was activation of group III mGluRs. These data show that myelin-induced microglial neurotoxicity can be prevented by the regulation of mGluRs and suggest these receptors on microglia may be promising targets for therapeutic intervention in multiple sclerosis. (Pinteaux-Jones et al. 2008). Regulation of glutamate transporters by inflammatory mediators In the late 1960s, it was recognized that, when present in excess, glutamate has the potential to be excitotoxic. That is why, glutamate uptake is the mechanism responsible for the long-term maintenance of low, nontoxic concentrations of glutamate in extracellular space, including the synaptic cleft. Glutamate transporter proteins, which use the

Fig. 1 Mechanisms of neuronal damage in neuroinflammation and neurodegeneration

123

I. R. Stojanovic et al.

electrochemical gradients across the plasma membranes, uptake glutamate from extracellular space into neurons and glial cells. In humans, there are five different subtypes of high-affinity glutamate transporters, named sodium- and potassium-coupled glutamate transporters or, preferably, excitatory amino acid transporters 1–5 (EAAT 1–5) (Danbolt 2001). The flux of transported molecules depends on the density of transporters in the cell plasma membrane, which determines whether synaptic independence is compromised by the synaptic transmitter cross talk. Glial cells express GLT-1 (GAAT2) and GLAST (GAAT1), which facilitate the uptake of glutamate and its accumulation in synaptic vesicles (Kanai et al. 1997). The dysfunction of these transporter systems in both neural and glial cells may result in excitotoxic damage, which has been proved in EAE (Ohgoh et al. 2002). Under pathologic conditions, these transporters could become either inoperative or acting reversibly and raise extracellular glutamate concentrations. Numerous studies indicate that in neuroinflammation, proinflammatory cytokines (TNFa and IL-1b) mediate negative regulation of the expression and activity of glutamate transporters (Tilleux and Hermans 2007). Besides, in pathological conditions, Na?-independent, high-affinity glutamate transport system that carries cystine into the cell in exchange for internal glutamate, named the glutamate/cystine exchanger or xc antiporter system, is of great importance. In human brain, it is present mostly in neurons, but it could be also found in glial cells (Burdo et al. 2006). In pathological occasions, astrocytes can release glutamate using multiple mechanisms, both Ca2?-dependent and Ca2?-independent. The second one includes reversed action of glutamate reuptake carriers, exchange with cystine, the essential substrate for astrocytic production of glutathione, mediated by cystine–glutamate antiporter. The study of Domercq et al. (2007) provided evidence that the concomitant glutamate release from activated microglia by the cystine/glutamate antiporter and the inhibition of Na?independent glutamate uptake by activated microglia could induce a local increase in extracellular glutamate, leading to excitotoxic oligodendrocyte death. During the clinically active phase of MS, activated monocytes, macrophages, and microglia infiltrating the CNS overexpress the xc antiporter system and intensively release glutamate (Pampliega et al., 2011). In high extracellular glutamate concentrations, xc antiporter function is inhibited, inducing a decrease in cystine uptake and intracellular glutathione level, thus favoring oxidative stress in neurons and oligodendrocytes. Glutamate and neuroinflammation Neuroinflammation is a complex process with multiple mediators, signaling pathways, and feedback loops. It

123

comprises the activation of glial cells, recruitment of peripheral immune cells, and production of cytokines, such as interferon-gamma (IFN-c) and tumor necrosis factoralpha (TNF-a) (Neumann 2001). TNF-a mediates cytotoxic damage to glial cells and neurons, while IFN-c induces cell surface molecules important in immune and brain cells interactions between (Imai et al. 2007). Reactive glia shift toward proinflammatory phenotype, thus releasing cytokines, chemokines, and neurotoxic molecules (Zindler and Zipp 2010). Activated glia affects neuronal injury and death through the production of neurotoxic factors like glutamate, S100B, TNF-a, IL-1b, prostaglandins, and reactive oxygen and nitrogen species. As disease progresses, inflammatory secretions engage neighboring cells, including astrocytes and endothelial cells, resulting in autocrine and paracrine amplification of inflammation, leading to neurodegeneration. Bender et al. (2005) documented that the increased levels of TNF-a and IL-1b could alter the activity of neurons. Using primary rat and human neuronal cultures, Ye et al. (2013) proved that these two proinflammatory cytokines induced cell death and apoptosis in vitro. By binding to neuronal TNF receptors, TNFa can cause cell death directly linked to death domains that activate caspase-dependent apoptosis (Zhao et al. 2001). This cytokine can induce additional release of ROS, by inducing NADPH oxidase activity (Li et al. 2005). Both intra- and extracellular glutamate levels were increased upon TNF-a and IL-1b treatment (Ye et al. 2013). In this study, pretreatment with NMDA receptor antagonist MK-801 blocked cytokine-induced glutamate production and alleviated neurotoxicity, indicating that these cytokines induced neurotoxicity through glutamate. It was documented that TNF-a was higher in CSF of progressive MS subjects (Rossi et al. 2014). In murine brain slices, incubated in the presence of CSF from progressive MS patients, these authors observed increased spontaneous excitatory postsynaptic currents and glutamate-mediated neuronal swelling through a mechanism dependent on enhanced TNF-a signaling, pointing out TNF-a as a primary neurotoxic molecule in progressive forms of MS. They also suggested a pathogenic role of B cells in TNF-a CSF increase, associated with exacerbation of glutamatergic transmission and neuronal damage. Besides, Ye et al. (2013) indentified glutaminase as an important player in glutamate overproduction during inflammatory cytokine stimulation, inducing dysregulation, translocation or release of glutaminase isoforms, which consequently induced neurotoxicity and apoptosis. In mouse cortical astrocytes, Fang et al. (2012) reported that the chemokine macrophage inflammatory protein-2c (MIP-2c) increased significantly upon stimulation with LPS or TNF-a in vitro. They suggested that MIP-2c mediated the pathogenesis of CNS disorders associated with neutrophil

The role of glutamate

infiltration in the brain and decreased GLT-1 activity. This chemokine reduces the expression of glutamate transporter-1 on astrocytes and increases neuronal sensitivity to glutamate excitotoxicity. Astrocytes overexpressing MIP-2c downregulated the expression of GLT-1 at the mRNA and protein level and caused redistribution of GLT-1 out of the lipid rafts that mediate glutamate uptake. The data of Tolosa et al. (2011) link two important pathogenic mechanisms, excitotoxicity and neuroinflammation, proving that TNF-a-induced nuclear factor-kappaB (NF-jB) activation potentiates glutamate excitotoxicity on spinal cord motononeurons. The authors reported that chronic glutamate excitotoxicity, induced by the glutamate uptake inhibitor threohydroxyaspartate (THA), resulted in motoneuron loss that was associated with a neuroinflammatory response, which was potentiated with TNF-a and mediated by the downregulation of the astroglial glutamate transporter-1 (GLT-1), which were prevented by NF-jB inhibition. Furthermore, TNF-a and THA also cooperated in the induction of oxidative stress in a mechanism involving the NF-jB signalling pathway as well. Furthermore, the treatment with TNF-a inhibitors showed a beneficial effect on EAE (Lim and Constantinescu 2010), while in MS patients this treatment had opposite effects with even worsening of the disease proved by magnetic resonance imaging (van Oosten et al. 1996). Also, the studies showed that anti-TNF-a agents may initiate or unmask and underly demyelinating disease (Kaltsonoudis et al. 2014). Glutamate excitotoxicity is known to contribute to autoimmune neuroinflammation (Melzer et al. 2008; Pitt et al. 2000). In inflammation, activated microglia and astrocytes release and maintain high level of extracellular glutamate (Takeuchi et al. 2006). Also, glutamate leakage from serum across the compromised BBB in CNS inflammation, plus infiltrating inflammatory leukocytes and activated resident microglia with the potential to synthesise and release glutamate provides continuous, local supply of this neurotransmitter. Microglia are known to generate reactive oxygen and nitrogen species that impair glutamate uptake mechanisms. The permanent increased availability of glutamate would induce upregulation of its receptors and the synthesis of molecules responsible for neuronal dysfunction (Ohgoh et al. 2002). In CNS, activated cells release proinflammatory cytokines, which may reinforce local glutamate excitotoxicity, such as TNFa, known to reduce the expression of EAATs and detoxifying enzymes in glial cells, thus limiting their capacity for glutamate uptake. Astrocytes play direct, active, and critical roles in mediating neuronal survival and function. The removal of glutamate from the extracellular space by astrocytes confers neuroprotection, while astrocyte release of potentially toxic molecules promotes neurodegeneration (Hauser and

Cookson 2011). Still, the exact mechanism of inflammatory mediators in the disease progression is still poorly understood. The key cells in neurodegeneration, formed on the pathogenic substrate of inflammation, are activated microglial cells. When stimulated by proinflammatory signals, microglia may undergo a reaction that includes a morphological transformation into ameboid shape, producing prostanoids, cytokines, chemokines, nitric oxide, inducing surface markers, including members of the major histocompatibility complex family and initiating oxidative burst (Block and Hong 2005; Decoursey and Ligeti 2005). Activated microglia and the release of molecules that are detrimental to oligodendrocyte have been suggested as mechanisms by which innate immunity causes demyelination in MS. In early inflammation, microglia initiate immune responses by enhancing the expression of toll-like receptors (TLR) and a wide range of proinflammatory mediators (TNFa, IL-1, and IL-6 for the removal of the CNS threat (Floden et al. 2005). They proliferate, migrate, phagocyte, produce oxidants, and induce gene expression (iNOS, COX-2, MHC class II, complement, etc.). Microglial cells are likely to play a dual role in MS, depending on signals present in their microenvironment. While early microglial activation could represent a beneficial response (promoting tissue repair and removal of misfolded protein), in chronic phase, microglia express detrimental effects and promote neuronal death, thus contributing to the progression of the disease. It has been hypothesized that microglial proliferation is followed by programmed cell death (Gehrmann and Banati 1995), possibly as a compensatory feedback phenomenon. Whether or not activation of microglia during the acute phase of EAE is sufficient to induce neuronal cell death is still a matter of debate (Aarum et al. 2003; Butovsky et al. 2006, Walton et al. 2006). Nevertheless, the morphological and functional changes of microglial cells observed in mice with EAE (Centonze et al. 2010) clearly support the idea that activated microglia exert a key role in synaptic alterations and possibly neuronal damage. The consequent damage to oligodendrocytes, neurons, and the BBB, plus inflammatory cytokine release from microglia, contribute considerably to the pathology of EAE and MS and could be considered as one of the major disturbances in neuroinflammation. Although it is obvious that neuroinflammation by itself is not the cause of neuronal cell death, nowadays, there is strong evidence that neuroinflammation contributes to neurodegeneration. Glutamate neurotoxicity and neurodegeneration in MS Chronic inflammation and the consequent damage of neurons are considered as the key processes in neurodegenerative

123

I. R. Stojanovic et al.

diseases (Huang et al. 2005). Neuroinflammation in MS is considered as one of the constitutive components of the disease pathogenesis and the lesions generation. Multiple sclerosis has been considered for a long time only as an inflammatory demyelinating disease. The evidence is now increasing that excessive glutamate is released at the site of demyelination and axonal degeneration in MS plaques, and the most probable candidates for this cellular release are infiltrating leukocytes and activated microglia. The molecular mechanisms linking systemic inflammation and neuronal excitotoxicity are still poorly understood. The study of Degos et al. (2013) provides experimental support that group I mGluRs are involved in the mechanisms underlying inflammation-mediated sensitization to excitotoxic neurodegeneration. The mechanisms of neurodegeneration in MS are likely multifactorial and include direct damage by T cells and humoral immunity as well as oxidative stress, glutamatemediated excitotoxicity, and neuronal and oligodendrocyte apoptosis. Excitotoxicity is the pathological process by which nerve cells are damaged or killed by abnormal stimulation by excitatory neurotransmitters. Dysregulation of glutamate signaling leads to neurodegeneration (Maragakis and Rothstein 2006). Excitotoxic neuronal death can be direct, as the result of excessive stimulation of NMDA receptor or indirect. The hypothesis of indirect excitotoxic death pathway suggests that bioenergetic deficit causes depolarization when the nontoxic levels of glutamate become lethal (Kroemer et al. 2007). The numerous triggers, such as oxidative stress, calcium overload, mitochondrial dysfunction, and energy depletion, can lead to changes in neuronal excitation process, which has been proved to be involved also in multiple sclerosis pathogenesis (van Horssen et al. 2012). These mechanisms lead to the damage of proteins, nucleic acids, and lipids and mitochondrial disruption followed by overproduction of free radicals, energy depletion, and pro-apoptotic factors activation, resulting in death of neuronal cells (Farooqui and Farooqui 2009). Oxidative stress and excitotoxicity Glutamate receptor overstimulation is the main mediator to intracellular oxidative stress (Kumar et al. 2011). The recent studies suggest strong relationship between excessive calcium influx and glutamate-triggered neuronal injury (Sendrowski et al. 2013). The prolonged elevation of intracellular calcium concentration occurs due to an excessive depolarization of neurons as well as due to release from internal stores, mitochondria, and endoplasmic reticulum, or the malfunction of receptors and channels present in their membranes. The increased intracellular calcium concentration can trigger a range of

123

downstream neurotoxic cascades, with mitochondria playing a central role in cell biology as ATP producer and regulators of calcium signal, resulting in increased formation of ROS and activation of both caspase-dependent and caspase-independent apoptotic-like cell death (Lipton 2008). Reactive oxygen species can exert multiple damaging reactions to proteins, lipids, carbohydrates, and nucleic acids, thereby disrupting cellular functions (Martindale and Holbrook 2002). Lipid peroxidation results from the chemical attack by free radicals on fatty acids in membranes (Allen et al. 2012) and leads to membrane damage and cell lyses. NO and other reactive nitrogen species Nitric oxide (NO), the free radical and intra- and intercellular messenger molecule, is synthesized from L-arginine in the reaction catalyzed by nitric oxide synthases (NOSs) family: neuronal (nNOS, NOS1), endothelial (eNOS, NOS3), and inducible (iNOS, NOS2) isoforms (Steinert et al. 2010, West and Tseng 2011). In the brain, eNOS is expressed in cerebral vascular endothelial cells, while iNOS is expressed in astrocytes and microglia cells in response to inflammatory stimuli. Due to specific cellular distribution, they play different roles in both physiologic and pathologic processes. Neuronal NOS activity increases upon intracellular calcium increase, so overactivation of NMDA receptors and the consequent flux of calcium provide a link between an excitotoxic insult and NO-mediated cell damage. Excitotoxicity is further intensified by NO, which stimulates glutamate release from astrocytes. The recent studies document nitric oxide involvement in the pathology of many neurodegenerative diseases (Kumar et al. 2011). There is evidence that NO and iNOS are elevated in CNS and plasma in both MS (Stojanovic et al. 2012; Bagasra et al. 1995) and experimental allergic EAE, an experimental model of MS (Ljubisavljevic et al. 2012). iNOS expression, by itself, is not able to induce cell death. The double key in the mechanism of neurodegeneration is simultaneous activation of iNOS and phagocyte NADPH oxidase with the consequent effects mediated by peroxynitrite consequent to the initiation of an oxidative burst that produces superoxide from an orchestrated mechanism involving NADPH oxidase (Block and Hong 2005; Decoursey and Ligeti 2005). Although free radical NO is not as reactive as some other ROS, so the predominant contribution of NO to excitotoxicity depends on increased superoxide ion (O_2) production. It rapidly reacts with NO forming peroxynitrite (ONOO-) (Kumar et al. 2012). This exposure to NO/O2_ with resultant ONOO- formation results in necrosis of neurons or apoptosis, depending on its intensity (Rossi et al. 2014).

The role of glutamate

Rose et al. (2004) have suggested a mechanism that operates through the actions of two enzymes, cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS), both of which have been documented to be present in MS lesions. COX-2-derived prostanoids, present at high concentrations in EAE and MS CNS, stimulate glutamate release. Additionally, nitric oxide (NO), from iNOS, can increase COX-2. It also reacts with reactive oxygen species (ROS), forming peroxynitrite (ONOO-) that inactivates the glutamate transporters (Gurwitz and Kloog 1998) and directly damages myelin, oligodendrocytes, and axons (Mattle et al. 2004). Calcium overload Although it is known that every class of GluRs is involved in excitotoxic cell death, it is broadly accepted that NMDA receptors play the most important role, due to their permeability for calcium (Kroemer et al. 2007). Upon their activation, a massive influx of extracellular Ca2? leads to the activation of a number of Ca2?-dependent enzymes, such as proteases, protein kinases, phosphatases, phospholipases, nitric oxide synthase, and endonucleases, that influence a wide variety of cellular components involved in a number of cellular processes (Nicholls 2004) that are associated with neuritic degeneration and cell death through different pathways, including membrane breakdown, cytoskeleton alterations, and nitric oxide-derived free radicals formation (Norenberg and Rao 2007). One of the hallmarks of glutamate excitotoxicity is the degradation of the neuronal cytoskeleton mediated by NMDA receptor activity. Chung et al. (2005) suggested that excitotoxicity triggered a progressive pathway of cytoskeletal degeneration within axons, initially characterized by the loss of neurofilament proteins. The state of axonal integrity depends on the adequate phosphorylation of cytoskeleton proteins, especially microtubule-associated proteins, and neurofilaments. An important second messenger system that regulates the phosphorylation of cytoskeletal proteins is Ca2?. Upon activation of glutamatergic receptors, the consequent intracellular Ca2? increase results in changes in microtubules and neurofilaments phosphorylation state. Chung et al. (2005) found that treatment with even nontoxic levels of glutamate resulted in dramatic alterations in the axonal cytoskeleton that ultimately led to the total degradation of axonal structure. Mitochondrial dysfunction There is an emerging evidence that mitochondrial dysfunction actively contributes to neurodegeneration and the damage of axons (Witte et al. 2013; Fiebiger et al. 2013; Mahler et al. 2012; Virgili et al. 2013). As ATP producers

and regulators of calcium signal, mitochondria play a central role in cell biology. The brain consumes about 20 % of total oxygen and, due to great energy demands, has an intensive oxidative metabolism. More than half of the energy in the brain is used to restore resting potential in excitatory cells (Ames, 2000). In physiological conditions, calcium induced depolarization and calcium uptake regulate activation of pyruvate, a-ketoglutarate, and isocitrate dehydrogenases (Yacoubian et al. 2010), as well as mitochondrial dehydrogenases and mitochondrial ATP synthase (Brandon et al. 2006). However, during overactivation of NMDA receptor and excessive influx of Ca2?, there is also Ca2? release from intracellular compartments (mitochondria and endoplasmic reticulum), which together overwhelm Ca2?-regulatory mechanisms, leading to these enzymes inhibition, as well as respiratory chain complex I and the subsequent spread of excitation and neuronal death (Sendrowski et al. 2013). There is accumulated evidence that mitochondrial dysfunction contributes to axonal degeneration in inflammatory phase of MS, as well as that it is an important mechanism of neuron degeneration in the chronic stage of the disease (Witte et al. 2010; Mahad et al. 2009; Trapp and Stys 2009). In active demyelinating MS lesions, in both axons and neurons, the increased mitochondrial density was associated with the enhanced expression of mtHSP70, the marker of mitochondrial stress (Witte et al. 2009; Mahad et al. 2008). In inflammatory MS lesions, macrophages and activated microglia produce ROS, which damage proteins, lipids, and DNA. Lu et al. (2000) documented that in these conditions, the most damage was found in mitochondrial DNA (mtDNA). It has been known that mtDNA is 10 times more vulnerable to oxidative damage than nuclear one (Mecocci et al. 1993). This was associated with decreased activity of mitochondrial respiratory chain complex I and the decrease in oxidative phosphorylation. Mahad et al. (2008) found that the decreased respiratory chain complex IV activity in fulminant MS lesions correlated with the number of activated microglia and infiltrated macrophages. The consequent decreased ATP production, together with an increased ROS production by respiratory chain itself, contributes to the degeneration of axons in MS (Browne et al. 1997). Furthermore, mitochondria in nondemyelinated MS gray matter neurons were also found to have decreased activity of complexes I and III (Dutta et al. 2006). Increased mitochondrial ROS production and decreased activity of complexes I and III strengthen each other, leading to an accumulation of oxidative damage and mitochondria-driven degenerative mechanisms of axonal injury. Apart from the inhibition of enzyme activity and respiratory chain disturbances, the opening of a large nonselective pore in the inner mitochondrial membrane,

123

I. R. Stojanovic et al.

termed ‘‘mitochondrial permeability transition pore’’ (MPTP), has also been suggested as a key mechanism in neuronal excitotoxicity (Choi et al. 2013). The induction of MPTP can consecutively trigger apoptotic cascades via liberation of apoptosis-induced factor or cytochrome C, leading to cell death due to mitochondrial depolarization (Veto et al. 2010). Forte et al. (2007) reported that cyclophilin D (a key regulator of the MPTP) knockout mice were less sensitive to oxidative and nitrosative axonal damage, indicating that their axonal mitochondria are more resistant to inflammation-derived ROS and subsequent mitochondrial dysfunction, resulting in less axonal degeneration. The mechanism of excitotoxicity is now accepted in terms of glutamate role in the pathogenesis of neurodegenerative diseases. The two key pathways that trigger excitotoxicity involve glutamate neurotoxicity and intracellular calcium overload. In MS, almost all aspects of glutamate homeostasis are pathologically altered, which point out glutamate excitotoxicity as an important mechanism in the pathogenesis of the disease. The present body of evidence suggests that immunoinflammatory and neurodegenerative processes coexist in MS and that glutamate excitotoxicity is a link between them. These observations have already been partially confirmed, beside in animal models, by postmortem studies and in vivo analyses in MS patients, thus raising the possibility that modulation of glutamate release and transport, as well as receptors blockade or glutamate metabolism modulation, might be relevant targets for the development of future therapeutic interventions (Frigo et al. 2012; Rahn et al. 2012). Positive outcomes (decreased neuronal loss, improved cognition) that have been demonstrated suggest that glutamate metabolism and transport modulation could be a promising target in the prevention and delay of neurodegeneration and cognitive impairment in multiple sclerosis. Acknowledgments This paper was supported by The Ministry of Education and Science of the Republic of Serbia under the project number 41018.

References Aarum J, Sandberg K, Haeberlein SL, Persson MA (2003) Migration and differentiation of neural precursor cells can be directed by microglia. Proc Natl Acad Sci USA 100:15983–15988 Abbott NJ (2000) Inflammatory mediators and modulation of bloodbrain barrier permeability. Cell Mol Neurobiol 20:131–147 Agulhon C, Fiacco TA, McCarthy KD (2010) Hippocampal shortand longterm plasticity are not modulated by astrocyte Ca2? signaling. Science 327:1250–1254 Alix JJ, Domingues AM (2011) White matter synapses: form, function, and dysfunction. Neurology 76:397–404

123

Allen M, Zou F, Chai HS, Younkin CS, Miles R, Nair AA, Crook JE, Pankratz VS, Carrasquillo MM, Rowley CN, Nguyen T, Ma L, Malphrus KG, Bisceglio G, Ortolaza AI, Palusak R, Middha S, Maharjan S, Georgescu C, Schultz D, Rakhshan F, Kolbert CP, Jen J, Sando SB, Aasly JO, Barcikowska M, Uitti RJ, Wszolek ZK, Ross OA, Petersen RC, Graff-Radford NR, Dickson DW, Younkin SG, Ertekin-Taner N (2012) Glutathione S-transferase omega genes in Alzheimer and Parkinson disease risk, age-atdiagnosis and brain gene expression: an association study with mechanistic implications. Mol Neurodegener 7:13 Ames AI (2000) CNS energy metabolism as related to function. Brain Res Brain Res Rev 34:42–68 Bagasra O, Michaels FH, Zheng YM, Bobroski LE, Spitsin SV, Fu ZF (1995) Activation of the inducible form of nitric oxide synthase in the brains of patients with multiple sclerosis. Proc Natl Acad Sci USA 92(26):12041–12045 Bender LM, Morgan MJ, Thomas LR, Liu ZG, Thorburn A (2005) The adaptor protein TRADD activates distinct mechanisms of apoptosis from the nucleus and the cytoplasm. Cell Death Differ 12:473–481 Block ML, Hong JS (2005) Microglia and inflammation-mediated neurodegeneration: multiple triggers with a common mechanism. Prog Neurobiol 76(2):77–98 Bogaert L, Scheller D, Moonen J, Sarre S, Smolders I, Ebinger G, Michotte Y (2000) Neurochemical changes and laser Doppler flowmetry in the endothelin-1 rat model for focal cerebral ischemia. Brain Res 887:266–275 Bolton C, Paul C (2006) Glutamate receptors in neuroinflammatory demyelinating disease. Mediat Inflamm 2:93684 Brandon M, Baldi P, Wallace DC (2006) Mitochondrial mutations in cancer. Oncogene 25(34):4647–4662 Browne SE, Bowling AC, MacGarvey U, Baik MJ, Berger SC, Muqit MM, Bird ED, Beal MF (1997) Oxidative damage and metabolic dysfunction in Huntington’s disease: selective vulnerability of the basal ganglia. Ann Neurol 41:646–653 Burdo J, Dargusch R, Schubert D (2006) Distribution of the cystine/ glutamate antiporter system xc- in the brain, kidney, and duodenum. J Histochem Cytochem 54:549–557 Butovsky O, Landa G, Kunis G, Ziv Y, Avidan H, Greenberg N (2006) Induction and blockage of oligodendrogenesis by differently activated microglia in an animal model of multiple sclerosis. J Clin Invest 116:905–915 Centonze D, Muzio L, Rossi S, Cavasinni F, De Chiara V, Bergami A (2009) Inflammation triggers synaptic alteration and degeneration in experimental autoimmune encephalomyelitis. J Neurosci 29:3442–3452 Centonze D, Muzio L, Rossi S, Furlan R, Bernardi G, Martino G (2010) The link between inflammation, synaptic transmission and neurodegeneration in multiple sclerosis. Cell Death Differ 17:1083–1091 Choi IY, Lim JH, Kim C, Song HY, Ju C, Kim WK (2013) 4-Hydroxy-2(E)-nonenal facilitates NMDA-induced neurotoxicity via triggering mitochondrial permeability transition pore opening and mitochondrial calcium overload. Exp Neurobiol 22(3):200–207 Chung RS, McCormack GH, King AE, West AK, Vickers JK (2005) Glutamate induces rapid loss of axonal neurofilament proteins from cortical neurons in vitro. Exp Neurol 193:481–488 Cianfoni A, Niku S, Imbesi SG (2007) Metabolite findings in tumefactive demyelinating lesions utilizing short echo time proton magnetic resonance spectroscopy. AJNR Am J Neuroradiol 28:272–277 Collard CD, Park KA, Montalto MC, Alapati S, Buras JA, Stahl GL, Colgan SP (2002) Neutrophil-derived glutamate regulates vascular endothelial barrier function. J Biol Chem 277:14801–14811 Danbolt NC (2001) Glutamate uptake. Prog Neurobiol 65(1):1–105

The role of glutamate Decoursey TE, Ligeti E (2005) Regulation and termination of NADPH oxidase activity. Cell Mol Life Sci 62(19–20): 2173–2193 Degos V, Peineau P, Kaindl AM, Sigaut S, Favrais G, Plaisant F, Teissier N, Gouadon E, Lombet A, Saliba E, Collingridge GL, Maze M, Nicoletti F, Heijnen C, Mantz J, Kavelaars A, Gressens P (2013) G protein-coupled receptor kinase 2 and group I metabotropic glutamate receptors mediate inflammation-induced sensitization to excitotoxic neurodegeneration. Ann Neurol 73(5):667–678 Dempsey RJ, Baskaya MK, Dogan A (2000) Attenuation of brain edema, blood-brain barrier breakdown, and injury volume by ifenprodil, a polyamine-site N-methyl-D-aspartate receptor antagonist, after experimental traumatic brain injury in rats. Neurosurgery 47:399–404 Domercq M, Sa´nchez-Go´mez MV, Sherwin C, Etxebarria E, Fern R, Matute C (2007) System xc and glutamate transporter inhibition mediates microglial toxicity to oligodendrocytes. J Immunol 178:6549–6556 Dutta R, McDonough J, Yin X, Peterson J, Chang A, Torres T, Gudz T, Maclin WB, Lewis DA, Fox RJ, Rudick R, Mirnics K, Trapp BD (2006) Mitochondrial dysfunction as a cause of axonal degeneration in multiple sclerosis patients. Ann Neurol 59(3):478–489 Fang J, Han D, Hong J, Tan Q, Tian Y (2012) The chemokine, macrophage inflammatory protein-2c, reduces the expression of glutamate transporter-1 on astrocytes and increases neuronal sensitivity to glutamate excitotoxicity. J Neuroinflammation 9:267 Farooqui T, Farooqui AA (2009) Aging: an important factor for the pathogenesis of neurodegenerative diseases. Mech Ageing Dev 130:203–215 Fiebiger SM, Bros H, Grobosch T, Janssen A, Chanvillard C, Paul F, Do¨rr J, Millward JM, Infante-Duarte C (2013) The antioxidant idebenone fails to prevent or attenuate chronic experimental autoimmune encephalomyelitis in the mouse. J Neuroimmunol 262:66–71 Floden AM, Li S, Combs CK (2005) Beta-amyloid-stimulated microglia induce neuron death via synergistic stimulation of tumor necrosis factor alpha and NMDA receptors. J Neurosci 25:2566–2575 Forte M, Gold BG, Marracci G, Chaudhary P, Basso E, Johnsen D, Yu X, Fowlkes J, Rahder M, Stem K, Bernardi P, Bourdette D (2007) Cyclophilin D inactivation protects axons in experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis. Proc Natl Acad Sci USA 104:7558–7563 Frigo M, Cogo MG, Fusco ML, Gardinetti M, Frigeni B (2012) Glutamate and multiple sclerosis. Curr Med Chem 19(9):1295–1299 Gehrmann J, Banati RB (1995) Microglial turnover in the injured CNS: activated microglia undergo delayed DNA fragmentation following peripheral nerve injury. J Neuropathol Exp Neurol 54:680–688 Gray E, Rice C, Nightingale H, Ginty M, Hares K, Kemp K, Cohen N, Love S, Scolding N, Wilkins A (2013) Accumulation of cortical hyperphosphorylated neurofilaments as a marker of neurodegeneration in multiple sclerosis. Mult Scler J 19(2):153–161 Gurwitz D, Kloog Y (1998) Peroxynitrite generation might explain elevated glutamate and aspartate levels in multiple sclerosis cerebrospinal fluid. Eur J Clin Invest 28(9):760–761 Hauser DN, Cookson MR (2011) Astrocytes in Parkinson’s disease and DJ-1. J Neurochem 117:357–358 Huang Y, Erdmann N, Peng H, Zhao Y, Zheng J (2005) The role of TNF related apoptosis-inducing ligand in neurodegenerative diseases. Cell Mol Immunol 2:113–122

Imai F, Suzuki H, Oda J, Ninomiya T, Ono K, Sano H, Sawada M (2007) Neuroprotective effect of exogenous microglia in global brain ischemia. J Cereb Blood Flow Metab 27(3):488–500 Julio-Pieper M, Flor PJ, Dinan TG, Cryan JF (2011) Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues. Pharmacol Rev 63:35–58 Kaltsonoudis E, Voulgari PV, Konitsiotis S, Drosos AA (2014) Demyelination and other neurological adverse events after antiTNF therapy. Autoimmun Rev 13:54–58 Kanai Y, Trotti D, Nussberger S, Hediger MA (1997) The high affinity glutamate transporter family: structure, function and physiological relevance. In: Reith MEA (ed) Neurotransmitter transporters: structure, function, and regulation. Humana Press, Totowa, NJ, pp 171–214 Kroemer G, Galluzzi L, Brenner C (2007) Mitochondrial membrane permeabilization in cell death. Physiol Rev 87(1):99–163 Kumar P, Kalonia H, Kumar A (2011) Role of LOX/COX pathways in 3-nitropropionic acid induced Huntington’s disease-like symptoms in rats: protective effect of licofelone. Br J Pharmacol 164(2b):644–654 Kumar P, Kalonia H, Kumar A (2012) Possible GABAergic mechanism in the neuroprotective effect of gabapentin and lamotrigine against 3-nitropropionic acid induced neurotoxicity. Eur J Pharmacol 674(2–3):265–274 Li JM, Fan LM, Christie MR, Shah AM (2005) Acute tumor necrosis factor alpha signaling via NADPH oxidase in microvascular endothelial cells: role of p47phox phosphorylation and binding to TRAF4. Mol Cell Biol 25(6):2320–2330 Lim SY, Constantinescu CS (2010) TNF-a: a paradigm of paradox and complexity in multiple sclerosis and its animal models. Open Autoimmun J 2:160–170 Lipton SA (2008) NMDA receptor activity regulates transcription of antioxidant pathways. Nat Neurosci 11(4):381–382 Ljubisavljevic S, Stojanovic I, Pavlovic D, Milojkovic M, Pavlovic D, Vojinovic S, Sokolovic D, Stevanovic I (2012) Correlation of nitric oxide levels in the cerebellum and spinal cord of experimental autoimmune encephalomyelitis rats with clinical symptoms. Acta Neurobiol Exp (Wars) 72:33–39 Love S (1999) Oxidative stress in brain ischemia. Brain Pathol 9:119–131 Lu F, Selak M, O’Connor J, Croul S, Lorenzana C, Butunoi C, Kalman B (2000) Oxidative damage to mitochondrial DNA and activity of mitochondrial enzymes in chronic active lesions of multiple sclerosis. J Neurol Sci 177:95–103 Lucchinetti C, Bruck W, Parisi J, Scheithauer B, Rodriguez M, Lassmann H (2000) Heterogeneity of multiple sclerosis lesions: implications for the pathogenesis of demyelination. Ann Neurol 47:707–717 Mahad D, Ziabreva I, Lassmann H, Turnbull D (2008) Mitochondrial defects in acute multiple sclerosis lesions. Brain 131:1722–1735 Mahad DJ, Ziabreva I, Campbell G, Lax N, White K, Hanson PS, Lassmann H, Turnbull DM (2009) Mitochondrial changes within axons in multiple sclerosis. Brain 132:1161–1174 Mahler A, Steiniger J, Bock M, Brandt AU, Haas V, Boschmann M, Paul F (2012) Is metabolic flexibility altered in multiple sclerosis patients? PLoS One 7(8):e43675 Maragakis NJ, Rothstein JD (2006) Mechanisms of disease: astrocytes in neurodegenerative disease. Nat Clin Prac Neurol 2:679–689 Martindale JL, Holbrook NJ (2002) Cellular response to oxidative stress: signaling for suicide and survival. J Cell Physiol 192:1–15 Mattle HP, Lienert C, Greeve I (2004) Uric acid and multiple sclerosis [in German]. Ther Umsch 61(9):553–555 Matute C (2006) Oligodendrocyte NMDA receptors: a novel therapeutic target. Trends Mol Med 12:289–292

123

I. R. Stojanovic et al. Mecocci P, MacGarvey U, Kaufman AE, Koontz D, Shoffner JM, Wallace DC, Beal MF (1993) Oxidative damage to mitochondrial DNA shows marked age dependent increases in human brain. Ann Neurol 34:609–616 Melzer N, Meuth SG, Torres-Salazar D, Bittner S, Zozulya AL, Weidenfeller C, Kotsiari A, Stangel M, Fahlke C, Wiendl H (2008) A beta-lactam antibiotic dampens excitotoxic inflammatory CNS damage in a mouse model of multiple sclerosis. PLoS One 3:e3149 Neumann H (2001) Control of glial immune function by neurons. Glia 36(2):191–199 Newcombe J, Uddin A, Dove R, Patel B, Turski L, Nishizawa Y (2008) Glutamate receptor expression in multiple sclerosis lesions. Brain Pathol 18:52–61 Newsholme EA, Calder PC (1997) The proposed role of glutamine in some cells of the immune system and speculative consequences for the whole animal. Nutrition 13(7–8):728–730 Nicholls DG (2004) Mitochondrial dysfunction and glutamate excitotoxicity studied in primary neuronal cultures. Curr Mol Med 4:149–177 Norenberg MD, Rao KVR (2007) The mitochondrial permeability transition in neurologic disease. Neurochem Int 50(7–8):983–997 Ohgoh M, Hanada T, Smith T, Hashimoto T, Ueno M, Yamanishi Y, Watanabe M, Nishizawa Y (2002) Altered expression of glutamate transporters in experimental autoimmune encephalomyelitis. J Neuroimmunol 125(1–2):170–178 Palmer GC (2001) Neuroprotection by NMDA receptor antagonists in a variety of neuropathologies. Curr Drug Targets 2:241–271 Pampliega O, Domercq M, Soria FN, Villoslada P, Rodrı´guezAntigu¨edad A, Matute CJ (2011) Increased expression of cystine/glutamate antiporter in multiple sclerosis. J Neuroinflammation 8:63 Perea G, Araque A (2010) Glia modulates synaptic transmission. Brain Res Rev 63(1–2):93–102 Piani D, Fontana A (1994) Involvement of the cystine transport system xc- in the macrophage-induced glutamate-dependent cytotoxicity to neurons. J Immunol 152(7):3578–3585 Pinteaux-Jones F, Sevastou IG, Fry VA, Heales S, Baker D, Pocock JM (2008) Myelin-induced microglial neurotoxicity can be controlled by microglial metabotropic glutamate receptors. J Neurochem 106(1):442–454 Pitt D, Werner P, Raine CS (2000) Glutamate excitotoxicity in a model of multiple sclerosis. Nat Med 6:67–70 Rahn KA, Slusher BS, Kaplin AI (2012) Glutamate in CNS neurodegeneration and cognition and its regulation by GCPII inhibition. Curr Med Chem 19(9):1335–1345 Ribeiro FM, Paquet M, Cregan SP, Ferguson SS (2010) Group I metabotropic glutamate receptor signalling and its implication in neurological disease. CNS Neurol Disord Drug Targets 9:574–595 Rose JW, Hill KE, Watt HE, Carlson NG (2004) Inflammatory cell expression of cyclooxygenase-2 in the multiple sclerosis lesion. J Neuroimmunol 149(1–2):40–49 Rossi S, Motta C, Studer V, Barbieri F, Buttari F, Bergami A, Sancesario G, Bernardini S, De Angelis G, Martino G, Furlan R, Centonze D (2014) Tumor necrosis factor is elevated in progressive multiple sclerosis and causes excitotoxic neurodegeneration. Mult Scler 20(3):304–312. doi:10.1177/135245851 3498128 Sarchielli P, Greco L, Floridi A, Floridi A, Gallai V (2003) Excitatory amino acids and multiple sclerosis: evidence from cerebrospinal fluid. Arch Neurol 60:1082–1088 Sendrowski K, Rusak M, Sobaniec P, Iłendo E, Da˛browska M, Boc´kowski L, Koput A, Sobaniec W (2013) Study of the protective effect of calcium channel blockers against neuronal

123

damage induced by glutamate in cultured hippocampal neurons. Pharmacol Rep 65(3):730–746 Sharp CD, Hines I, Houghton J, Warren A, Jackson TH, Jawahar A, Nanda A, Elrod JW, Long A, Chi A, Minagar A, Alexander JS (2003) Glutamate causes a loss in human cerebral endothelial barrier integrity through activation of NMDA receptor. Am J Physiol Heart Circ Physiol 285:H2592–H2598 Sinnecker T, Mittelstaedt P, Do¨rr J, Pfueller CF, Harms L, Niendorf T, Paul F, Wuerfel J (2012) Multiple sclerosis lesions and irreversible brain tissue damage a comparative ultrahigh-field strength magnetic resonance imaging study. Arch Neurol 69(6):739–745 Steinert JR, Chernova T, Forsythe ID (2010) Nitric oxide signaling in brain function, dysfunction, and dementia. Neuroscientist 16:435–452 Stojanovic I, Vojinovic S, Ljubisavljevic S, Pavlovic R, Basic J, Pavlovic D, Ilic A, Cvetkovic T, Stukalov M (2012) INF-b1b therapy modulates L-arginine and nitric oxide metabolism in patients with relapse remittens multiple sclerosis. J Neurol Sci 323(1–2):187–192 Sulkowski G, Da˛browska-Bouta B, Chalimoniuk M, Stru_zyn´ska (2013) Effects of antagonists of glutamate receptors on proinflammatory cytokines in the brain cortex of rats subjected to experimental autoimmune encephalomyelitis. J Neuroimmunol 261(1–2):67–76 Takeuchi H, Jin S, Wang J, Zhang G, Kawanokuchi J, Kuno R, Sonobe Y, Mizuno T, Suzumura A (2006) Tumor necrosis factor-alpha induces neurotoxicity via glutamate release from hemichannels of activated microglia in an autocrine manner. J Biol Chem 281:21362–21368 Tilleux S, Hermans E (2007) Neuroinflammation and regulation of glial glutamate uptake in neurological disorders. J Neurosci Res 85:2059–2070 Tolosa L, Caraballo-Miralles V, Olmos G, Llado´ J (2011) TNF-a potentiates glutamate-induced spinal cord motoneuron death via NF-jB. Mol Cell Neurosci 46(1):176–186 Trapp BD, Stys PK (2009) Virtual hypoxia and chronic necrosis of demyelinated axons in multiple sclerosis. Lancet Neurol 8:280–291 Traynelis SF, Wollmuth LP, McBain CJ, Menniti FS, Vance KM, Ogden KK, Hansen KB, Yuan H, Myers SJ, Dingledine R (2010) Glutamate receptor ion channels: structure, regulation, and function. Pharmacol Rev 62:405–496 Vallejo-Illarramendi A, Domercq M, Pe´rez-Cerda´ F, Ravid R, Matute C (2006) Increased expression and function of glutamate transporters in multiple sclerosis. Neurobiol Dis 21:154–164 van Horssen J, Witte ME, Ciccarelli O (2012) The role of mitochondria in axonal degeneration and tissue repair in MS. Mult Scler J 18(8):1058–1067 van Oosten BW, Barkhof F, Truyen L, Boringa JB, Bertelsmann FW, von Blomberg BM, Woody JN, Hartung HP, Polman CH (1996) Increased MRI activity and immune activation in two multiple sclerosis patients treated with the monoclonal anti-tumor necrosis factor antibody cA2. Neurology 47:1531–1534 Vercellino M, Merola A, Piacentino C, Votta B, Capello E, Mancardi GL (2007) Altered glutamate reuptake in relapsing-remitting and secondary progressive multiple sclerosis cortex: correlation with microglia infiltration, demyelination, and neuronal and synaptic damage. J Neuropathol Exp Neurol 66:732–739 Verkhratsky A (2010) Physiology of neuronal-glial networking. Neurochem Int 57:332–343 Veto S, Acs P, Bauer J, Berente Z, Setalo G Jr, Borgulya G, Sumegi B, Komoly S, Gallyas F Jr, Illes Z (2010) Inhibiting poly(ADPribose) polymerase: a potential therapy against oligodendrocyte death. Brain 133(Pt 3):822–834

The role of glutamate Virgili N, Mancera P, Wappenhans B, Sorrosal G, Biber K, Pugliese M, Espinosa-Parrilla JF (2013) KATP channel opener diazoxide prevents neurodegeneration: a new mechanism of action via antioxidative pathway activation. PLoS ONE 8(9):e75189 Walton NM, Sutter BM, Laywell ED, Levkoff LH, Kearns SM, Marshall GP 2nd, Scheffler B, Steindler DA (2006) Microglia instruct subventricular zone neurogenesis. Glia 54:815–825 West AR, Tseng KY (2011) Nitric oxide-soluble guanylyl cyclasecyclic GMP signaling in the striatum: new targets for the treatment of Parkinson’s disease? Front Syst Neurosci 5:1–55 Willard SS, Koochekpour S (2013) Glutamate, glutamate receptors, and downstream signaling pathways. Int J Biol Sci 9(9):948–959 Witte ME, Bø L, Rodenburg RJ, Belien JA, Musters R, Hazes T, Wintjes LT, Smeitink JA, Geurts JJ, De Vries HE, van der Valk P, van Horssen J (2009) Enhanced number and activity of mitochondria in multiple sclerosis lesions. J Pathol 219(2):193–204 Witte ME, Geurts JJG, de Vries HE, van der Valk P, van Horssen J (2010) Mitochondrial dysfunction: a potential link between neuroinflammation and neurodegeneration? Mitochondrion 10:411–418 Witte ME, Nijland PG, Drexhage JAR, Gerritsen W, Geerts D, van het Hof B, de Reijerkerk A, Vries HE, van der Valk P, van

Horssen J (2013) Reduced expression of PGC-1a partly underlies mitochondrial changes and correlates with neuronal loss in multiple sclerosis cortex. Acta Neuropathol 125:231–243 Yacoubian TA, Slone SR, Harrington AJ, Hamamichi S, Schieltz JM, Caldwell KA (2010) Differential neuroprotective effects of 14-33 proteins in models of Parkinson’s disease. Cell Death Dis 1:e2 Yawata I, Takeuchi H, Doi Y, Liang J, Mizuno T, Suzumura A (2008) Macrophage-induced neurotoxicity is mediated by glutamate and attenuated by glutaminase inhibitors and gap junction inhibitors. Life Sci 82(21–22):1111–1116 Ye L, Huang Y, Zhao L, Li Y, Sun L, Zhou Y, Qian G, Zheng JC (2013) IL-1b and TNF-a induce neurotoxicity through glutamate production: a potential role of glutaminase. J Neurochem 125:897–908 Zhao X, Bausano B, Pike BR, Newcomb-Fernandez JK, Wang KK, Shohami E, Ringger NC, DeFord SM, Anderson DK, Hayes RL (2001) TNF-alpha stimulates caspase-3 activation and apoptotic cell death in primary septo-hippocampal cultures. J Neurosci Res 64(2):121–131 Zindler E, Zipp F (2010) Neuronal injury in chronic CNS inflammation. Best Pract Res Clin Anaesthesiol 24(4):551–562

123