Expression and potential role of inducible nitric oxide synthase in the central nervous system of Theiler's murine encephalomyelitis virus-induced demyelinating disease

Axonal response of mitochondria to demyelination and complex IV activity within demyelinated axons in experimental models of multiple sclerosis

AIMS

Axonal injury in multiple sclerosis (MS) and experimental models is most frequently detected in acutely demyelinating lesions. We recently reported a compensatory neuronal response, where mitochondria move to the acutely demyelinated axon and increase the mitochondrial content following lysolecithin-induced demyelination. We termed this homeostatic phenomenon, which is also evident in MS, the axonal response of mitochondria to demyelination (ARMD). The aim of this study is to determine whether ARMD is consistently evident in experimental demyelination and how its perturbation relates to axonal injury.

METHODS

In the present study, we assessed axonal mitochondrial content as well as axonal mitochondrial respiratory chain complex IV activity (cytochrome c oxidase or COX) of axons and related these to axonal injury in nine different experimental disease models. We used immunofluorescent histochemistry as well as sequential COX histochemistry followed by immunofluorescent labelling of mitochondria and axons.

RESULTS

We found ARMD a consistent and robust phenomenon in all experimental disease models. The increase in mitochondrial content within demyelinated axons, however, was not always accompanied by a proportionate increase in complex IV activity, particularly in highly inflammatory models such as experimental autoimmune encephalomyelitis (EAE). Axonal complex IV activity inversely correlated with the extent of axonal injury in experimental disease models.

CONCLUSIONS

Our findings indicate that ARMD is a consistent and prominent feature and emphasise the importance of complex IV activity in the context of ARMD, especially in autoimmune inflammatory demyelination, paving the way for the development of novel neuroprotective therapies.

Cathepsin-B inhibitor CA-074 attenuates retinopathy and optic neuritis in experimental autoimmune encephalomyelitis induced in SJL/J mice

The complicated multiple sclerosis (MS) can exhibit subacute sight deterioration and can lead to total deprivation of vision. In the current work, we explored the therapeutic outcome of Cathepsin B inhibitor (CA-074) against retinopathy and optic neuritis (ON) caused by experimental autoimmune encephalomyelitis (EAE) induced by proteolipid protein peptide (PLP) in female SJL/J mice. A daily dose of 10 mg/kg CA-074 was administered to the EAE mice intraperitoneally for 14 days from day 14 post-immunization until day 28. The Western blot and immunofluorescence analyses show inflammation in the optic nerve through the elevation of iNOS and NFkB markers in EAE mice. Optic neuritis was reported which is a consequence of demyelination and axon injury, estimated with the reduction in myelin basic protein (MBP). The glial fibrillary acidic protein (GFAP) expression level was found to be elevated in the retina of EAE mice which confirmed the retinopathy. The administration of CA-074 ameliorated optic neuritis and retinopathy by reducing inflammation. The treatment with CA-074 also reduced the demyelination and axonal injuries in the EAE mice. The findings of this study have shown the protective effect of CA-074 in the case of retinopathy and ON inflicted by EAE in SJL/J mice.

Theiler’s virus-induced demyelinating disease as an infectious model of progressive multiple sclerosis

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disease of unknown etiology. However, several studies suggest that infectious agents, e.g., Human Herpes Viruses (HHV), may be involved in triggering the disease. Molecular mimicry, bystander effect, and epitope spreading are three mechanisms that can initiate immunoreactivity leading to CNS autoimmunity in MS. Theiler’s murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD) is a pre-clinical model of MS in which intracerebral inoculation of TMEV results in a CNS autoimmune disease that causes demyelination, neuroaxonal damage, and progressive clinical disability. Given the spectra of different murine models used to study MS, this review highlights why TMEV-IDD represents a valuable tool for testing the viral hypotheses of MS. We initially describe how the main mechanisms of CNS autoimmunity have been identified across both MS and TMEV-IDD etiology. Next, we discuss how adaptive, innate, and CNS resident immune cells contribute to TMEV-IDD immunopathology and how this relates to MS. Lastly, we highlight the sexual dimorphism observed in TMEV-IDD and MS and how this may be tied to sexually dimorphic responses to viral infections. In summary, TMEV-IDD is an underutilized murine model that recapitulates many unique aspects of MS; as we learn more about the nature of viral infections in MS, TMEV-IDD will be critical in testing the future therapeutics that aim to intervene with disease onset and progression.

Celastrol Attenuates Multiple Sclerosis and Optic Neuritis in an Experimental Autoimmune Encephalomyelitis Model

This study was aimed to evaluate the effects of celastrol, a natural compound with multiple bioactivities, on multiple sclerosis and optic neuritis (ON) in rat experimental autoimmune encephalomyelitis (EAE). EAE was induced in Sprague Dawley rats using myelin basic protein, and the animals received daily intraperitoneal injections of celastrol or vehicle for 13 days. The EAE rats showed abnormal neurobehavior and inflammatory infiltration and demyelination in the spinal cord. Significantly upregulated mRNA expression of pro-inflammatory cytokines interferon-γ and interleukin-17 and downregulated anti-inflammatory cytokines interleukin-4 were found in the spinal cord of EAE rats. In the study of ON, severely inflammatory responses like in the spinal cord were also seen in the optic nerve, as well as obvious microgliosis. Furthermore, activation of nuclear factor kappa-B and upregulated inducible nitric oxide synthase was observed in the optic nerve. In addition, apoptosis of retinal ganglion cells and dysregulation of apoptotic-associated proteins in the optic nerve were found in EAE rats. Treatment of celastrol potently restored these changes. In most of the indexes, the effects of high dose of celastrol were better than the low dose. Our data conclude that administration of celastrol attenuates multiple sclerosis and ON in EAE via anti-inflammatory and anti-apoptotic effects. These findings provide new pre-clinical evidence for the use of celastrol in treatment of multiple sclerosis.

Dimethyl fumarate suppresses Theiler's murine encephalomyelitis virus-induced demyelinating disease by modifying the Nrf2-Keap1 pathway

Dimethyl fumarate (DMF) is a modifier of the nuclear factor (erythroid-derived 2)-2 (Nrf2)-kelch-like ECH-associated protein 1 (Keap1) pathway. DMF treatment in the effector phase significantly suppressed the development of Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) both clinically and histologically. DMF treatment leads to an enhanced Nrf2 antioxidant response in TMEV-IDD mice. DMF treatment in the effector phase significantly suppressed the level of IL-17A mRNA. DMF is known to inhibit differentiation of T helper 17 (Th17) cells via suppressing NF-κB. Taken together, our data suggest that DMF treatment in the effector phase may suppress TMEV-IDD not only via enhancing the antioxidant response but also via suppressing IL-17A.

Expression and activity of nitric oxide synthase isoforms in methamphetamine-induced striatal dopamine toxicity

Nitric oxide is implicated in methamphetamine (METH)-induced neurotoxicity; however, the source of the nitric oxide has not been identified. Previous work has also revealed that animals with partial dopamine loss induced by a neurotoxic regimen of methamphetamine fail to exhibit further decreases in striatal dopamine when re-exposed to methamphetamine 7-30 days later. The current study examined nitric oxide synthase expression and activity and protein nitration in striata of animals administered saline or neurotoxic regimens of methamphetamine at postnatal days 60 and/or 90, resulting in four treatment groups: Saline:Saline, METH:Saline, Saline:METH, and METH:METH. Acute administration of methamphetamine on postnatal day 90 (Saline:METH and METH:METH) increased nitric oxide production, as evidenced by increased protein nitration. Methamphetamine did not, however, change the expression of endothelial or inducible isoforms of nitric oxide synthase, nor did it change the number of cells positive for neuronal nitric oxide synthase mRNA expression or the amount of neuronal nitric oxide synthase mRNA per cell. However, nitric oxide synthase activity in striatal interneurons was increased in the Saline:METH and METH:METH animals. These data suggest that increased nitric oxide production after a neurotoxic regimen of methamphetamine results from increased nitric oxide synthase activity, rather than an induction of mRNA, and that constitutively expressed neuronal nitric oxide synthase is the most likely source of nitric oxide after methamphetamine administration. Of interest, animals rendered resistant to further methamphetamine-induced dopamine depletions still show equivalent degrees of methamphetamine-induced nitric oxide production, suggesting that nitric oxide production alone in response to methamphetamine is not sufficient to induce acute neurotoxic injury.

Astrocyte response to Junín virus infection

In a previous study of experimental murine encephalitis induced by Junín virus (JV), an arenavirus, we showed increased expression of iNOS by unidentified cells, concomitant with the astrocyte reaction. The specific inhibition of iNOS was associated with greater mortality but lower astrocytosis, suggesting that the protective role of nitric oxide (NO) synthesized by iNOS was related to enhanced astrocyte activation, representing a beneficial cellular response to virus-induced central nervous system damage. In the present work, cultured astrocytes were used to study whether JV infection could trigger iNOS expression and assess its eventual relationship with viral replication, glial fibrilary acidic protein (GFAP) expression levels and the presence of apoptosis. We found that JV infection of astrocytes did not induce apoptosis but produced both increased iNOS synthesis, detected by immunocytochemistry and fluorescence activated cell sorting (FACS) analysis, and increased NO, which was indirectly measured by nitrite/nitrate levels. These changes occurred early relative to the increases in GFAP expression, as detected by immunocytochemistry, FACS analysis and RT-PCR. The fact that iNOS inhibition abolished enhanced GFAP expression in infected monolayers suggests that NO was directly involved. In addition, iNOS inhibition enhanced virus replication. Together with data from confocal microscopy, these results suggest that JV induces iNOS expression in infected astrocytes and that the resulting NO has an important role both in reducing viral replication and in enhancing subsequent astrocyte activation.

Immunological aspects of axon injury in multiple sclerosis

The role of immune-mediated axonal injury in the induction of nonremitting functional deficits associated with multiple sclerosis is an area of active research that promises to substantially alter our understanding of the pathogenesis of this disease and modify or change our therapeutic focus. This review summarizes the current state of research regarding changes in axonal function during demyelination, provides evidence of axonal dysmorphia and degeneration associated with demyelination, and identifies the cellular and molecular effectors of immune-mediated axonal injury. Finally, a unifying hypothesis that links neuronal stress associated with demyelination-induced axonal dysfunction to immune recognition and immunopathology is provided in an effort to shape future experimentation.

Immunohistochemical evidence of inducible nitric oxide synthase and nitrotyrosine in a case of clinically isolated optic neuritis

BACKGROUND

Optic neuritis (ON) is a demyelinating inflammation of the optic nerve that may occur as an isolated disease or related to multiple sclerosis (MS). There is little evidence of whether the immunohistochemistry of ON resembles that of typical cerebral MS lesions.

METHODS

Pathologic optic nerves were obtained from a patient who died of causes unrelated to ON after clinical recovery from clinically isolated ON. Normal control optic nerves were obtained from an eye bank. Normal and pathologic tissues were probed with antibodies to pathologic proteins including myelin basic protein (MBP) fragment, the inducible form of nitric oxide synthase (iNOS), macrophage markers CD14 and CD64, nitrotyrosine, and cyclooxygenase (COX-2). We also examined MBP, the oligodendrocyte marker cyclic nucleotide phosphodiesterase (CNPase), and glial fibrillary acidic protein.

RESULTS

In the affected pathologic nerve, iNOS-positive macrophages/microglia, iNOS-positive astrocytes, COX-2, and nitrotyrosine were observed. iNOS and COX-2 were occasionally observed in the unaffected nerve. Decreased expression of MBP and CNPase was seen in the pathologic optic nerves, along with evidence of gliosis and ongoing myelin degradation indicated by the presence of MBP fragment.

CONCLUSIONS

The immunohistochemistry of clinically isolated optic neuritis, as judged by this single case, resembles that of cerebral lesions of MS in showing abnormally high levels of iNOS and nitrotyrosine as well as other mediators of immune damage.

Differential astrocyte response to Theiler's murine encephalomyelitis virus infection

OBJECTIVES

We aimed to address if selective astrocyte apoptosis is involved in the lack of murine demyelinating disease following infection by the L*-1 variant of Theiler's virus. In addition, we investigated whether L*-1-infected astrocytes were able to selectively express molecules whose effects would play a role as pathogenic factors.

METHODS

Murine cultured astrocytes were infected with two Theiler viruses, the DA strain and the mutated DA variant L*-1, which does not synthesize the out of frame L* protein.

RESULTS

Neither DA nor L*-1 provoked apoptosis, although they replicated in astrocytes inducing GFAP and iNOS expression, as well as subsequent nitric oxide production. In addition, both viruses caused an enhanced expression of ICAM-1, VCAM-1 and decay accelerating factor (DAF). In this connection, values of VCAM-1 and DAF induced by L*-1 were higher and lower, respectively, than those induced by DA.

CONCLUSIONS

Since no apoptosis was found, such mechanism would not be involved in the lack of TMEV-induced demyelinating disease by L*-1. In contrast, selective expression of VCAM-1 and DAF molecules induced by L*-1 could have a role in virus clearance from the central nervous system.

Activation of the endocannabinoid system as a therapeutic approach in a murine model of multiple sclerosis

Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) is a well-characterized murine model of human multiple sclerosis (MS) that closely resembles the chronic and progressive clinical form of the disease. Recent studies have described the involvement of the cannabinoid system in the progression of the disease and the benefits associated with the administration of cannabinoid agonists. With the objective to study whether "indirect" agonists, that is, compounds able to reinforce the physiological endocannabinoid transmission and, therefore, devoid of the psychotropic effects of "direct" agonists, could be suitable agents for the amelioration of MS neurological deficits, we administered the potent and selective anandamide uptake inhibitor UCM707 to TMEV-infected mice. Our results indicate that treatment during established disease significantly improves the motor function of the diseased mice. At the histological level, UCM707 is able to reduce microglial activation, diminish major histocompatibility complex class II antigen expression, and decrease cellular infiltrates in the spinal cord. Additionally, in microglial cells, UCM707 decreases the production of the proinflammatory cytokines tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6; reduces nitric oxide levels and inducible nitric oxide synthase expression; and is able to potentiate the action of a subeffective dose of the endocannabinoid anandamide. Overall, these results suggest that agents able to activate the endocannabinoid system could constitute a new series of drugs for the treatment of MS.

Cytokines, nitric oxide, and cGMP modulate the permeability of an in vitro model of the human blood-brain barrier

The endothelial cells (EC) of the microvasculature in the brain form the anatomical basis of the blood-brain barrier (BBB). In the present study, the effects of agents that modify the permeability of a well-established in vitro model of the human BBB were studied. The monolayers formed by confluent human brain microvessel endothelial cell (HBMEC) cultures are impermeable to the macromolecule tracer horseradish peroxidase (HRP) and have high electrical resistance. Exposure of HBMEC to various cytokines including TNF-alpha, IL-1beta, interferon gamma (IFN-gamma), or lipopolysaccharide (LPS) decreased transendothelial electrical resistance (TEER) mainly by increasing the permeability of the tight junctions. Primary cultures of HBMEC express endothelial nitric oxide synthase (eNOS) and produce low levels of NO. Treatment with the NO donors sodium nitroprusside (SNP) and DETA NONOate or the cGMP agonist 8-Br-cGMP significantly increased monolayer resistance. Conversely, inhibition of soluble guanylyl cyclase with ODQ rapidly decreased the resistance, and pretreatment of HBMEC with Rp-8-CPT-cGMPS, an inhibitor of cGMP-dependent protein kinase, partially prevented the 8-Br-cGMP-induced increase in resistance. Furthermore, NO donors and 8-Br-cGMP could also reverse the increased permeability of the monolayers induced by IL-1beta, IFN-gamma, and LPS. These results indicate that NO can decrease the permeability of the human BBB through a mechanism at least partly dependent on cGMP production and cGMP-dependent protein kinase activation.

Inducible nitric oxide synthase in chronic active multiple sclerosis plaques: distribution, cellular expression and association with myelin damage

Inducible nitric oxide synthase (iNOS) is an enzyme that produces nitric oxide (NO) and is thought to contribute to the pathogenesis of multiple sclerosis (MS). The extent of iNOS expression was examined using laser scanning confocal microscopy of 13 chronic active plaques from seven MS patients displaying both acute demyelination and active inflammation. iNOS expression in these plaques was substantial and diverse in cellular distribution. Expression of iNOS was observed in ependymal cells located in periventricular lesions, inflammatory cells, and occasionally in astrocytes. iNOS was found in microglial/macrophage cells that expressed CD64, the high affinity Fc gamma receptor associated with cells that have phagocytic function and participate in antibody-dependent cellular cytotoxicity (ADCC). Scavenger microglial/macrophage cells that expressed the marker CD14 were also present and may express iNOS. The markers for myelin damage, nitrotyrosine (an index of iNOS mediated damage via peroxynitrite formation), along with MBP fragments, were also observed associated with iNOS in MS plaques. Together, these findings support a central role for iNOS in the pathogenesis of multiple sclerosis.

Theiler's virus infection: a model for multiple sclerosis

Both genetic background and environmental factors, very probably viruses, appear to play a role in the etiology of multiple sclerosis (MS). Lessons from viral experimental models suggest that many different viruses may trigger inflammatory demyelinating diseases resembling MS. Theiler's virus, a picornavirus, induces in susceptible strains of mice early acute disease resembling encephalomyelitis followed by late chronic demyelinating disease, which is one of the best, if not the best, animal model for MS. During early acute disease the virus replicates in gray matter of the central nervous system but is eliminated to very low titers 2 weeks postinfection. Late chronic demyelinating disease becomes clinically apparent approximately 2 weeks later and is characterized by extensive demyelinating lesions and mononuclear cell infiltrates, progressive spinal cord atrophy, and axonal loss. Myelin damage is immunologically mediated, but it is not clear whether it is due to molecular mimicry or epitope spreading. Cytokines, nitric oxide/reactive nitrogen species, and costimulatory molecules are involved in the pathogenesis of both diseases. Close similarities between Theiler's virus-induced demyelinating disease in mice and MS in humans, include the following: major histocompatibility complex-dependent susceptibility; substantial similarities in neuropathology, including axonal damage and remyelination; and paucity of T-cell apoptosis in demyelinating disease. Both diseases are immunologically mediated. These common features emphasize the close similarities of Theiler's virus-induced demyelinating disease in mice and MS in humans.

Immunohistochemical detection of osteopontin in the spinal cords of mice with Theiler's murine encephalomyelitis virus-induced demyelinating disease

The spinal cords of mice that were infected with the BeAn 8386 strain of Theiler's murine encephalomyelitis virus (TMEV) were studied to elucidate the involvement of osteopontin in the course of TMEV-induced demyelination. Immunohistochemistry showed staining for osteopontin in the vessels of the normal spinal cords, and more intense immunoreactivity in the vessels within the demyelinating lesions. Intense osteopontin immunoreactivity was observed in the cell bodies, as well as in the extracellular space of the demyelinating lesions, where some glial cells, which included activated microglia/macrophages, were also immunopositive for osteopontin. These findings suggest that osteopontin is upregulated in the demyelinating spinal cord, and that osteopontin from either microglia or astrocytes may be involved in the chemotaxis of inflammatory cells and astrocytes, which ultimately leads to chronic inflammation and astrogliosis in this model system.

Apoptosis of infiltrating T cells in the central nervous system of mice infected with Theiler's murine encephalomyelitis virus

Theiler murine encephalomyelitis virus (TMEV), DA strain, induces in susceptible strain of mice a biphasic disease consisting of early acute disease followed by late chronic demyelinating disease. Both phases of the disease are associated with inflammatory infiltrates of the central nervous system (CNS). Late chronic demyelinating disease induced by TMEV serves as an excellent model to study human demyelinating disease, multiple sclerosis. During early acute disease, the virus is partially cleared from the CNS by CD3(+) T cells. These T cells express Fas, FasL, negligible levels of Bcl-2 proteins and undergo activation-induced cell death as determined by TUNEL assay leading to resolution of the inflammatory response. In contrast, during late chronic demyelinating disease, and despite dense perivascular and leptomeningeal infiltrates, only very few cells undergo apoptosis. Mononuclear cells infiltrating the CNS express Bcl-2. It appears that the lack of apoptosis of T cells during late chronic demyelinating disease leads to the accumulation of these cells in the CNS. These cells may play a role in the pathogenesis of the demyelinating disease.

Production of nitric oxide by glial cells: regulation and potential roles in the CNS

Roles proposed for nitric oxide (NO) in CNS pathophysiology are increasingly diverse and range from intercellular signaling, through necrotic killing of cells and invading pathogens, to the involvement of NO in apoptosis and tissue remodeling. In vitro evidence and observations from experimental animal models of a variety of human neuropathologies, including stroke, indicate that glial cells can produce NO. Regulation of at least one of the NO synthase genes (NOS-2) in glia has been well described; however, apart from hints emerging out of co-culture studies and extrapolation based upon the reactivity of NO, we are a long way from identifying functions for glial-derived NO in the CNS. Although the assumption is that NO is very often cytotoxic, it is evident that NO production does not always equate with tissue damage, and that both the cellular source of NO and the timing of NO production are important factors in terms of its effects. With the development of strategies to transfer or manipulate expression of the NOS genes in specific cells in situ, the ability to deliver NO into the CNS via long-lived chemical donors, and the emergence of more selective NOS inhibitors, an appreciation of the significance of glial-derived NO will change.