Oligodendrocytes are damaged by neuromyelitis optica immunoglobulin G via astrocyte injury

Abstract

Devic's neuromyelitis optica is an inflammatory demyelinating disorder normally restricted to the optic nerves and spinal cord. Since the identification of a specific autoantibody directed against aquaporin 4, neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody, neuromyelitis optica has been considered an entity distinct from multiple sclerosis. Recent findings indicate that the neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody has a pathogenic role through complementdependent astrocyte toxicity. However, the link with demyelination remains elusive. Autoantibodies can act as receptor agonists/antagonists or alter antigen density in their target cells. We hypothesized that the neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody impairs astrocytic function and secondarily leads to demyelination. Rat astrocytes and oligodendrocytes from primary cultures and rat optic nerves were exposed long-term (24 h) to immunoglobulin G in the absence of complement. Immunoglobulin G was purified from the serum of patients with neuromyelitis optica who were either neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody positive or negative, as well as from healthy controls. Flow cytometry analysis showed a reduction of membrane aquaporin 4 and glutamate transporter type 1 on astrocytes following contact with immunoglobulin G purified from neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody positive serum only. The activity of glutamine synthetase, an astrocyte enzyme converting glutamate into glutamine, decreased in parallel, indicating astrocyte dysfunction. Treatment also reduced oligodendrocytic cell processes and approximately 30% oligodendrocytes died. This deleterious effect was confirmed ex vivo; exposed optic nerves showed reduction of myelin basic protein. Immunoglobulin G from neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody seronegative patients and from healthy controls had no similar effect. Neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody did not directly injure oligodendrocytes cultured without astrocytes. A toxic bystander effect of astrocytes damaged by neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody on oligodendrocytes was identified. Progressive accumulation of glutamate in the culture medium of neuromyelitis opticaimmunoglobulin G/aquaporin 4-antibody-treated glial cells supported the hypothesis of a glutamate-mediated excitotoxic death of oligodendrocytes in our models. Moreover, co-treatment of glial cultures with neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody and D+2-amino-5-phosphonopentanoic acid, a competitive antagonist at the N-methyl-D-aspartate/ glutamate receptor, partially protected oligodendrocytes. Co-immunolabelling of oligodendrocyte markers and neuromyelitis optica-immunoglobulin G/aquaporin 4 antibody showed that astrocytic positive processes were in close contact with oligodendrocytes and myelin in rat optic nerves and spinal cord, but far less so in other parts of the central nervous system. This suggests a bystande...

In vivo imaging reveals rapid astrocyte depletion and axon damage in a model of neuromyelitis optica‐related pathology

“…Based on our experience,21, 22, 32 use of recombinant reagents, and detailed controls, neither additional components in human samples, imaging, or transgenic labeling can account for the axonal swellings (for details, see Materials and Methods). Although we cannot exclude a possible direct “by‐stander” attack of activated complement factors or an indirect effect by another cell type,39 the timing, lack of associated pathology, and localization of the axonal changes make these unlikely, and suggest that axonal swelling is the consequence of astrocyte loss. Indeed, this is not entirely surprising,40, 41, 42 given that numerous vital interactions between astrocytes and axons have been documented43, 44, 45, 46: For example, astrocytes are known to scavenge glutamate,47 and excitotoxicity has been implicated in axonal pathology during neuroinflammation 48, 49, 50.…”

Elevated cerebrospinal fluid uric acid during relapse of neuromyelitis optica spectrum disorders

“…Oxidative stress and glutamate‐mediated excitotoxicity plays an important role in NMOSD pathogeny (Marignier et al., 2010). CSF UA levels could reflect oxidative stress and glutamate‐mediated excitotoxicity in neurological patients (Stover et al., 1997), and it has been suggested (Stover et al., 1997) that increasing oxidative stress is reflected by UA and elevated uric acid levels are associated with glutamate‐mediated excitotoxicity in CSF.…”

Evidence against Cellular Internalization in Vivo of NMO-IgG, Aquaporin-4, and Excitatory Amino Acid Transporter 2 in Neuromyelitis Optica

“…One study also reported internalization of glutamate transporter EAAT2 together with AQP4 (13) and concluded that NMO pathogenesis involves glutamate excitotoxicity by a mechanism involving NMO-IgG-induced internalization of EAAT2 and consequent impairment in glutamate uptake from the extracellular space following neuroexcitation. Marignier et al (15) showed that elevated extracellular glutamate could injure oligodendrocytes in an astrocyte/oligodendrocyte mixed primary cell culture. They suggested glutamate excitotoxicity as a possible cause of demyelination in NMO.…”