Neurological autoimmunity targeting aquaporin-4

Update on aquaporin-4 antibody detection: the early diagnosis of neuromyelitis optica spectrum disorders

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune-mediated primary inflammatory myelinopathy of the central nervous system that primarily affects the optic nerve and spinal cord. The aquaporin 4 antibody (AQP4-Ab) is a specific autoantibody marker for NMOSD. Most patients with NMOSD are seropositive for AQP4-Ab, thus aiding physicians in identifying ways to treat NMOSD. AQP4-Ab has been tested in many clinical and laboratory studies, demonstrating effectiveness in diagnosing NMOSD. Recently, novel assays have been developed for the rapid and accurate detection of AQP4-Ab, providing further guidance for the diagnosis and treatment of NMOSD. This article summarizes the importance of rapid and accurate diagnosis for treating NMOSD based on a review of the latest relevant literature. We discussed current challenges and methods for improvement to offer new ideas for exploring rapid and accurate AQP4-Ab detection methods, aiming for early diagnosis of NMOSD.

Functional changes in neuronal circuits due to antibody-driven autoimmune response

Autoimmune-mediated encephalitis syndromes are increasingly being recognized as important clinical entities. They need to be thought of as differential diagnosis in any patient presenting with fast-onset psychosis or psychiatric problems, memory deficits or other cognitive problems, including aphasias, as well as seizures or motor automatisms, but also rigidity, paresis, ataxia or dystonic / parkinsonian symptoms. Diagnosis including imaging and CSF search for antibodies needs to be fast, as progression of these inflammatory processes is often causing scarring of brain tissue, with hypergliosis and atrophy. As these symptoms show, the autoantibodies present in these cases appear to act within the CNS. Several of such antibodies have by now been identified such as IgG directed against NMDA-receptors, AMPA receptors, GABA_A and GABA_B receptors, and voltage gated potassium channels and proteins of the potassium channel complex (i.e. LGI1 and CASPR2). These are neuropil / surface antigens where antibody interaction can well be envisaged to cause dysfunction of the target protein, including internalization. Others, such as antibodies directed against GAD65 (an intracellular enzyme responsible for GABA-synthesis from glutamate), are discussed to constitute epiphenomena, but not causal agents in disease progression. This review will focus on the current knowledge of antibody interaction mechanisms, especially discussing cellular excitability changes and synaptic interactions in hippocampal and other brain networks. One challenge in this context is to find viable hypotheses for the emergence of both, hyperexcitability and seizures, and presumably reduced synaptic plasticity and underlying cognitive dysfunction.

Dynamics of AQP4 upon exposure to seropositive patient serum before and after Rituximab therapy in Neuromyelitis Optica: A cell-based study

Neuromyelitis Optica (NMO) is an autoimmune inflammatory disease that affects the optic nerves and spinal cord. The autoantibody is generated against the abundant water channel protein of the brain, Aquaporin 4 (AQP4). Of the two isoforms of AQP4, the shorter one (M23) often exists as a supramolecular assembly known as an orthogonal array of particles (OAPs). There have been debates about the fate of these AQP4 clusters upon binding to the antibody, the exact mechanism of its turnover, and the proteins associated with the process. Recently several clinical cases of NMO were reported delineating the effect of Rituximab (RTX) therapy. Extending these reports at the cell signaling level, we developed a glioma based cellular model that mimicked antibody binding and helped us track the subsequent events including a variation of AQP4 levels, alterations in cellular morphology, and the changes in downstream signaling cascades. Our results revealed the extent of perturbations in the signaling pathways related to stress involving ERK, JNK, and AKT1 together with markers for cell death. We could also decipher the possible routes of degradation of AQP4, post-exposure to antibody. We further investigated the effect of autoantibody on AQP4 transcriptional level and involvement of FOXO3a and miRNA-145 in the regulation of transcription. This study highlights the differential outcome at the cellular level when treated with the serum of the same patient pre and post RTX therapy and for the first time mechanistically describes the effect of RTX.

Molecular Level Characterization of Circulating Aquaporin-4 Antibodies in Neuromyelitis Optica Spectrum Disorder

OBJECTIVE

To determine whether distinct aquaporin-4 (AQP4)-IgG lineages play a role in neuromyelitis optica spectrum disorder (NMOSD) pathogenesis, we profiled the AQP4-IgG polyclonal serum repertoire and identified, quantified, and functionally characterized distinct AQP4-IgG lineages circulating in 2 patients with NMOSD.

METHODS

We combined high-throughput sequencing and quantitative immunoproteomics to simultaneously determine the constituents of both the B-cell receptor (BCR) and the serologic (IgG) anti-AQP4 antibody repertoires in the peripheral blood of patients with NMOSD. The monoclonal antibodies identified by this platform were recombinantly expressed and functionally characterized in vitro.

RESULTS

Multiple antibody lineages comprise serum AQP4-IgG repertoires. Their distribution, however, can be strikingly different in polarization (polyclonal vs pauciclonal). Among the 4 serum AQP4-IgG monoclonal antibodies we identified in 2 patients, 3 induced complement-dependent cytotoxicity in a model mammalian cell line (p < 0.01).

CONCLUSIONS

The composition and polarization of AQP4-IgG antibody repertoires may play an important role in NMOSD pathogenesis and clinical presentation. Here, we present a means of coupling both cellular (BCR) and serologic (IgG) antibody repertoire analysis, which has not previously been performed in NMOSD. Our analysis could be applied in the future to clinical management of patients with NMOSD to monitor disease activity over time as well as applied to other autoimmune diseases to facilitate a deeper understanding of disease pathogenesis relative to autoantibody clones.

Connexins in neuromyelitis optica: a link between astrocytopathy and demyelination

Neuromyelitis optica, a rare neuroinflammatory demyelinating disease of the CNS, is characterized by the presence of specific pathogenic autoantibodies directed against the astrocytic water channel aquaporin 4 (AQP4) and is now considered as an astrocytopathy associated either with complement-dependent astrocyte death or with astrocyte dysfunction. However, the link between astrocyte dysfunction and demyelination remains unclear. We propose glial intercellular communication, supported by connexin hemichannels and gap junctions, to be involved in demyelination process in neuromyelitis optica. Using mature myelinated cultures, we demonstrate that a treatment of 1 h to 48 h with immunoglobulins purified from patients with neuromyelitis optica (NMO-IgG) is responsible for a complement independent demyelination, compared to healthy donors' immunoglobulins (P < 0.001). In parallel, patients' immunoglobulins induce an alteration of connexin expression characterized by a rapid loss of astrocytic connexins at the membrane followed by an increased size of gap junction plaques (+60%; P < 0.01). This was co-observed with connexin dysfunction with gap junction disruption (-57%; P < 0.001) and increased hemichannel opening (+17%; P < 0.001), associated with glutamate release. Blocking connexin 43 hemichannels with a specific peptide was able to prevent demyelination in co-treatment with patients compared to healthy donors' immunoglobulins. By contrast, the blockade of connexin 43 gap junctions with another peptide was detrimental for myelin (myelin density -48%; P < 0.001). Overall, our results suggest that dysregulation of connexins would play a pathogenetic role in neuromyelitis optica. The further identification of mechanisms leading to connexin dysfunction and soluble factors implicated, would provide interesting therapeutic strategies for demyelinating disorders.

Intractable Vomiting and Hiccups: An Atypical Presentation of Neuromyelitis Optica

Neuromyelitis optica is an inflammatory disorder of the central nervous system. It involves the immune-mediated demyelination of predominantly the optic nerves and the spinal cord, which can lead to optic neuritis and transverse myelitis, respectively. Patients usually present with symptoms related to the eyes or the spinal cord, like loss of vision, pain in the eyes, visual field defects or numbness and weakness of limbs. Vomiting and hiccups are common cases encountered in medicine clinics and can sometimes be an atypical presentation of this disorder. Here we present a case of a 33-year-old female who initially presented to our tertiary care centre with repeated episodes of bilious vomiting and intractable hiccups for 10 days. After multiple investigations over a couple of days, the patient was found to be positive for anti-NMO antibodies and displayed neuro-radiological findings on MRI brain and spine, which finally led to the diagnosis of neuromyelitis optica spectrum disorder (NMOSD). Through this case we highlight the importance of suspecting NMO in a patient with complaints of intractable vomiting and hiccups, so that early intervention and treatment can prevent further disabling complications of the disease.

Role of complement and potential of complement inhibitors in myasthenia gravis and neuromyelitis optica spectrum disorders: a brief review

The complement system is a powerful member of the innate immune system. It is highly adept at protecting against pathogens, but exists in a delicate balance between its protective functions and overactivity, which can result in autoimmune disease. A cascade of complement proteins that requires sequential activation, and numerous complement regulators, exists to regulate a proportionate response to pathogens. In spite of these mechanisms there is significant evidence for involvement of the complement system in driving the pathogenesis of variety of diseases including neuromyelitis optica spectrum disorders (NMOSD) and myasthenia gravis (MG). As an amplification cascade, there are an abundance of molecular targets that could be utilized for therapeutic intervention. Clinical trials assessing complement pathway inhibition in both these conditions have recently been completed and include the first randomized placebo-controlled trial in NMOSD showing positive results. This review aims to review and update the reader on the complement system and the evolution of complement-based therapeutics in these two disorders.

Complement-independent bystander injury in AQP4-IgG seropositive neuromyelitis optica produced by antibody-dependent cellular cytotoxicity

Cellular injury in AQP4-IgG seropositive neuromyelitis spectrum disorder (herein called NMO) involves AQP4-IgG binding to astrocytes, resulting in astrocyte injury by complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) mechanisms. The rapid disease progression, severe tissue damage, and abundant leukocyte infiltration seen in some NMO patients suggest a more direct mechanism for demyelination and neurologic deficit than secondary injury from astrocyte loss. Here, we report evidence for an ‘ADCC bystander mechanism’ in NMO involving injury to nearby cells by leukocytes following their activation by AQP4-bound AQP4-IgG on astrocytes. In model cocultures containing AQP4-expressing and null CHO cells, AQP4-IgG and complement killed bystander null cells to ~ 100 μm away from AQP4-expressing cells; AQP4-IgG and NK cells produced bystander killing to ~ 300 μm, with perforin deposition seen on injured null cells. Bystander cytotoxicity was also seen with neutrophil-mediated ADCC and in astrocyte-neuron cocultures. Mechanistic studies, including real-time imaging, suggested that leukocytes activated by an AQP4-dependent ADCC mechanism injure bystander cells by direct targeted exocytosis on neighboring cells and not by diffusion of soluble granule contents. In support of this conclusion, ADCC bystander injury was preferentially reduced by an RGDS peptide that inhibits integrin adhesion. Evidence for ADCC bystander injury to oligodendrocytes and neurons was also found in mice following intracerebral injection of AQP4-IgG and NK cells, which was inhibited by RGDS peptide. These results establish a novel cellular pathogenesis mechanism in AQP4-IgG seropositive NMO and provide evidence that inflammatory mechanisms can cause widespread tissue damage in NMO independently of the secondary effects from astrocyte loss.

Role of Glutamatergic Excitotoxicity in Neuromyelitis Optica Spectrum Disorders

Neuromyelitis optica spectrum disorder (NMOSD) is an inflammatory disorder mediated by immune-humoral responses directed against central nervous system (CNS) antigens. Most patients are positive for specific immunoglobulin G (IgG) auto-antibodies for aquaporin-4 (AQP4), a water channel present in astrocytes. Antigen-antibody binding promotes complement system cascade activation, immune system cell infiltration, IgG deposition, loss of AQP4 and excitatory amino acid transporter 2 (EAAT2) expression on the astrocytic plasma membrane, triggering necrotic destruction of spinal cord tissue and optic nerves. Astrocytes are very important cells in the CNS and, in addition to supporting other nerve cells, they also regulate cerebral homeostasis and control glutamatergic synapses by modulating neurotransmission in the cleft through the high-affinity glutamate transporters present in their cell membrane. Specific IgG binding to AQP4 in astrocytes blocks protein functions and reduces EAAT2 activity. Once compromised, EAAT2 cannot take up free glutamate from the extracellular space, triggering excitotoxicity in the cells, which is characterized by overactivation of glutamate receptors in postsynaptic neurons. Therefore, the longitudinally extensive myelitis and optic neuritis lesions observed in patients with NMOSD may be the result of primary astrocytic damage triggered by IgG binding to AQP4, which can activate the immune-system cascade and, in addition, downregulate EAAT2. All these processes may explain the destructive lesions in NMOSD secondary to neuroinflammation and glutamatergic excitotoxicity. New or repurposed existing drugs capable of controlling glutamatergic excitotoxicity may provide new therapeutic options to reduce tissue damage and permanent disability after NMOSD attacks.

Disruption of blood-brain barrier integrity associated with brain lesions in Chinese neuromyelitis optica spectrum disorder patients

OBJECTIVE

The aims of this study were to report brain characteristic abnormalities and to evaluate the relationship of blood-brain barrier (BBB) disruption and brain lesions in Chinese patients with NMOSD.

METHODS

Brain magnetic resonance imaging characteristics and cerebrospinal fluid (CSF) laboratory tests of 121 patients with NMOSD at acute attack were reviewed retrospectively. Qalb (CSF albumin/serum albumin) was used for assessment of disruption of BBB.

RESULTS

Brain MRI abnormalities were observed in 36.4% (44/121) of the NMOSD patients. Thirty patients (25%) showed typical-NMOSD abnormalities, including dorsal medulla lesions (n = 16, 13.2%), brainstem/cerebellum (n = 11, 9.1%), thalamus/hypothalamus (n = 3, 2.5%), periventricular white matter lesions (n = 4, 3.3%) hemispheric white matter (n = 4, 3.3%). Twenty-five patients (20.7%) had nonspecific lesions. Compared to the NMOSD patients without brain lesion, the proportion of patients who had abnormal BBB permeability was significantly higher in the abnormal brain MRI group (47.7% vs. 27.3%, P < 0.05). BBB permeability was not correlated to distribution of brain lesions or enhancement lesions. Qalb was associated with higher Expanded Disability Status Scale scores (r = 0.689, P < 0.05).

CONCLUSIONS

Brain lesions are common in NMOSD patients. Marker of BBB permeability is associated with brain lesion and EDSS scores of NMOSD.

Prognostic factors and disease course in aquaporin-4 antibody-positive Chinese patients with acute optic neuritis

The objective is to assess the frequency of AQP4-Ab in Chinese patients with optic neuritis (ON) and to investigate the prognostic implications of AQP4-Ab seropositivity in such patients. The presence of aquaporin-4 (AQP4) antibodies in human AQP4-transfected cells was determined by indirect immunofluorescence, and the diagnostic and prognostic relevance of AQP4 antibodies in 215 Chinese patients with optic neuritis was evaluated. The patients were enrolled from the Chinese People's Liberation Army General Hospital (PLAGH) and followed up to 1-228 months. Kaplan-Meier survival analysis was used to investigate the visual outcomes and prognostic characteristics of 70 patients who were aquaporin-4 antibody seropositive, and altogether 115 eyes were involved. AQP4-Ab was detectable in 70/215 (32.6%) patients with acute monosymptomatic optic neuritis. These AQP4-Ab seropositive patients were diagnosed as sufferers of neuromyelitis optica (NMO) (n = 10), NMO spectrum disease (NMOSD) (n = 41), multiple sclerosis (MS) (n = 1), and acute optic neuritis (AON) (n = 18). Moreover, AQP4-Ab levels did not vary between seropositive AON and NMO-ON and did not correlate with disease severity. Finally, the male gender, older age at onset, number of relapses, concomitant autoimmune antibodies, and optic lesions in magnetic resonance imaging (MRI) were associated with the AQP4-Ab seropositive status and the risk of developing permanent visual disability. Acute optic neuritis has a higher incidence of anti-aquaporin-4 antibody in China than in Western countries, and AQP4 antibody is a sensitive and specific biomarker for discriminating NMO-ON from other types of optic neuritis during the early stage of the disease course.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

Large-Scale in-House Cell-Based Assay for Evaluating the Serostatus in Patients with Neuromyelitis Optica Spectrum Disorder Based on New Diagnostic Criteria

BACKGROUND AND PURPOSE

The detection of aquaporin 4-IgG (AQP4-IgG) is now a critical diagnostic criterion for neuromyelitis optica spectrum disorder (NMOSD). To evaluate the serostatus of NMOSD patients based on the 2015 new diagnostic criteria using a new in-house cell-based assay (CBA).

METHODS

We generated a stable cell line using internal ribosome entry site-containing bicistronic vectors, which allow the simultaneous expression of two proteins (AQP4 and green fluorescent protein) separately from the same RNA transcript. We performed in-house CBA using serum from 386 patients: 178 NMOSD patients diagnosed according to the new diagnostic criteria without AQP4-IgG, 63 high risk NMOSD patients presenting 1 of the 6 core clinical characteristics of NMOSD but not fulfilling dissemination in space, and 145 patients with other neurological diseases, including 66 with multiple sclerosis. The serostatus of 111 definite and high risk NMOSD patients were also tested using a commercial CBA kit with identical serum to evaluate the correlation between the 2 methods. All assays were performed by two independent and blinded investigators.

RESULTS

Our in-house assay yielded a specificity of 100% and sensitivities of 80% (142 of 178) and 76% (48 of 63) when detecting definite- and high risk NMOSD patients, respectively. The comparison with the commercial CBA kit revealed a correlation for 102 of the 111 patients: no correlation was present in 7 patients who were seronegative using the commercial method but seropositive using the in-house method, and in 2 patients who were seropositive using the commercial method but seronegative using the in-house method.

CONCLUSIONS

These results demonstrate that our in-house CBA is a highly specific and sensitive method for detecting AQP4-IgG in NMOSD patients.

Injectable Hydrogels for Spinal Cord Repair: A Focus on Swelling and Intraspinal Pressure

Spinal cord injury (SCI) is a devastating condition that leaves patients with limited motor and sensory function at and below the injury site, with little to no hope of a meaningful recovery. Because of their ability to mimic multiple features of central nervous system (CNS) tissues, injectable hydrogels are being developed that can participate as therapeutic agents in reducing secondary injury and in the regeneration of spinal cord tissue. Injectable biomaterials can provide a supportive substrate for tissue regeneration, deliver therapeutic factors, and regulate local tissue physiology. Recent reports of increasing intraspinal pressure after SCI suggest that this physiological change can contribute to injury expansion, also known as secondary injury. Hydrogels contain high water content similar to native tissue, and many hydrogels absorb water and swell after formation. In the case of injectable hydrogels for the spinal cord, this process often occurs in or around the spinal cord tissue, and thus may affect intraspinal pressure. In the future, predictable swelling properties of hydrogels may be leveraged to control intraspinal pressure after injury. Here, we review the physiology of SCI, with special attention to the current clinical and experimental literature, underscoring the importance of controlling intraspinal pressure after SCI. We then discuss how hydrogel fabrication, injection, and swelling can impact intraspinal pressure in the context of developing injectable biomaterials for SCI treatment.

Restoring immune tolerance in neuromyelitis optica: Part I

Neuromyelitis optica (NMO) and spectrum disorder (NMO/SD) represent a vexing process and its clinical variants appear to have at their pathogenic core the loss of immune tolerance to the aquaporin-4 water channel protein. This process results in a characteristic pattern of astrocyte dysfunction, loss, and demyelination that predominantly affects the spinal cord and optic nerves. Although several empirical therapies are currently used in the treatment of NMO/SD, none has been proven effective in prospective, adequately powered, randomized trials. Furthermore, most of the current therapies subject patients to long-term immunologic suppression that can cause serious infections and development of cancers. The following is the first of a 2-part description of several key immune mechanisms in NMO/SD that might be amenable to therapeutic restoration of immune tolerance. It is intended to provide a roadmap for how potential immune tolerance restorative techniques might be applied to patients with NMO/SD. This initial installment provides a background rationale underlying attempts at immune tolerization. It provides specific examples of innovative approaches that have emerged recently as a consequence of technical advances. In several autoimmune diseases, these strategies have been reduced to practice. Therefore, in theory, the identification of aquaporin-4 as the dominant autoantigen makes NMO/SD an ideal candidate for the development of tolerizing therapies or cures for this increasingly recognized disease.

The Immunology of Neuromyelitis Optica-Current Knowledge, Clinical Implications, Controversies and Future Perspectives

Neuromyelitis optica (NMO) is an autoimmune, demyelinating disorder of the central nervous system (CNS) with typical clinical manifestations of optic neuritis and acute transverse myelitis attacks. Previously believed to be a variant of multiple sclerosis (MS), it is now considered an independent disorder which needs to be differentiated from MS. The discovery of autoantibodies against aquaporin-4 (AQP4-IgGs) changed our understanding of NMO immunopathogenesis and revolutionized the diagnostic process. AQP4-IgG is currently regarded as a specific biomarker of NMO and NMO spectrum disorders (NMOsd) and a key factor in its pathogenesis. Nevertheless, AQP4-IgG seronegativity in 10%-25% of NMO patients suggests that there are several other factors involved in NMO immunopathogenesis, i.e., autoantibodies against aquaporin-1 (AQP1-Abs) and antibodies against myelin oligodendrocyte glycoprotein (MOG-IgGs). This manuscript reviews current knowledge about NMO immunopathogenesis, pointing out the controversial issues and showing potential directions for future research. Further efforts should be made to broaden our knowledge of NMO immunology which could have important implications for clinical practice, including the use of potential novel biomarkers to facilitate an early and accurate diagnosis, and modern treatment strategies improving long-term outcome of NMO patients.

Differences in distribution and regulation of astrocytic aquaporin-4 in human and rat hydrocephalic brain

AIMS

Aquaporin-4 (AQP4) is the most abundant cellular water channel in brain and could be a molecular basis for a cerebrospinal fluid absorption route additional to the arachnoid villi. In the search for 'alternative' cerebrospinal fluid absorption pathways it is important to compare experimental findings with human pathophysiology. This study compares expression of AQP4 in hydrocephalic human brain with human controls and hydrocephalic rat brain.

METHODS

Cortical biopsies from patients with chronic hydrocephalus (n = 29) were sampled secondary to planned surgical intervention. AQP4 in human hydrocephalic cortex relative to controls was quantified by Western blotting (n = 28). A second biopsy (n = 13) was processed for immunohistochemistry [glial fibrillary acidic protein (GFAP), CD68, CD34 and AQP4] and double immunofluorescence (AQP4 + GFAP and AQP4 + CD34). Brain tissue from human controls and kaolin-induced hydrocephalic rats was processed in parallel. Immunohistochemistry and immunofluorescence were assessed qualitatively.

RESULTS

Western blotting showed that AQP4 abundance was significantly increased (P < 0.05) in hydrocephalic human brain compared with controls. AQP4 immunoreactivity was present in both white and grey matter. In human brain (hydrocephalic and controls) AQP4 immunoreactivity was found on the entire astrocyte membrane, unlike hydrocephalic rat brain where pronounced endfeet polarization was present. Endothelial AQP4 immunoreactivity was not observed.

CONCLUSIONS

This study shows a significant increase in astrocytic AQP4 in human hydrocephalic cortex compared with control. Cell type specific expression in astrocytes is conserved between rat and human, although differences of expression in specific membrane domains are seen. This study addresses direct translational aspects from rat to human, hereby emphasizing the relevance and use of models in hydrocephalus research.

Peripheral blood biomarkers in multiple sclerosis

Multiple sclerosis is the most common autoimmune disorder affecting the central nervous system. The heterogeneity of pathophysiological processes in MS contributes to the highly variable course of the disease and unpredictable response to therapies. The major focus of the research on MS is the identification of biomarkers in biological fluids, such as cerebrospinal fluid or blood, to guide patient management reliably. Because of the difficulties in obtaining spinal fluid samples and the necessity for lumbar puncture to make a diagnosis has reduced, the research of blood-based biomarkers may provide increasingly important tools for clinical practice. However, currently there are no clearly established MS blood-based biomarkers. The availability of reliable biomarkers could radically alter the management of MS at critical phases of the disease spectrum, allowing for intervention strategies that may prevent evolution to long-term neurological disability. This article provides an overview of this research field and focuses on recent advances in blood-based biomarker research.

Aquaporin-4 autoantibodies increase vasogenic edema formation and infarct size in a rat stroke model

Background

Neuromyelitis optica (NMO) is an autoimmune disorder of the central nervous system, which is characterized by autoantibodies directed against the water channel aquaporin-4 (AQP4). As one of the main water regulators in the central nervous system, APQ4 is supposed to be involved in the dynamics of brain edema. Cerebral edema seriously affects clinical outcome after ischemic stroke; we therefore aimed to investigate whether NMO-antibodies may exert the same functional effects as an AQP4-inhibitor in-vivo in acute ischemic stroke.

Methods

Sixteen male Wistar rats were randomized into two groups twice receiving either purified NMO-IgG or immune globulin from healthy controls, 24 hours and 30 minutes before middle cerebral artery occlusion (MCAO) was performed. T2-weighted MRI was carried out 24 hours after MCAO.

Results

MRI-examination showed a significant increase of infarct size in relation to the cerebral hemisphere volume with NMO-IgG treated animals (27.1% ± 11.1% vs. 14.3% ± 7.2%; p < 0.05) when corrected for the space-occupying effect of vasogenic edema formation and similar results without edema correction (34.4% ± 16.4% vs. 17.5% ± 9.3%; p < 0.05). Furthermore, T2-RT revealed a significant increase in cortical brain water content of the treatment group (19.5 ms ± 9.7 ms vs. 9.2 ms ± 5.2 ms; p < 0.05).

Conclusions

These results support the functional impact of NMO-antibodies and also offer an in-vivo-applicable animal model to investigate the properties of AQP4 in ischemic stroke.

Regulation of neurovascular coupling in autoimmunity to water and ion channels

Much progress has been made in understanding autoimmune channelopathies, but the underlying pathogenic mechanisms are not always clear due to broad expression of some channel proteins. Recent studies show that autoimmune conditions that interfere with neurovascular coupling in the central nervous system (CNS) can lead to neurodegeneration. Cerebral blood flow that meets neuronal activity and metabolic demand is tightly regulated by local neural activity. This process of reciprocal regulation involves coordinated actions of a number of cell types, including neurons, glia, and vascular cells. In particular, astrocytic endfeet cover more than 90% of brain capillaries to assist blood-brain barrier (BBB) function, and wrap around synapses and nodes of Ranvier to communicate with neuronal activity. In this review, we highlight four types of channel proteins that are expressed in astrocytes, regarding their structures, biophysical properties, expression and distribution patterns, and related diseases including autoimmune disorders. Water channel aquaporin 4 (AQP4) and inwardly rectifying potassium (Kir4.1) channels are concentrated in astrocytic endfeet, whereas some voltage-gated Ca(2+) and two-pore domain K(+) channels are expressed throughout the cell body of reactive astrocytes. More channel proteins are found in astrocytes under normal and abnormal conditions. This research field will contribute to a better understanding of pathogenic mechanisms underlying autoimmune disorders.

An immunoassay that distinguishes real neuromyelitis optica signals from a labeling detected in patients receiving natalizumab

BACKGROUND

Cell-based assays for neuromyelitis optica (NMO) diagnosis are the most sensitive and specific methods to detect anti-aquaporin 4 (AQP4) antibodies in serum, but some improvements in their quantitative and specificity capacities would be desirable. Thus the aim of the present work was to develop a sensitive quantitative method for detection of anti-AQP4 antibodies that allows clear diagnosis of NMO and distinction of false labeling produced by natalizumab treatment.

METHODS

Sera from 167 individuals, patients diagnosed with NMO (16), multiple sclerosis (85), optic neuritis (24), idiopathic myelitis (21), or other neurological disorders (13) and healthy controls (8), were used as the primary antibody in an immunofluorescence assay on HEK cells transfected with the M23 isoform of human AQP4 fused with enhanced green fluorescent protein. Cells used were freshly transfected or stored frozen and then thawed just before adding the serum.

RESULTS

Microscopic observation and fluorescence quantification produced similar results in fresh and frozen samples. Serum samples from patients diagnosed with NMO were 100% positive for anti-AQP4 antibodies, while all the other sera were negative. Using serum from patients treated with natalizumab, a small and unspecific fluorescent signal was produced from all HEK cells, regardless of AQP4 expression.

CONCLUSIONS

Our cell-based double-label fluorescence immunoassay protocol significantly increases the signal specificity and reduces false diagnosis of NMO patients, especially in those receiving natalizumab treatment. Frozen pretreated cells allow faster detection of anti-AQP4 antibodies.

Frequency of autoantibodies and connective tissue diseases in Chinese patients with optic neuritis

Background

Optic neuritis (ON) is often associated with other clinical or serological markers of connective tissue diseases (CTDs). To date, the effects of autoantibodies on ON are not clear.

Purpose

To assess the prevalence, clinical patterns, and short outcomes of autoantibodies and Sjögren’s syndrome (SS) involvement in Chinese ON patients and evaluate the relationship between ON, including their subtypes, and autoantibodies.

Methods

A total of 190 ON patients were divided into recurrent ON (RON), bilateral ON (BON), and isolated monocular ON (ION). Demographic, clinical, and serum autoantibodies data were compared between them with and without SS involvement. Serum was drawn for antinuclear antibody (ANA), extractable nuclear antigen antibodies (SSA/SSB), rheumatoid factor (RF), anticardiolipin antibodies (ACA), and anti-double-stranded DNA antibody (A-ds DNA), anticardiolipin antibody (ACLs), anti-β2-glycoprotein I (β2-GPI) and Aquaporin-4 antibodies (AQP4-Ab). Spectral-domain optical coherence tomography (SD-OCT) was used to evaluate the atrophy of the optic nerve.

Results

68 patients (35.79%) had abnormal autoantibodies, 26(13.68%) patients met diagnostic criteria for CTDs, including 15(7.89%) patients meeting the criteria for SS. Antibodies including SSA/SSB 23 (30.26%) (p1 and p 2<0.001) and AQP4–Ab10 (13.16%) (p1 = 0.044, p2 = 0.01) were significantly different in patients in the RON group when compared with those in the BON (P1 = RON VS ION) and ION (p2 = RON VS ION) groups. SS was more common in RON patients (p1 = 0.04, p2 = 0.028). There was no significant difference between SSA/SSB positive and negative patients in disease characteristics or severity. Similar results were obtained when SS was diagnosed in SSA/SSB positive patients.

Conclusion

RON and BON were more likely associated with abnormal autoantibodies; furthermore, AQP4 antibody, SSA/SSB and SS were more common in the RON patients. AQP4 antibodydetermination is crucial in RON patients who will develop NMO. However, when compared with other autoantibodies, SSA/SSB detected in patients was not significantly associated with disease characteristics or severity.

Paraneoplastic neuromyelitis optica spectrum disorder associated with metastatic carcinoid expressing aquaporin-4

IMPORTANCE

Reports of neuromyelitis optica spectrum disorder (NMOSD) occurring in the setting of neoplasia suggest that aquaporin-4 autoimmunity may in some cases have a paraneoplastic basis.

OBSERVATIONS

In this case report, we describe a patient with NMOSD whose test results were seropositive for aquaporin-4 IgG and who had a hepatic metastasis from a small-bowel neuroendocrine tumor. The tumor cells expressed aquaporin-4 immunoreactivity. She presented to the Neurology Department at Wayne State University with bilateral leg weakness, ascending paresthesias, and decreased sensation.

CONCLUSIONS AND RELEVANCE

This case extends the context of NMOSD as a paraneoplastic disorder.

Development, maintenance and disruption of the blood-brain barrier

The interface between the blood circulation and the neural tissue features unique characteristics that are encompassed by the term 'blood-brain barrier' (BBB). The main functions of this barrier, namely maintenance of brain homeostasis, regulation of influx and efflux transport, and protection from harm, are determined by its specialized multicellular structure. Every constituent cell type makes an indispensable contribution to the BBB's integrity. But if one member of the BBB fails, and as a result the barrier breaks down, there can be dramatic consequences and neuroinflammation and neurodegeneration can occur. In this Review, we highlight recently gained mechanistic insights into the development and maintenance of the BBB. We then discuss how BBB disruption can cause or contribute to neurological disease. Finally, we examine how this knowledge can be used to explore new possibilities for BBB repair.

Paraneoplastic and other autoimmune disorders of the central nervous system

As a result of the burgeoning growth of disease-specific neural autoantibody markers available for diagnostic patient evaluation, there has been increasing awareness of autoimmune central nervous system (CNS) disorders in hospital practice. Hospital-based neurologists have also taken great interest in these disorders since many occur in the setting of an occult systemic cancer which can be detected and treated at an early stage, and many affected patients are responsive to immunotherapy. Associated neurological disorders are typically subacute in onset, some are common or classic (eg, limbic encephalitis, cerebellar degeneration), but others have atypical or multifocal presentations. For patients with a suspected paraneoplastic disorder, many and costly oncological evaluations may be required for diagnosis. Comprehensive serological and cerebrospinal fluid (CSF) evaluation for neural autoantibodies may permit a focused cancer evaluation (eg, antineuronal nuclear antibody type 1 [ANNA-1] is associated with small cell lung carcinoma), and in some circumstances may indicate the likelihood of a good response to therapy (eg, voltage-gated potassium channel complex antibody) or poor neurological prognosis (eg, purkinje cell cytoplasmic antibody type 1 [antiYo]). Positron-emission tomography-computed tomography (PET-CT) imaging of trunk may increase the diagnostic yield for certain cancers where other modalities have been negative. For some patients, rapid treatment with immunotherapy may facilitate marked improvement, or full recovery; multiple sequential trials of one or more of steroids, intravenous immunoglobulin or plasma exchange, or combination therapy are often required. For patients with N-methyl-d-aspartate receptor antibody encephalitis, early treatment with immunosuppressants and weeks or months of supportive intensive care may additionally be required. One or more of clinical examination, electroencephalogram (including video telemetry), and imaging provide objective parameters to which posttreatment outcomes can be compared.

Brain-reactive antibodies and disease

Autoimmune diseases currently affect 5-7% of the world's population; in most diseases there are circulating autoantibodies. Brain-reactive antibodies are present in approximately 2-3% of the general population but do not usually contribute to brain pathology. These antibodies penetrate brain tissue only early in development or under pathologic conditions. This restriction on their pathogenicity and the lack of correlation between serum titers and brain pathology have, no doubt, contributed to a delayed appreciation of the contribution of autoantibodies in diseases of the central nervous system. Nonetheless, it is increasingly clear that antibodies can cause damage in the brain and likely initiate or aggravate multiple neurologic conditions; brain-reactive antibodies contribute to symptomatology in autoimmune disease, infectious disease, and malignancy.

Presence of six different lesion types suggests diverse mechanisms of tissue injury in neuromyelitis optica

Neuromyelitis optica (NMO) is an autoimmune disease targeting aquaporin 4 (AQP4), localized mainly at the astrocytic foot processes. Loss of AQP4 and glial fibrillary acidic protein (GFAP) was reported, but the pathological significance of astrocytopathy is still controversial. Here we show that active lesions in NMO display a wide spectrum of pathology even within a single tissue block of an individual patient. We have distinguished six different lesion types. The first reflects complement deposition at the surface of astrocytes, associated with granulocyte infiltration and astrocyte necrosis and followed by demyelination, global tissue destruction and the formation of cystic, necrotic lesions (lesion type 2). Such destructive lesions lead to Wallerian degeneration in lesion-related tracts (lesion type 3). Around active NMO lesions AQP4 may selectively be lost in the absence of aquaporin 1 (AQP1) loss or other structural damage (lesion type 4). Another pattern is characterized by clasmatodendrosis of astrocytes, defined by cytoplasmic swelling and vacuolation, beading and dissolution of their processes and nuclear alterations resembling apoptosis, which was associated with internalization of AQP4 and AQP1 and astrocyte apoptosis in the absence of complement activation. Such lesions give rise to extensive astrocyte loss, which may occur in part in the absence of any other tissue injury, such as demyelination or axonal degeneration (lesion type 5). Finally, lesions with a variable degree of astrocyte clasmatodendrosis are found, which show plaque-like primary demyelination that is associated with oligodendrocyte apoptosis, but with preservation of axons (lesion type 6). In active multiple sclerosis (MS) lesions astrocytes reveal changes of reactive protoplasmatic or fibrillary gliosis. Only in a subset of lesions, in patients with aggressive disease, loss of AQP4 is observed in the initial stage of their formation, which is associated with retraction of astrocyte processes in the absence of complement deposition, granulocyte infiltration or loss of AQP1 or astrocytes. Our data underline the primary assault of astrocytes in NMO lesions, but also indicate that different mechanisms of tissue injury operate in parallel in the same patient and even within the same lesion.

Intractable nausea and vomiting from autoantibodies against a brain water channel

BACKGROUND & AIMS

Antibodies against the water channel protein aquaporin (AQP)-4 cause a spectrum of inflammatory, demyelinating, central nervous system disorders called neuromyelitis optica spectrum disorders (NMOSDs); these primarily affect the optic nerves and spinal cord but also the brain. Symptoms of intractable nausea, vomiting, and hiccups reflect involvement of AQP4 in the brainstem area postrema and account for gastroenterological presentations. We investigated the frequency of intractable nausea, vomiting, or hiccups in patients with NMOSD who tested positive for immunoglobulin G against AQP4 (AQP4-IgG). We also analyzed sera from patients with idiopathic nausea or vomiting for the presence of AQP4-IgG.

METHODS

We reviewed the Mayo Clinic AQP4-IgG positive NMOSD database (n = 70) to identify patients who presented with vomiting, focusing on results from gastroenterological evaluations. We also tested serum samples (from the Gastroparesis Clinical Research Consortium repository) from patients who presented with idiopathic nausea or vomiting for AQP4-IgG (controls, n = 318 with gastroparesis and 117 without gastroparesis).

RESULTS

Ten AQP4-IgG-positive patients diagnosed with NMOSD (14% of patients in the database) initially presented with intractable vomiting. Extensive gastroenterological evaluation was noninformative. AQP4-IgG was not detected in any of the controls.

CONCLUSIONS

Although NMOSDs are rare, tests for AQP4-IgG should be considered for patients who present with unexplained, intractable vomiting. Detection of the antibody before the development of optic neuritis or transverse myelitis allows patients to receive immunosuppressive therapy before the development of neurologic disabilities.

Blood-brain barrier disruption is more severe in neuromyelitis optica than in multiple sclerosis and correlates with clinical disability

OBJECTIVES

This study evaluated blood-brain barrier (BBB) integrity, using blood and cerebrospinal fluid (CSF) markers, and assessed the practicality of these markers in the differential diagnosis of neuromyelitis optica (NMO) and multiple sclerosis (MS).

METHODS

This was a retrospective observational study of consecutive patients presenting with acute phase NMO or MS (first attack or relapse). Haematological tests (including antiaquaporin-4 antibody levels) and CSF parameters (using primary component analyses) were undertaken; the correlation between BBB permeability and disease severity (by Expanded Disability Status Scale [EDSS] score) was examined.

RESULTS

Levels of several markers of BBB permeability were higher in patients with NMO (n=21) than in those with MS (n=52). The CSF:serum albumin ratio (AR) was the one of the main differentiators of NMO and MS. Additionally, there was a significant correlation between AR and clinical severity for NMO but not for MS.

CONCLUSIONS

Markers of BBB permeability were significantly higher in NMO patients than in MS patients. AR was the best marker for differentiating NMO and MS. Thus, measurement of BBB disruption markers (such as AR) might help to differentiate the diagnosis of NMO and MS in acute clinical settings.

Striatal adenosine signaling regulates EAAT2 and astrocytic AQP4 expression and alcohol drinking in mice

Adenosine signaling is implicated in several neuropsychiatric disorders, including alcoholism. Among its diverse functions in the brain, adenosine regulates glutamate release and has an essential role in ethanol sensitivity and preference. However, the molecular mechanisms underlying adenosine-mediated glutamate signaling in neuroglial interaction remain elusive. We have previously shown that mice lacking the ethanol-sensitive adenosine transporter, type 1 equilibrative nucleoside transporter (ENT1), drink more ethanol compared with wild-type mice and have elevated striatal glutamate levels. In addition, ENT1 inhibition or knockdown reduces glutamate transporter expression in cultured astrocytes. Here, we examined how adenosine signaling in astrocytes contributes to ethanol drinking. Inhibition or deletion of ENT1 reduced the expression of type 2 excitatory amino-acid transporter (EAAT2) and the astrocyte-specific water channel, aquaporin 4 (AQP4). EAAT2 and AQP4 colocalization was also reduced in the striatum of ENT1 null mice. Ceftriaxone, an antibiotic compound known to increase EAAT2 expression and function, elevated not only EAAT2 but also AQP4 expression in the striatum. Furthermore, ceftriaxone reduced ethanol drinking, suggesting that ENT1-mediated downregulation of EAAT2 and AQP4 expression contributes to excessive ethanol consumption in our mouse model. Overall, our findings indicate that adenosine signaling regulates EAAT2 and astrocytic AQP4 expressions, which control ethanol drinking in mice.

ASTROCYTES: EMERGING STARS IN LEUKODYSTROPHY PATHOGENESIS

Astrocytes are the predominant glial cell population in the central nervous system (CNS). Once considered only passive scaffolding elements, astrocytes are now recognised as cells playing essential roles in CNS development and function. They control extracellular water and ion homeostasis, provide substrates for energy metabolism, and regulate neurogenesis, myelination and synaptic transmission. Due to these multiple activities astrocytes have been implicated in almost all brain pathologies, contributing to various aspects of disease initiation, progression and resolution. Evidence is emerging that astrocyte dysfunction can be the direct cause of neurodegeneration, as shown in Alexander's disease where myelin degeneration is caused by mutations in the gene encoding the astrocyte-specific cytoskeleton protein glial fibrillary acidic protein. Recent studies point to a primary role for astrocytes in the pathogenesis of other genetic leukodystrophies such as megalencephalic leukoencephalopathy with subcortical cysts and vanishing white matter disease. The aim of this review is to summarize current knowledge of the pathophysiological role of astrocytes focusing on their contribution to the development of the above mentioned leukodystrophies and on new perspectives for the treatment of neurological disorders.

Myelination: do astrocytes play a role?

Astrocytes are the most abundant cell type in the adult central nervous system (CNS), and their functional diversity in response to injury is now being appreciated. Astrocytes have long been considered the main player in the inhibition of CNS repair via the formation of the gliotic scar, but now it is accepted that astrocyte can play an important role in CNS repair and remyelination. Interest in the relationship between astrocytes and myelination focused initially on attempts to understand how the development of plaques of astroglial scar tissue in multiple sclerosis was related to the failure of these lesions to remyelinate. It is now considered that this is an end stage pathological response to injury, and that normally astrocytes play important roles in supporting the development and maintenance of CNS myelin. This review will focus on how this new understanding may be exploited to develop new strategies to enhance remyelination in multiple sclerosis and other diseases.

Aggravated inflammation and increased expression of cysteinyl leukotriene receptors in the brain after focal cerebral ischemia in AQP4-deficient mice

OBJECTIVE

Aquaporin-4 (AQP4), the main water channel protein in the brain, plays a critical role in water homeostasis and brain edema. Here, we investigated its role in the inflammatory responses after focal cerebral ischemia.

METHODS

In AQP4-knockout (KO) and wild-type mice, focal cerebral ischemia was induced by 30 min of middle cerebral arterial occlusion (MCAO). Ischemic neuronal injury and cellular inflammatory responses, as well as the expression and localization of cysteinyl leukotriene CysLT(2) and CysLT(1) receptors, were determined at 24 and 72 h after MCAO.

RESULTS

AQP4-KO mice showed more neuronal loss, more severe microglial activation and neutrophil infiltration, but less astrocyte proliferation in the brain after MCAO than wild-type mice. In addition, the protein levels of both CysLT(1) and CysLT(2) receptors were up-regulated in the ischemic brain, and the up-regulation was more pronounced in AQP4-KO mice. The CysLT(1) and CysLT(2) receptors were primarily localized in neurons, microglia and neutrophils; those localized in microglia and neutrophils were enhanced in AQP4-KO mice.

CONCLUSION

AQP4 may play an inhibitory role in postischemic inflammation.

Personalized medicine in multiple sclerosis: hope or reality?

Personalized treatment is highly desirable in multiple sclerosis because it is an immensely heterogeneous disease. This heterogeneity is seen in both the disease course and the treatment responses. Currently, a combination of clinical features and imaging parameters in magnetic resonance imaging is used to classify active and non-active patients and treatment responders and non-responders. Although this classification works on a group level, individual patients often behave differently from the group. Therefore additional biomarkers are needed to provide better indicators for prognosis and treatment response. Basic and clinical research have discovered different promising targets. It is now essential to verify the utility and accuracy of these markers in large, prospectively sampled patient cohorts.

IκBα deficiency in brain leads to elevated basal neuroinflammation and attenuated response following traumatic brain injury: implications for functional recovery

Background

The transcription factor NFκB is an important mediator of cell survival and inflammation in the immune system. In the central nervous system (CNS), NFκB signaling has been implicated in regulating neuronal survival following acute pathologic damage such as traumatic brain injury (TBI) and stroke. NFκB is normally bound by the principal inhibitory protein, IκBα, and sequestered in the cytoplasm. Activation of NFκB requires the degradation of IκBα, thereby freeing NFκB to translocate to the nucleus and activate the target genes. Mice deficient in IκBα display deregulated and sustained NFκB activation and early postnatal lethality, highlighting a critical role of IκBα in NFκB regulation.

Results

We investigated the role of IκBα in regulating NFκB activity in the brain and the effects of the NFκB/IκBα pathway in mediating neuroinflammation under both physiological and brain injury conditions. We report that astrocytes, but not neurons, exhibit prominent NFκB activity, and that basal NFκB activity in astrocytes is elevated in the absence of IκBα. By generating mice with brain-specific deletion of IκBα, we show that IκBα deficiency does not compromise normal brain development. However, basal neuroinflammation detected by GFAP and Iba1 immunoreactivity is elevated. This leads to impaired inflammatory responses following TBI and worsened brain damage including higher blood brain barrier permeability, increased injury volumes and enlarged ventricle volumes.

Conclusions

We conclude that, in the CNS, astrocyte is the primary cell type subject to NFκB regulation. We further demonstrate that IκBα plays an important role in regulating NFκB activity in the brain and a robust NFκB/IκBα-mediated neuroinflammatory response immediately following TBI is beneficial.

Clinical usefulness of cell-based indirect immunofluorescence assay for the detection of aquaporin-4 antibodies in neuromyelitis optica spectrum disorder

Background

The presence of antibodies to aquaporin-4 (AQP4) has been identified as a key characteristic of neuromyelitis optica spectrum disorder (NMOSD), an autoimmune inflammatory demyelinating central nervous system (CNS) disorder. We evaluated the performance of a cell-based indirect immunofluorescence assay (CIIFA) for detecting AQP4 antibodies using antigen prepared with a recombinant AQP4 peptide transfection technique and assessed the usefulness of CIIFA for diagnosis of NMOSD in routine clinical practice.

Methods

Forty-six serum samples from 36 patients as a comparison set and another 101 patients enrolled consecutively from a neurology clinic were included. CIIFA and fluorescence immunoprecipitation assays (FIPA) were performed. CIIFA was performed at 2 different institutions for comparison purposes.

Results

CIIFA and FIPA sensitivity in the comparison set was 86% and 79% in neuromyelitis optica (NMO) patients and 55% and 36% in high-risk NMO patients, respectively. The semiquantitative titer measured by CIIFA correlated well with the arbitrary unit (fluorescence units [FU]) derived from FIPA (r=0.66). Titers measured by CIIFA and FIPA were elevated in NMO patients compared to high-risk NMO patients (1:240 vs. 1:180 and 8,390 vs. 4,059 FU, respectively). The frequency of AQP4 antibody detection by CIIFA in 101 consecutively enrolled patients was 100% in NMO and 23% in high-risk NMO patients, while only 4.6% in control patients, including those with multiple sclerosis.

Conclusions

Detection of AQP4 antibodies by CIIFA provides sensitive and highly specific diagnostic information for NMO and high-risk NMO patients, which can be used to differentiate these conditions from other demyelinating CNS diseases.

Brain connexins in demyelinating diseases: therapeutic potential of glial targets

Several demyelinating syndromes have been linked to mutations in glial gap junction proteins, the connexins. Although mutations in connexins of the myelinating cells, Schwann cells and oligodendrocytes, were initially described, recent data have shown that astrocytes also play a major role in the demyelination process. Alterations in astrocytic proteins directly affect the oligodendrocytes' ability to maintain myelin structure, and associated astrocytic proteins that regulate water and ionic fluxes, including aquaporins, can also regulate myelin integrity. Here, we will review the main evidence from human disorders and transgenic mouse models that implicate glial gap junction proteins in demyelinating diseases and the therapeutic potential of some of these targets. This article is part of a Special Issue entitled Electrical Synapses.

Central nervous system aquaporin-4 autoimmunity presenting with an isolated cerebral abnormality

Advances in the understanding of central nervous system aquarporin-4 autoimmunity have promoted the recognition of diverse clinical presentations beyond the traditional view of neuromyelitis optica. We describe a patient who developed hemiparesis caused by an extensive cerebral lesion as an initial manifestation of central nervous system aquarporin-4 autoimmunity. Although the patient had no history of optic neuritis or myelitis, not only was serum anti-aquarporin-4 antibody positive, but an imaging, treatment response and histopathological features also revealed characteristic findings suggestive of central nervous system aquarporin-4 autoimmunity. The present case highlights the importance of a comprehensive evaluation for anti-aquarporin-4 antibody even in patients presenting with isolated cerebral lesions.

Neuromyelitis optica IgG and natural killer cells produce NMO lesions in mice without myelin loss

The pathogenesis of neuromyelitis optica (NMO) involves targeting of NMO-immunoglobulin G (NMO-IgG) to aquaporin-4 (AQP4) on astrocytes in the central nervous system. Prior work provided evidence for complement-dependent cytotoxicity (CDC) in NMO lesion development. Here, we show that antibody-dependent cellular cytotoxicity (ADCC), in the absence of complement, can also produce NMO-like lesions. Antibody-dependent cellular cytotoxicity was produced in vitro by incubation of mouse astrocyte cultures with human recombinant monoclonal NMO-IgG and human natural killer cells (NK-cells). Injection of NMO-IgG and NK-cells in mouse brain caused loss of AQP4 and GFAP, two characteristic features of NMO lesions, but little myelin loss. Lesions were minimal or absent following injection of: (1) control (non-NMO) IgG with NK-cells; (2) NMO-IgG and NK-cells in AQP4-deficient mice; or (3) NMO-IgG and NK-cells in wild-type mice together with an excess of mutated NMO-IgG lacking ADCC effector function. NK-cells greatly exacerbated NMO lesions produced by NMO-IgG and complement in an ex vivo spinal cord slice model of NMO, causing marked myelin loss. NMO-IgG can thus produce astrocyte injury by ADCC in a complement-independent and dependent manner, suggesting the potential involvement of ADCC in NMO pathogenesis.

Aggravated chronic brain injury after focal cerebral ischemia in aquaporin-4-deficient mice

The water channel aquaporin-4 (AQP4) is important in brain water homeostasis, and is also involved in astrocyte growth and glial scar formation. It has been reported that AQP4 deficiency attenuates acute ischemic brain injury as a result of reducing cytotoxic edema. Here, we determined whether AQP4 deficiency influences chronic brain injury after focal cerebral ischemia induced by 30 min of middle cerebral artery occlusion (MCAO). AQP4(-/-) mice exhibited a lower survival rate and less body weight gain than wild-type mice, but their neurological deficits were similar to wild-type mice during 35 days after MCAO. At 35 days after MCAO, AQP4(-/-) mice showed more severe brain atrophy and cavity formation in the ischemic hemisphere as well as more neuronal loss in the hippocampus. Furthermore, astrocyte proliferation and glial scar formation were impaired in AQP4(-/-) mice. Therefore, AQP4 deficiency complicated by astrocyte dysfunction aggravates chronic brain injury after focal cerebral ischemia, suggesting that AQP4 may be important in the chronic phase of the post-ischemic recovery process.

Aquaporin-4 deficiency attenuates acute lesions but aggravates delayed lesions and microgliosis after cryoinjury to mouse brain

OBJECTIVE

To determine whether aquaporin-4 (AQP4) regulates acute lesions, delayed lesions, and the associated microglial activation after cryoinjury to the brain.

METHODS

Brain cryoinjury was applied to AQP4 knockout (KO) and wild-type mice. At 24 h and on days 7 and 14 after cryoinjury, lesion volume, neuronal loss, and densities of microglia and astrocytes were determined, and their changes were compared between AQP4 KO and wild-type mice.

RESULTS

Lesion volume and neuronal loss in AQP4 KO mice were milder at 24 h following cryoinjury, but worsened on days 7 and 14, compared to those in wild-type mice. Besides, microglial density increased more, and astrocyte proliferation and glial scar formation were attenuated on days 7 and 14 in AQP4 KO mice.

CONCLUSION

AQP4 deficiency ameliorates acute lesions, but worsens delayed lesions, perhaps due to the microgliosis in the late phase.

Phagocytized corpora amylacea as a histological hallmark of astrocytic injury in neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory demyelinating and necrotizing disorder of the CNS that mainly affects the optic nerve and spinal cord. The etiology is still uncertain; however, the discovery of serum anti-aquaporin-4 (AQP4) autoantibody is becoming the center of attention, and a new hypothesis is emerging that NMO is essentially astrocytopathy provoked by this autoantibody. In this study, we focused on corpora amylacea (CA), glycoproteinaceous inclusions in astrocytic processes. We examined 57 lesions in nine cases of NMO spectrum disorder, and demonstrated that CA were phagocytized by macrophages in 42 lesions (74%) of eight cases, while phagocytized figures were not seen in unaffected areas. Phagocytized CA were frequently encountered in early-phase lesions still retaining myelin structures, while fewer or none were found in chronic destructive lesions. Moreover, phagocytized CA were significantly smaller in diameter than intact ones, and CA were decreased or absent in most lesions assessed. These findings suggest the following pathophysiological process: the astrocytes are affected at an early phase in NMO, CA are expelled from the astrocytes and phagocytized by macrophages finally leading to clearance. A phagocytized figure and subsequent loss of CA can be a histological hallmark of astrocytic injury of NMO.

Review: the architecture of inflammatory demyelinating lesions: implications for studies on pathogenesis

Recent technological advances provided the chance to analyse the molecular events involved in the pathogenesis of lesions in human disease. A major prerequisite for such studies is, however, that the pathological material used is exactly defined and characterized. In multiple sclerosis (MS), this is difficult, as several types of active lesions exist, depending upon the stage of the disease, the age and location of these lesions and the inter-individual differences between patients. In addition, within an active lesion, different closely adjacent zones are present reflecting initial tissue injury, debris removal or repair. Here evidence is reviewed, showing that distinct subareas of active MS lesions reflect different pathological hallmarks of lesion evolution. These data provide the basis for our understanding of the pathogenesis of tissue injury in MS and imply that studies on MS pathogenesis have to rely on a clear definition of the lesions analysed and have to focus on specific lesion areas, isolated by microdissection. In addition, these data also imply that molecules, identified in these studies, must be confirmed and validated in the correct context of lesion initiation and/or progression.

Evidence against cellular internalization in vivo of NMO-IgG, aquaporin-4, and excitatory amino acid transporter 2 in neuromyelitis optica

Autoantibodies against astrocyte water channel aquaporin-4 (AQP4) are thought to be pathogenic in neuromyelitis optica (NMO). Prior work has suggested that a key component of NMO autoantibody (NMO-IgG) pathogenesis is internalization of AQP4 and the associated glutamate transporter EAAT2, leading to glutamate excitotoxicity. Here, we show selective endocytosis of NMO-IgG and AQP4 in transfected cell cultures, but little internalization in brain in vivo. AQP4-dependent endocytosis of NMO-IgG occurred rapidly in various AQP4-transfected cell lines, with efficient transport from early endosomes to lysosomes. Cell surface AQP4 was also reduced following NMO-IgG exposure. However, little or no internalization of NMO-IgG, AQP4, or EAAT2 was found in primary astrocyte cultures, nor was glutamate uptake affected by NMO-IgG exposure. Following injection of NMO-IgG into mouse brain, NMO-IgG binding and AQP4 expression showed a perivascular astrocyte distribution, without detectable cellular internalization over 24 h. We conclude that astrocyte endocytosis of NMO-IgG, AQP4, and EAAT2 is not a significant consequence of AQP4 autoantibody in vivo, challenging generally accepted views about NMO pathogenesis.