The astrocytic aquaporin-4 (AQP4) water channel is the target of pathogenic antibodies in a spectrum of relapsing autoimmune inflammatory central nervous system disorders of varying severity that is unified by detection of the serum biomarker neuromyelitis optica (NMO)-IgG. Neuromyelitis optica is the most severe of these disorders. The two major AQP4 isoforms, M1 and M23, have identical extracellular residues. This report identifies two novel properties of NMO-IgG as determinants of pathogenicity. First, the binding of NMO-IgG to the ectodomain of astrocytic AQP4 has isoform-specific outcomes. M1 is completely internalized, but M23 resists internalization and is aggregated into larger-order orthogonal arrays of particles that activate complement more effectively than M1 when bound by NMO-IgG. Second, NMO-IgG binding to either isoform impairs water flux directly, independently of antigen down-regulation. We identified, in nondestructive central nervous system lesions of two NMO patients, two previously unappreciated histopathological correlates supporting the clinical relevance of our in vitro findings: (i) reactive astrocytes with persistent foci of surface AQP4 and (ii) vacuolation in adjacent myelin consistent with edema. The multiple molecular outcomes identified as a consequence of NMO-IgG interaction with AQP4 plausibly account for the diverse pathological features of NMO: edema, inflammation, demyelination, and necrosis. Differences in the nature and anatomical distribution of NMO lesions, and in the clinical and imaging manifestations of disease documented in pediatric and adult patients, may be influenced by regional and maturational differences in the ratio of M1 to M23 proteins in astrocytic membranes.T he most abundant water channel in the central nervous system (CNS) is aquaporin-4 (AQP4), which is confined to astrocytes and ependyma; is enriched at glial-pial and glialendothelial interfaces; and surrounds nodes of Ranvier and paranodes, adjacent oligodendroglial loops, and synapses (1). In 2005, we identified AQP4 as the target of pathogenic autoantibodies in a spectrum of inflammatory CNS disorders of varying severity that is unified by detection of the serum biomarker neuromyelitis optica (NMO)-IgG (2, 3). These disorders are now recognized collectively as IgG-mediated autoimmune astrocytopathies. Before discovery of this antibody, NMO spectrum disorders were misclassified as multiple sclerosis variants. NMOIgG is centrally involved in the pathogenesis of NMO spectrum disorders. Its detection predicts frequent relapses that cause cumulative neurological impairment. Lesions characteristically affect the spinal cord and optic nerve, but do not spare the brain. Independent laboratories have demonstrated that NMO-IgG binding initiates AQP4 down-regulation with accompanying endocytosis of its physically associated glutamate transporter, EAAT2, complement activation, impairment of blood-brain barrier integrity, inflammation, and astrocyte injury (4-8). Demyelination is a proposed consequence of both pa...

We present and discuss the role of endothelial and astroglial cells in managing the blood-brain barrier (BBB) and aspects of pathological alterations in the BBB. The impact of astrocytes, pericytes, and perivascular cells on the induction and maintenance of the gliovascular unit is largely unidentified so far. An understanding of the signaling pathways that lie between these cell types and the endothelium and that possibly are mediated by components of the basal lamina is just beginning to emerge. The metabolism for the maintenance of the endothelial barrier is intimately linked to and dependent on the microenvironment of the brain parenchyma. We report the structure and function of the endothelial cells of brain capillaries by describing structures involved in the regulation of permeability, including transporter systems, caveolae, and tight junctions. There is increasing evidence that caveolae are not only vehicles for endo- and transcytosis, but also important regulators of tight-junction-based permeability. Tight junctions separate the luminal from the abluminal membrane domains of the endothelial cell ("fence function") and control the paracellular pathway ("gate function") thus representing the most significant structure of the BBB. In addition, the extracellular matrix between astrocytes/pericytes and endothelial cells contains numerous molecules with inherent signaling properties that have to be considered if we are to improve our knowledge of the complex and closely regulated BBB.

Cerebral gliomas of World Health Organization (WHO) grade II and III represent a major challenge in terms of histological classification and clinical management. Here, we asked whether largescale genomic and transcriptomic profiling improves the definition of prognostically distinct entities. We performed microarray-based genome-and transcriptome-wide analyses of primary tumor samples from a prospective German Glioma Network cohort of 137 patients with cerebral gliomas, including 61 WHO grade II and 76 WHO grade III tumors. Integrative bioinformatic analyses were employed to define molecular subgroups, which were then related to histology, molecular biomarkers, including isocitrate dehydrogenase 1 or 2 (IDH1/2) mutation, 1p/19q co-deletion and telomerase reverse transcriptase (TERT) promoter mutations, and patient outcome. Genomic profiling identified five distinct glioma groups, including three IDH1/2 mutant and two IDH1/2 wild-type groups. Expression profiling revealed evidence for eight transcriptionally different groups (five IDH1/2 mutant, three IDH1/2 wild type), which were only partially linked to the genomic groups. Correlation of DNA-based molecular stratification with clinical outcome allowed to define three major prognostic groups with characteristic genomic aberrations. The best prognosis was found in patients with IDH1/2 mutant and 1p/19q co-deleted tumors. Patients with IDH1/2 wild-type gliomas and glioblastoma-like genomic alterations, including gain on chromosome arm 7q (+7q), loss on chromosome arm 10q (-10q), TERT promoter mutation and oncogene amplification, displayed the worst outcome. Intermediate survival was seen in patients with IDH1/2 mutant, but 1p/19q intact, mostly astrocytic gliomas, and in patients with IDH1/2 wild-type gliomas lacking the +7q/-10q genotype and TERT promoter mutation. This molecular subgrouping stratified patients into prognostically distinct groups better than histological classification. Addition of gene expression data to this genomic classifier did not further improve prognostic stratification. In summary, DNA-based molecular profiling of WHO grade II and III gliomas distinguishes biologically distinct tumor groups and provides prognostically relevant information beyond histological classification as well as IDH1/2 mutation and 1p/19q co-deletion status.

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The dystrophin–dystroglycan complex (DDC) is a molecular array of proteins in muscle and brain cells. The central component of the DDC is dystroglycan, which comprises α- and β-subunits. α-Dystroglycan (α-DG) binds to extracellular matrix components such as agrin, whereas β-dystroglycan (β-DG) is a membrane-spanning protein linking α-DG to the cytoskeleton and other intracellular components such as α-syntrophin. In astrocytes, α-syntrophin binds to the water channel protein aquaporin-4 (AQP4). Recently, it has been shown that AQP4 expression is unaltered in agrin-knockout mice, but that formation of orthogonal arrays of particles (OAPs), consisting of AQP4, is abnormal. As the brain-selective deletion of the DG gene causes a disorganization of the astroglial endfeet, we investigated whether DG deletion has an impact on AQP4. Western blotting revealed reduced AQP4 in the parenchymal but not in the superficial compartment of the astrocyte-conditioned DG-knockout mouse brain. Accordingly, immunohistochemical stainings of AQP4 revealed a selective loss of AQP4 in perivascular but not in superficial astroglial endfeet. In both superficial and perivascular endfeet of the DG-knockout brain, we observed a loss of OAPs. We conclude that in the absence of DG the majority of superficial AQP4 molecules did not form OAPs, and that expression of AQP4 in perivascular endfeet is compromised. However, the decreased number of perivascular AQP4 molecules obviously did form a few OAPs, even in the absence of DG.

The blood-brain barrier (BBB) does not exclusively refer to brain endothelial cells, which are the site of the barrier proper. In the past few years, it has become increasingly clear that BBB endothelial cells depend considerably on the brain microenvironment to a degree exceeding the environmental influence in other organs. The concept of the BBB has been continuously developed over the decades, culminating now in the recognition that endothelial cell function in the brain is not limited to simply mediating energy and oxygen transfer between blood and neural tissue. Endothelial cells are rather "Janus-headed beings" that are active partners of both luminal molecules and cells, as well as subendothelial cells such as pericytes, astrocytes, and neurons. In this overview, the authors present and discuss both the role of astroglial cells in managing the BBB and aspects of pathological alterations in the brain as far as the BBB is involved. After a brief introduction of the BBB that describes the structure and function of the brain capillary endothelial cells, the authors report on both the water channel protein aquaporin-4 (AQP4) in astrocytes and the extracellular matrix between astrocytes/pericytes and endothelial cells. The AQP4 has an important impact on the homeostasis in the brain parenchyma; however, the mechanistic cascade from the composition of the astrocyte membrane to the maintenance of BBB properties in the endothelial cells, including their tight junction formation, is still completely unknown.

In human glioblastoma, the blood-brain barrier (BBB) is disturbed. According to our concept, the glio-vascular relationships and thus the control of the BBB are essentially dependent on the polarity of astroglial cells. This polarity is characterized by the uneven distribution of the water channel protein aquaporin-4 (AQP4), dystroglycan and other molecules. Recently, we were able to show that the extracellular matrix component agrin is important for the construction and localization of the so-called orthogonal arrays of particles (OAPs), which consist in AQP4. Here, combining freeze-fracture electron microscopy, immunohistochemistry and Western blotting, we describe alterations of expression and distribution of AQP4, dystroglycan, agrin and the matrix metalloproteinases (MMP) 2, 3 and 9 in human primary glioblastomas (eight primary tumours, six recurrent tumours). Increase of MMP3- and MMP2/9 immunoreactivities went along with loss of agrin and dystroglycan respectively. On the protein level, AQP4 expression was increased in glioblastoma compared to control tissue. This was not accompanied by an increase of OAPs, suggesting that AQP4 can also occur without forming OAPs. The results underline our concept of the loss of glioma cell polarity as one of the factors responsible for the disturbance of the neurovascular unit and as an explanation for the formation of edemas in the glioblastoma.