Aquaporin water channels in the nervous system

Inhibition of Aquaporin-4 Improves the Outcome of Ischaemic Stroke and Modulates Brain Paravascular Drainage Pathways

Aquaporin-4 (AQP4) is the most abundant water channel in the brain, and its inhibition before inducing focal ischemia, using the AQP4 inhibitor TGN-020, has been showed to reduce oedema in imaging studies. Here, we aimed to evaluate, for the first time, the histopathological effects of a single dose of TGN-020 administered after the occlusion of the medial cerebral artery (MCAO). On a rat model of non-reperfusion ischemia, we have assessed vascular densities, albumin extravasation, gliosis, and apoptosis at 3 and 7 days after MCAO. TGN-020 significantly reduced oedema, glial scar, albumin effusion, and apoptosis, at both 3 and 7 days after MCAO. The area of GFAP-positive gliotic rim decreased, and 3D fractal analysis of astrocytic processes revealed a less complex architecture, possibly indicating water accumulating in the cytoplasm. Evaluation of the blood vessels revealed thicker basement membranes colocalizing with exudated albumin in the treated animals, suggesting that inhibition of AQP4 blocks fluid flow towards the parenchyma in the paravascular drainage pathways of the interstitial fluid. These findings suggest that a single dose of an AQP4 inhibitor can reduce brain oedema, even if administered after the onset of ischemia, and AQP4 agonists/antagonists might be effective modulators of the paravascular drainage flow.

Molecular Determinants of Malignant Brain Cancers: From Intracellular Alterations to Invasion Mediated by Extracellular Vesicles

Malignant glioma cells invade the surrounding brain parenchyma, by migrating along the blood vessels, thus promoting cancer growth. The biological bases of these activities are grounded in profound alterations of the metabolism and the structural organization of the cells, which consequently acquire the ability to modify the surrounding microenvironment, by altering the extracellular matrix and affecting the properties of the other cells present in the brain, such as normal glial-, endothelial- and immune-cells. Most of the effects on the surrounding environment are probably exerted through the release of a variety of extracellular vesicles (EVs), which contain many different classes of molecules, from genetic material to defined species of lipids and enzymes. EV-associated molecules can be either released into the extracellular matrix (ECM) and/or transferred to neighboring cells: as a consequence, both deep modifications of the recipient cell phenotype and digestion of ECM components are obtained, thus causing cancer propagation, as well as a general brain dysfunction. In this review, we first analyze the main intracellular and extracellular transformations required for glioma cell invasion into the brain parenchyma; then we discuss how these events may be attributed, at least in part, to EVs that, like the pawns of a dramatic chess game with cancer, open the way to the tumor cells themselves.

Migratory potential of transplanted glial progenitors as critical factor for successful translation of glia replacement therapy: The gap between mice and men

Neurological disorders are a major threat to public health. Stem cell-based regenerative medicine is now a promising experimental paradigm for its treatment, as shown in pre-clinical animal studies. Initial attempts have been on the replacement of neuronal cells only, but glial progenitors (GPs) are now becoming strong alternative cellular therapeutic candidates to replace oligodendrocytes and astrocytes as knowledge accumulates about their important emerging role in various disease processes. There are many examples of successful therapeutic outcomes for transplanted GPs in small animal models, but clinical translation has proved to be challenging due to the 1,000-fold larger volume of the human brain compared to mice. Human GPs transplanted into the mouse brain migrate extensively and can induce global cell replacement, but a similar extent of migration in the human brain would only allow for local rather than global cell replacement. We review here the mechanisms that govern cell migration, which could potentially be exploited to enhance the migratory properties of GPs through cell engineering pre-transplantation. We furthermore discuss the (dis)advantages of the various cell delivery routes that are available, with particular emphasis on intra-arterial injection as the most suitable route for achieving global cell distribution in the larger brain. Now that therapeutic success has proven to be feasible in small animal models, future efforts will need to be directed to enhance global cell delivery and migration to make bench-to-bedside translation a reality.

Subject-specific multi-poroelastic model for exploring the risk factors associated with the early stages of Alzheimer's disease

There is emerging evidence suggesting that Alzheimer's disease is a vascular disorder, caused by impaired cerebral perfusion, which may be promoted by cardiovascular risk factors that are strongly influenced by lifestyle. In order to develop an understanding of the exact nature of such a hypothesis, a biomechanical understanding of the influence of lifestyle factors is pursued. An extended poroelastic model of perfused parenchymal tissue coupled with separate workflows concerning subject-specific meshes, permeability tensor maps and cerebral blood flow variability is used. The subject-specific datasets used in the modelling of this paper were collected as part of prospective data collection. Two cases were simulated involving male, non-smokers (control and mild cognitive impairment (MCI) case) during two states of activity (high and low). Results showed a marginally reduced clearance of cerebrospinal fluid (CSF)/interstitial fluid (ISF), elevated parenchymal tissue displacement and CSF/ISF accumulation and drainage in the MCI case. The peak perfusion remained at 8 mm s⁻¹ between the two cases.

Alpha-1 antichymotrypsin is involved in astrocyte injury in concert with arginine-vasopressin during the development of acute hepatic encephalopathy

Background and aims

We developed a bio-artificial liver (BAL) using a radial-flow bioreactor and rescued mini-pig models with lethal acute liver failure (ALF). The point of the rescue is the recovery from hepatic encephalopathy (HE). HE on ALF has sometimes resulted in brain death following brain edema with astrocyte swelling. Several factors, including ammonia and glutamine, have been reported to induce astrocyte swelling and injury. However, many clinicians believe that there are any other factors involved in the development of HE. Therefore, the aim of this study was to identify novel HE-inducible factors, particularly those inducing astrocyte dysfunction.

Methods

Mini-pig plasma samples were collected at three time points: before the administration of toxins (α-amanitin and LPS), when HE occurred after the administration of toxins, and after treatment with extracorporeal circulation (EC) by the BAL. To identify the causative factors of HE, each plasma sample was subjected to a comparative proteome analysis with two-dimensional gel electrophoresis and mass spectrometry. To assess the direct effects of candidate factors on the astrocyte function and injury, in vitro experiments with human astrocytes were performed.

Results

Using a proteome analysis, we identified alpha-1 antichymotrypsin (ACT), which was increased in plasma samples from mini-pigs with HE and decreased in those after treatment with EC by BAL. In in vitro experiments with human astrocytes, ACT showed growth-inhibitory and cytotoxic effects on astrocytes. In addition, the expression of water channel protein aquaporin-4, which is induced in injured astrocytes, was increased following ACT treatment. Interestingly, these effects of ACT were additively enhanced by adding arginine-vasopressin (AVP) and were canceled by adding an AVP receptor antagonist.

Conclusions

These results suggest that ACT is involved in astrocyte injury and dysfunction in concert with AVP during the development of acute HE.

Intermittent Fasting Protects against Alzheimer's Disease Possible through Restoring Aquaporin-4 Polarity

The impairment of amyloid-β (Aβ) clearance in the brain plays a causative role in Alzheimer's disease (AD). Polarity distribution of aquaporin-4 (AQP4) is important to remove Aβ from brain. AQP4 polarity can be influenced by the ratio of two AQP4 isoforms M1 and M23 (AQP4-M1/M23), however, it is unknown whether the ratio of AQP4-M1/M23 changes in AD. Histone deacetylase 3 has been reported to be significantly increased in AD brain. Moreover, evidence indicated that microRNA-130a (miR-130a) possibly mediates the regulation of histone deacetylase 3 on AQP4-M1/M23 ratio by repressing the transcriptional activity of AQP4-M1 in AD. This study aimed to investigate whether intermittent fasting (IF), increasing the level of an endogenous histone deacetylases inhibitor β-hydroxybutyrate, restores AQP4 polarity via miR-130a mediated reduction of AQP4-M1/M23 ratio in protection against AD. The results showed that IF ameliorated cognitive dysfunction, prevented brain from Aβ deposition, and restored the AQP4 polarity in a mouse model of AD (APP/PS1 double-transgenic mice). Additionally, IF down-regulated the expression of AQP4-M1 and histone deacetylase 3, reduced AQP4-M1/M23 ratio, and increased miR-130a expression in the cerebral cortex of APP/PS1 mice. In vitro, β-hydroxybutyrate was found to down-regulate the expression of AQP4-M1 and histone deacetylase 3, reduce AQP4-M1/M23 ratio, and increase AQP4-M23 and miR-130a expression in 2 μM Aβ-treated U251 cells. Interestingly, on the contrary to the result observed in 2 μM Aβ-treated cells, AQP4 expression was obviously decreased in cells exposed to 10 μM Aβ. miR-130a mimic decreased the expression of AQP4-M1 and the ratio of AQP4-M1/M23, as well as silencing histone deacetylase 3 caused the up-regulation of AQP4 and miR-130a, and the reduction of AQP4-M1/M23 ratio in U251 cells. In conclusion, IF exhibits beneficial effects against AD. The mechanism may be associated with recovery of AQP4 polarity, resulting from the reduction of AQP4-M1/M23 ratio. Furthermore, β-hydroxybutyrate may partly mediate the effect of IF on the reduction of AQP4-M1/M23 ratio in AD, in which miR-130a and histone deacetylase 3 may be implicated.

Neuroprotective Effects and Mechanism of β-Asarone against Aβ1-42-Induced Injury in Astrocytes

Emerging evidence suggests that activated astrocytes play important roles in AD, and β-asarone, a major component of Acorus tatarinowii Schott, was shown to be a potential therapeutic candidate for AD. While our previous study found that β-asarone could improve the cognitive function of rats hippocampally injected with Aβ, the effects of β-asarone on astrocytes remain unclear, and this study aimed to investigate these effects. A rat model of Aβ1-42 (10 μg) was established, and the rats were intragastrically treated with β-asarone at doses of 10, 20, and 30 mg/kg or donepezil at a dose of 0.75 mg/kg. The sham and model groups were intragastrically injected with an equal volume of saline. Animals were sacrificed on the 28th day after administration of the drugs. In addition, a cellular model of Aβ1-42 (1.1 μM, 6 h) was established, and cells were treated with β-asarone at doses of 0, 2.06, 6.17, 18.5, 55.6, and 166.7 μg/mL. β-Asarone improved cognitive impairment, alleviated Aβ deposition and hippocampal damage, and inhibited GFAP, AQP4, IL-1β, and TNF-α expression. These results suggested that β-asarone could alleviate the symptoms of AD by protecting astrocytes, possibly by inhibiting TNF-α and IL-1β secretion and then downregulating AQP4 expression.

Brain extracellular space, hyaluronan, and the prevention of epileptic seizures

Mutant mice deficient in hyaluronan (HA) have an epileptic phenotype. HA is one of the major constituents of the brain extracellular matrix. HA has a remarkable hydration capacity, and a lack of HA causes reduced extracellular space (ECS) volume in the brain. Reducing ECS volume can initiate or exacerbate epileptiform activity in many in vitro models of epilepsy. There is both in vitro and in vivo evidence of a positive feedback loop between reduced ECS volume and synchronous neuronal activity. Reduced ECS volume promotes epileptiform activity primarily via enhanced ephaptic interactions and increased extracellular potassium concentration; however, the epileptiform activity in many models, including the brain slices from HA synthase-3 knockout mice, may still require glutamate-mediated synaptic activity. In brain slice epilepsy models, hyperosmotic solution can effectively shrink cells and thus increase ECS volume and block epileptiform activity. However, in vivo, the intravenous administration of hyperosmotic solution shrinks both brain cells and brain ECS volume. Instead, manipulations that increase the synthesis of high-molecular-weight HA or decrease its breakdown may be used in the future to increase brain ECS volume and prevent seizures in patients with epilepsy. The prevention of epileptogenesis is also a future target of HA manipulation. Head trauma, ischemic stroke, and other brain insults that initiate epileptogenesis are known to be associated with an early decrease in high-molecular-weight HA, and preventing that decrease in HA may prevent the epileptogenesis.

Fast, Na+ /K+ pump driven, steady-state transcytolemmal water exchange in neuronal tissue: A study of rat brain cortical cultures

PURPOSE

Water homeostasis and transport play important roles in brain function (e.g., ion homeostasis, neuronal excitability, cell volume regulation, etc.). However, specific mechanisms of water transport across cell membranes in neuronal tissue have not been completely elaborated.

METHODS

The kinetics of transcytolemmal water exchange were measured in neuronal tissue using simultaneous, real-time fluorescence and nuclear magnetic resonance (NMR) measurements of perfused, active brain organotypic cortical cultures. Perfusion with a paramagnetic MRI contrast agent, gadoteridol, allows NMR determination of the unidirectional rate constant for steady-state cellular water efflux (k_io ), and the mole fraction of intracellular water ( pi), related to the average cell volume (V). Changes in intracellular calcium concentration [Cai2+] were used as a proxy for neuronal activity and were monitored by fluorescence imaging.

RESULTS

The k_io value, averaged over all cultures (N = 99) at baseline, was 2.02 (±1.72) s^-1 , indicating that on average, the equivalent of the entire intracellular water volume turns over twice each second. To probe possible molecular pathways, the specific Na⁺ -K⁺ -ATPase (NKA) inhibitor, ouabain (1 mM), was transiently introduced into the perfusate. This caused significant transient changes (N = 8): [Cai2+] rose ∼250%, V rose ∼89%, and k_io fell ∼45%, with a metabolically active k_io contribution probably eliminated by ouabain saturation.

CONCLUSIONS

These results suggest that transcytolemmal water exchange in neuronal tissue involves mechanisms affected by NKA activity as well as passive pathways. The active pathway may account for half of the basal homeostatic water flux. Magn Reson Med 79:3207-3217, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Expression of Aquaporin 1 and 4 in the Choroid Plexus and Brain Parenchyma of Kaolin-Induced Hydrocephalic Rats

Objective

Aquaporin (AQP) is a recently discovered protein that regulates water homeostasis. The present study examines changes in AQP 1 and 4 in kaolin induced experimental hydrocephalic rats to elucidate the pathophysiology of water homeostasis in the disease.

Methods

Hydrocephalus was induced by percutaneous intracisternal injection of kaolin. The brain parenchyma and choroid plexus were obtained at 3, 7, 14 and 30 days after injection. Protein expressions of AQP 1 and 4 were measured by western blot, immunohistochemistry (IHC) and immunofluorescence (IF) stains.

Results

In the choroid plexus of the kaolin-induced hydrocephalus group, AQP 1 expression identified by western blot exhibited sharp decrease in the early stage (55% by the 3rd day and 22% by the 7th day), but indicated a 2.2-fold increase in the later stage (30th day) in comparison with control groups. In the parenchyma, a quantitative measurement of AQP 4 expression revealed variable results on the 3rd and 7th days, but indicated expression 2.1 times higher than the control in the later stage (30th day). In addition, the IHC and IF findings supported the patterns of expression of AQP 1 in the choroid plexus and AQP 4 in the parenchyma.

Conclusion

Expression of AQP 1 decreased sharply in the choroid plexus of acute hydrocephalus rats and increased at later stages. Expression of AQP 4 in the brain parenchyma was variable in the early stage in the hydrocephalus group, but was higher than in the control in the later stage. These findings suggest a compensating role of AQPs in water physiology in hydrocephalus.

Supramolecular aggregation of aquaporin-4 is different in muscle and brain: correlation with tissue susceptibility in neuromyelitis optica

Neuromyelitis optica (NMO) is an autoimmune demyelinating disease of the central nervous system (CNS) caused by autoantibodies (NMO‐IgG) against the water channel aquaporin‐4 (AQP4). Though AQP4 is also expressed outside the CNS, for example in skeletal muscle, patients with NMO generally do not show clinical/diagnostic evidence of skeletal muscle damage. Here, we have evaluated whether AQP4 supramolecular organization is at the basis of the different tissue susceptibility. Using immunofluorescence we found that while the sera of our cohort of patients with NMO gave typical perivascular staining in the CNS, they were largely negative in the skeletal muscle. This conclusion was obtained using human, rat and mouse skeletal muscle including the AQP4‐KO mouse. A biochemical analysis using a new size exclusion chromatography approach for AQP4 suprastructure fractionation revealed substantial differences in supramolecular AQP4 assemblies and isoform abundance between brain and skeletal muscle matching a lower binding affinity of NMO‐IgG to muscle compared to the brain. Super‐resolution microscopy analysis with g‐STED revealed different AQP4 organization in native tissues, while in the brain perivascular astrocyte endfoot membrane AQP4 was mainly organized in large interconnected and raft‐like clusters, in the sarcolemma of fast‐twitch fibres AQP4 aggregates often appeared as small, relatively isolated linear entities. In conclusion, our results provide evidence that AQP4 supramolecular structure is different in brain and skeletal muscle, which is likely to result in different tissues susceptibility to the NMO disease.

Hippocampal and Cortical Pyramidal Neurons Swell in Parallel with Astrocytes during Acute Hypoosmolar Stress

Normal nervous system function is critically dependent on the balance of water and ions in the extracellular space (ECS). Pathological reduction in brain interstitial osmolarity results in osmotically-driven flux of water into cells, causing cellular edema which reduces the ECS and increases neuronal excitability and risk of seizures. Astrocytes are widely considered to be particularly susceptible to cellular edema due to selective expression of the water channel aquaporin-4 (AQP4). The apparent resistance of pyramidal neurons to osmotic swelling has been attributed to lack of functional water channels. In this study we report rapid volume changes in CA1 pyramidal cells in hypoosmolar ACSF (hACSF) that are equivalent to volume changes in astrocytes across a variety of conditions. Astrocyte and neuronal swelling was significant within 1 min of exposure to 17 or 40% hACSF, was rapidly reversible upon return to normosmolar ACSF, and repeatable upon re-exposure to hACSF. Neuronal swelling was not an artifact of patch clamp, occurred deep in tissue, was similar at physiological vs. room temperature, and occurred in both juvenile and adult hippocampal slices. Neuronal swelling was neither inhibited by TTX, nor by antagonists of NMDA or AMPA receptors, suggesting that it was not occurring as a result of excitotoxicity. Surprisingly, genetic deletion of AQP4 did not inhibit, but rather augmented, astrocyte swelling in severe hypoosmolar conditions. Taken together, our results indicate that neurons are not osmoresistant as previously reported, and that osmotic swelling is driven by an AQP4-independent mechanism.

Selective knockout of astrocytic Na+ /H+ exchanger isoform 1 reduces astrogliosis, BBB damage, infarction, and improves neurological function after ischemic stroke

Stimulation of Na⁺ /H⁺ exchanger isoform 1 (NHE1) in astrocytes causes ionic dysregulation under ischemic conditions. In this study, we created a Nhe1^flox/flox (Nhe1^f/f ) mouse line with exon 5 of Nhe1 flanked with two loxP sites and selective ablation of Nhe1 in astrocytes was achieved by crossing Nhe1^f/f mice with Gfap-Cre^ERT2 Cre-recombinase mice. Gfap-Cre^ERT2+/- ;Nhe1^f/f mice at postnatal day 60-90 were treated with either corn oil or tamoxifen (Tam, 75 mg/kg/day, i.p.) for 5 days. After 30 days post-injection, mice underwent transient middle cerebral artery occlusion (tMCAO) to induce ischemic stroke. Compared with the oil-vehicle group (control), Tam-treated Gfap-Cre^ERT2+/- ;Nhe1^f/f (Nhe1 KO) mice developed significantly smaller ischemic infarction, less edema, and less neurological function deficits at 1-5 days after tMCAO. Immunocytochemical analysis revealed less astrocytic proliferation, less cellular hypertrophy, and less peri-lesion gliosis in Nhe1 KO mouse brains. Selective deletion of Nhe1 in astrocytes also reduced cerebral microvessel damage and blood-brain barrier (BBB) injury in ischemic brains. The BBB microvessels of the control brains show swollen endothelial cells, opened tight junctions, increased expression of proinflammatory protease MMP-9, and significant loss of tight junction protein occludin. In contrast, the Nhe1 KO mice exhibited reduced BBB breakdown and normal tight junction structure, with increased expression of occludin and reduced MMP-9. Most importantly, deletion of astrocytic Nhe1 gene significantly increased regional cerebral blood flow in the ischemic hemisphere at 24 hr post-MCAO. Taken together, our study provides the first line of evidence for a causative role of astrocytic NHE1 protein in reactive astrogliosis and ischemic neurovascular damage.

Comparative proteomic analyses reveal the proteome response to short-term drought in Italian ryegrass (Lolium multiflorum)

Drought is a major abiotic stress that impairs growth and productivity of Italian ryegrass. Comparative analysis of drought responsive proteins will provide insight into molecular mechanism in Lolium multiflorum drought tolerance. Using the iTRAQ-based approach, proteomic changes in tolerant and susceptible lines were examined in response to drought condition. A total of 950 differentially accumulated proteins was found to be involved in carbohydrate metabolism, amino acid metabolism, biosynthesis of secondary metabolites, and signal transduction pathway, such as β-D-xylosidase, β-D-glucan glucohydrolase, glycerate dehydrogenase, Cobalamin-independent methionine synthase, glutamine synthetase 1a, Farnesyl pyrophosphate synthase, diacylglycerol, and inositol 1, 4, 5-trisphosphate, which might contributed to enhance drought tolerance or adaption in Lolium multiflorum. Interestingly, the two specific metabolic pathways, arachidonic acid and inositol phosphate metabolism including differentially accumulated proteins, were observed only in the tolerant lines. Cysteine protease cathepsin B, Cysteine proteinase, lipid transfer protein and Aquaporin were observed as drought-regulated proteins participating in hydrolysis and transmembrane transport. The activities of phospholipid hydroperoxide glutathione peroxidase, peroxiredoxin, dehydroascorbate reductase, peroxisomal ascorbate peroxidase and monodehydroascorbate reductase associated with alleviating the accumulation of reactive oxygen species in stress inducing environments. Our results showed that drought-responsive proteins were closely related to metabolic processes including signal transduction, antioxidant defenses, hydrolysis, and transmembrane transport.

The Role of Brain-Reactive Autoantibodies in Brain Pathology and Cognitive Impairment

Antibodies to different brain proteins have been recently found to be associated with an increasing number of different autoimmune diseases. They need to penetrate the blood–brain barrier (BBB) in order to bind antigens within the central nervous system (CNS). They can target either neuronal or non-neuronal antigen and result in damage either by themselves or in synergy with other inflammatory mediators. Antibodies can lead to acute brain pathology, which may be reversible; alternatively, they may trigger irreversible damage that persists even though the antibodies are no longer present. In this review, we will describe two different autoimmune conditions and the role of their antibodies in causing brain pathology. In systemic lupus erythematosus (SLE), patients can have double stranded DNA antibodies that cross react with the neuronal N-methyl-d-aspartate receptor (NMDAR), which have been recently linked to neurocognitive dysfunction. In neuromyelitis optica (NMO), antibodies to astrocytic aquaporin-4 (AQP4) are diagnostic of disease. There is emerging evidence that pathogenic T cells also play an important role for the disease pathogenesis in NMO since they infiltrate in the CNS. In order to enable appropriate and less invasive treatment for antibody-mediated diseases, we need to understand the mechanisms of antibody-mediated pathology, the acute and chronic effects of antibody exposure, if the antibodies are produced intrathecally or systemically, their target antigen, and what triggers their production. Emerging data also show that in utero exposure to some brain-reactive antibodies, such as those found in SLE, can cause neurodevelopmental impairment since they can penetrate the embryonic BBB. If the antibody exposure occurs at a critical time of development, this can result in irreversible damage of the offspring that persists throughout adulthood.

Effects of erythropoietin and methylprednisolone on AQP4 expression in astrocytes

Methylprednisolone sodium succinate (MPSS) has been suggested as a treatment for spinal cord injury (SCI), but its use has been limited due to its adverse effects. Erythropoietin (EPO) has been suggested as a promising candidate for limiting SCI in mammals. The aim of the present study was to investigate the effects of EPO in combination with MPSS on astrocytes following ischemic injury in vitro. Astrocytes were isolated from the cerebral cortex of postnatal day 3 Sprague-Dawley rats and cultured in vitro. Astrocyte ischemic injury was induced by oxygen and glucose deprivation for 4 h, and reperfusion was simulated by subsequent culture under normoxic conditions. The effects of EPO and MPSS on the expression of aquaporin-4 (AQP4) were investigated. Ischemic astrocytes were treated with EPO (10 U/ml), MPSS (10 µg/ml), or EPO (10 U/ml) in combination with MPSS (10 µg/ml) during reperfusion. The cell viability of astrocytes was assessed using an MTT assay. The mRNA and protein expression levels of AQP4 were determined using reverse transcription-quantitative polymerase chain reaction and western blot analysis, respectively. The role of the protein kinase C (PKC) signaling pathway in the molecular mechanisms underlying the effects of EPO and MPSS was also investigated. The present results demonstrated that following treatment with EPO and MPSS, the mRNA expression levels of AQP4 were upregulated and cell viability was enhanced. EPO and MPSS effectively inhibited the oxygen and glucose deprivation-mediated downregulation of AQP4 following reperfusion. In addition, the combined treatment with EPO and MPSS exhibited higher AQP4 expression levels and cell viability compared with each treatment alone. Finally, the effects of EPO and MPSS on AQP4 expression were partially reversed by pretreatment with the PKC inhibitor Ro 31–8220. The present study indicated that EPO and MPSS had a synergistic effect on AQP4 expression following reperfusion, and suggest that they may be combined in the treatment of SCI.

Transcriptome analysis suggests a role for the differential expression of cerebral aquaporins and the MAPK signalling pathway in human temporal lobe epilepsy

Epilepsies are common disorders of the central nervous system (CNS), affecting up to 2% of the global population. Pharmaco-resistance is a major clinical challenge affecting about 30% of temporal lobe epilepsy (TLE) patients. Water homeostasis has been shown crucial for regulation of neuronal excitability. The control of water movement is achieved through a family of small integral membrane channel proteins called aquaporins (AQPs). Despite the fact that changes in water homeostasis occur in sclerotic hippocampi of people with TLE, the expression of AQPs in the epileptic brain is not fully characterised. This study uses microarray and ELISA methods to analyse the mRNA and protein expression of the human cerebral AQPs in sclerotic hippocampi (TLE-HS) and adjacent neocortex tissue (TLE-NC) of TLE patients. The expression of AQP1 and AQP4 transcripts was significantly increased, while that of the AQP9 transcript was significantly reduced in TLE-HS compared to TLE-NC. AQP4 protein expression was also increased while expression of AQP1 protein remained unchanged, and AQP9 was undetected. Microarray data analysis identified 3333 differentially regulated genes and suggested the involvement of the MAPK signalling pathway in TLE pathogenesis. Proteome array data validated the translational profile for 26 genes and within the MAPK pathway (e.g. p38, JNK) that were identified as differentially expressed from microarray analysis. ELISA data showed that p38 and JNK inhibitors decrease AQP4 protein levels in cultured human primary cortical astrocytes. Elucidating the mechanism of selective regulation of different AQPs and associated regulatory proteins may provide a new therapeutic approach to epilepsy treatment.

Aquaporin-4 Functionality and Virchow-Robin Space Water Dynamics: Physiological Model for Neurovascular Coupling and Glymphatic Flow

The unique properties of brain capillary endothelium, critical in maintaining the blood-brain barrier (BBB) and restricting water permeability across the BBB, have important consequences on fluid hydrodynamics inside the BBB hereto inadequately recognized. Recent studies indicate that the mechanisms underlying brain water dynamics are distinct from systemic tissue water dynamics. Hydrostatic pressure created by the systolic force of the heart, essential for interstitial circulation and lymphatic flow in systemic circulation, is effectively impeded from propagating into the interstitial fluid inside the BBB by the tightly sealed endothelium of brain capillaries. Instead, fluid dynamics inside the BBB is realized by aquaporin-4 (AQP-4), the water channel that connects astrocyte cytoplasm and extracellular (interstitial) fluid. Brain interstitial fluid dynamics, and therefore AQP-4, are now recognized as essential for two unique functions, namely, neurovascular coupling and glymphatic flow, the brain equivalent of systemic lymphatics.

Astroglia as a cellular target for neuroprotection and treatment of neuro-psychiatric disorders

Astrocytes are key homeostatic cells of the central nervous system. They cooperate with neurons at several levels, including ion and water homeostasis, chemical signal transmission, blood flow regulation, immune and oxidative stress defense, supply of metabolites and neurogenesis. Astroglia is also important for viability and maturation of stem-cell derived neurons. Neurons critically depend on intrinsic protective and supportive properties of astrocytes. Conversely, all forms of pathogenic stimuli which disturb astrocytic functions compromise neuronal functionality and viability. Support of neuroprotective functions of astrocytes is thus an important strategy for enhancing neuronal survival and improving outcomes in disease states. In this review, we first briefly examine how astrocytic dysfunction contributes to major neurological disorders, which are traditionally associated with malfunctioning of processes residing in neurons. Possible molecular entities within astrocytes that could underpin the cause, initiation and/or progression of various disorders are outlined. In the second section, we explore opportunities enhancing neuroprotective function of astroglia. We consider targeting astrocyte-specific molecular pathways which are involved in neuroprotection or could be expected to have a therapeutic value. Examples of those are oxidative stress defense mechanisms, glutamate uptake, purinergic signaling, water and ion homeostasis, connexin gap junctions, neurotrophic factors and the Nrf2-ARE pathway. We propose that enhancing the neuroprotective capacity of astrocytes is a viable strategy for improving brain resilience and developing new therapeutic approaches.

Complement regulator CD59 prevents peripheral organ injury in rats made seropositive for neuromyelitis optica immunoglobulin G

Pathogenesis in aquaporin-4 immunoglobulin G (AQP4-IgG) seropositive neuromyelitis optica spectrum disorders (herein called NMO) involves complement-dependent cytotoxicity initiated by AQP4-IgG binding to astrocyte AQP4. We recently reported that rats lacking complement inhibitor protein CD59 were highly susceptible to development of NMO pathology in brain and spinal cord following direct AQP4-IgG administration (Yao and Verkman, Acta Neuropath Commun 2017, 5:15). Here, we report evidence that CD59 is responsible for protection of peripheral, AQP4-expressing tissues in seropositive NMO. Rats made seropositive by intraperitoneal injection of AQP4-IgG developed marked weakness by 24 h and died soon thereafter. Serum creatine phosphokinase at 24 h was >900-fold greater in seropositive CD59^-/- rats than in seropositive CD59^+/+ (or control) rats. AQP4-expressing cells in skeletal muscle and kidney, but not in stomach, of seropositive CD59^-/- rats showed injury with deposition of AQP4-IgG and activated complement C5b-9, and inflammation. Organ injury in seropositive CD59^-/- rats was prevented by a complement inhibitor. Significant pathological changes in seropositive CD59^-/- rats were not seen in optic nerve, spinal cord or brain, including circumventricular tissue. These results implicate a major protective role of CD59 outside of the central nervous system in seropositive NMO, and hence offer an explanation as to why peripheral, AQP4-expressing cells are largely unaffected in NMO.

Upregulation of miR-130b protects against cerebral ischemic injury by targeting water channel protein aquaporin 4 (AQP4)

Altered microRNA regulation has been implicated in the pathogenesis of various disorders, including cerebral ischemia/reperfusion injury (I/RI). However, the regulatory mechanism of miR-130b in cerebral ischemia injury has not been reported. In this study, we explored the role of miR-130b in cerebral ischemia injury and investigated its potential mechanism. Levels of miR-130b were quantified by real-time PCR, and the protein level of AQP4 was detected by Western blotting. Cell apoptosis was detected by flow cytometry. In vitro, miR-130b levels in astrocytes were found significantly downregulated after OGD. Overexpression of miR-130b by miR-130b mimic decreased LDH release and apoptosis, but promoted cell health of astrocytes with OGD, thus playing a protective role in astrocyte I/RI. The level of miR-130b was also downregulated in ischemic tissues in MCAO model compared with the sham group, and the expression of miR-130b was gradually downregulated over time after reperfusion. AQP4 was upregulated both in two models, and as the reperfusion went on, AQP4 expression gradually upregulated. Our results indicated knockdown of AQP4 could ameliorate astrocyte injury induced by OGD. Finally, we found that miR-130b regulated astrocyte expression of AQP4, and rescue experiments further proved the protective role of miR-130b was mediated by AQP4 downregulation. Our study demonstrated that miR-130b might exert a neuroprotective effect following cerebral I/RI by regulating AQP4 expression at the post-transcriptional level. Therefore, miR-130b may be a potential therapeutic target for stroke treatment.

Intracellular water preexchange lifetime in neurons and astrocytes

PURPOSE

To determine the intracellular water preexchange lifetime, τ_i , the "average residence time" of water, in the intracellular milieu of neurons and astrocytes. The preexchange lifetime is important for modeling a variety of MR data sets, including relaxation, diffusion-sensitive, and dynamic contrast-enhanced data sets.

METHODS

Herein, τ_i in neurons and astrocytes is determined in a microbead-adherent, cultured cell system. In concert with thin-slice selection, rapid flow of extracellular media suppresses extracellular signal, allowing determination of the transcytolemmal-exchange-dominated, intracellular T₁ . With this knowledge, and that of the intracellular T₁ in the absence of exchange, τ_i can be derived.

RESULTS

Under normal culture conditions, τ_i for neurons is 0.75 ± 0.05 s versus 0.57 ± 0.03 s for astrocytes. Both neuronal and astrocytic τ_i s decrease within 30 min after the onset of oxygen-glucose deprivation, with the astrocytic τ_i showing a substantially greater decrease than the neuronal τ_i .

CONCLUSIONS

Given an approximate intra- to extracellular volume ratio of 4:1 in the brain, these data imply that, under normal physiological conditions, an MR experimental characteristic time of less than 0.012 s is required for a nonexchanging, two-compartment (intra- and extracellular) model to be valid for MR studies. This characteristic time shortens significantly (i.e., 0.004 s) under injury conditions. Magn Reson Med 79:1616-1627, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Diagnostic algorithm for relapsing acquired demyelinating syndromes in children

Objective:

To establish whether children with relapsing acquired demyelinating syndromes (RDS) and myelin oligodendrocyte glycoprotein antibodies (MOG-Ab) show distinctive clinical and radiologic features and to generate a diagnostic algorithm for the main RDS for clinical use.

Methods:

A panel reviewed the clinical characteristics, MOG-Ab and aquaporin-4 (AQP4) Ab, intrathecal oligoclonal bands, and Epstein-Barr virus serology results of 110 children with RDS. A neuroradiologist blinded to the diagnosis scored the MRI scans. Clinical, radiologic, and serologic tests results were compared.

Results:

The findings showed that 56.4% of children were diagnosed with multiple sclerosis (MS), 25.4% with neuromyelitis optica spectrum disorder (NMOSD), 12.7% with multiphasic disseminated encephalomyelitis (MDEM), and 5.5% with relapsing optic neuritis (RON). Blinded analysis defined baseline MRI as typical of MS in 93.5% of children with MS. Acute disseminated encephalomyelitis presentation was seen only in the non-MS group. Of NMOSD cases, 30.7% were AQP4-Ab positive. MOG-Ab were found in 83.3% of AQP4-Ab–negative NMOSD, 100% of MDEM, and 33.3% of RON. Children with MOG-Ab were younger, were less likely to present with area postrema syndrome, and had lower disability, longer time to relapse, and more cerebellar peduncle lesions than children with AQP4-Ab NMOSD. A diagnostic algorithm applicable to any episode of CNS demyelination leads to 4 main phenotypes: MS, AQP4-Ab NMOSD, MOG-Ab–associated disease, and antibody-negative RDS.

Conclusions:

Children with MS and AQP4-Ab NMOSD showed features typical of adult cases. Because MOG-Ab–positive children showed notable and distinctive clinical and MRI features, they were grouped into a unified phenotype (MOG-Ab–associated disease), included in a new diagnostic algorithm.

Mechanisms of Autoantibody-Induced Pathology

Autoantibodies are frequently observed in healthy individuals. In a minority of these individuals, they lead to manifestation of autoimmune diseases, such as rheumatoid arthritis or Graves' disease. Overall, more than 2.5% of the population is affected by autoantibody-driven autoimmune disease. Pathways leading to autoantibody-induced pathology greatly differ among different diseases, and autoantibodies directed against the same antigen, depending on the targeted epitope, can have diverse effects. To foster knowledge in autoantibody-induced pathology and to encourage development of urgently needed novel therapeutic strategies, we here categorized autoantibodies according to their effects. According to our algorithm, autoantibodies can be classified into the following categories: (1) mimic receptor stimulation, (2) blocking of neural transmission, (3) induction of altered signaling, triggering uncontrolled (4) microthrombosis, (5) cell lysis, (6) neutrophil activation, and (7) induction of inflammation. These mechanisms in relation to disease, as well as principles of autoantibody generation and detection, are reviewed herein.

Osmotherapy With Hypertonic Saline Attenuates Global Cerebral Edema Following Experimental Cardiac Arrest via Perivascular Pool of Aquaporin-4

OBJECTIVES

We tested the hypothesis that osmotherapy with hypertonic saline attenuates cerebral edema following experimental cardiac arrest and cardiopulmonary resuscitation by exerting its effect via the perivascular pool of aquaporin-4. We used mice with targeted disruption of the gene encoding α-syntrophin (α-Syn) that demonstrate diminished perivascular aquaporin-4 pool but retain the non-endfoot and ependymal pools.

DESIGN

Laboratory animal study.

SETTING

University animal research laboratory.

INTERVENTIONS

Isoflurane-anesthetized adult male wild-type C57B/6 or α-Syn mice were subjected to cardiac arrest/cardiopulmonary resuscitation and treated with either a continuous IV infusion of 0.9% saline or various concentrations of hypertonic saline. Serum osmolality, regional brain water content, blood-brain barrier disruption, and aquaporin-4 protein expression were determined at 24 hours after cardiac arrest/cardiopulmonary resuscitation.

MEASUREMENTS AND MAIN RESULTS

Hypertonic saline (7.5%) treatment significantly attenuated water content in the caudoputamen complex and cortex compared with 0.9% saline treatment in wild-type mice subjected to cardiac arrest/cardiopulmonary resuscitation. In contrast, in α-Syn mice subjected to cardiac arrest/cardiopulmonary resuscitation, 7.5% hypertonic saline treatment did not attenuate water content. Treatment with 7.5% hypertonic saline attenuated blood-brain barrier disruption at 24 hours following cardiac arrest/cardiopulmonary resuscitation in wild-type mice but not in α-Syn mice. Total aquaporin-4 protein expression was not different between 0.9% saline and hypertonic saline-treated wild-type mice.

CONCLUSIONS

Following experimental cardiac arrest/cardiopulmonary resuscitation: 1) continuous hypertonic saline therapy maintained to achieve serum osmolality of ≈ 350 mOsm/L is beneficial for the treatment of cerebral edema; 2) perivascular pool of aquaporin-4 plays a critical role in water egress from brain; and 3) hypertonic saline attenuates blood-brain barrier disruption via perivascular aquaporin-4 pool.

Neuronal pannexin-1 channels are not molecular routes of water influx during spreading depolarization-induced dendritic beading

Spreading depolarization-induced focal dendritic swelling (beading) is an early hallmark of neuronal cytotoxic edema. Pyramidal neurons lack membrane-bound aquaporins posing a question of how water enters neurons during spreading depolarization. Recently, we have identified chloride-coupled transport mechanisms that can, at least in part, participate in dendritic beading. Yet transporter-mediated ion and water fluxes could be paralleled by water entry through additional pathways such as large-pore pannexin-1 channels opened by spreading depolarization. Using real-time in vivo two-photon imaging in mice with pharmacological inhibition or conditional genetic deletion of pannexin-1, we showed that pannexin-1 channels are not required for spreading depolarization-induced focal dendritic swelling.

Aquaporin 4-Mediated Glutamate-Induced Astrocyte Swelling Is Partially Mediated through Metabotropic Glutamate Receptor 5 Activation

Astrocytes are one of the most abundant cell types in the mammalian central nervous system (CNS), and astrocyte swelling is the primary event associated with brain edema. Glutamate, the principal excitatory amino acid neurotransmitter in the CNS, is released at high levels after brain injury including cerebral ischemia. This leads to astrocyte swelling, which we previously demonstrated is related to metabotropic glutamate receptor (mGluR) activation. Aquaporin 4 (AQP4), the predominant water channel in the brain, is expressed in astrocyte endfeet and plays an important role in brain edema following ischemia. Studies recently showed that mGluR5 is also expressed on astrocytes. Therefore, it is worth investigating whether AQP4 mediates the glutamate-induced swelling of astrocytes via mGluR5. In the present study, we found that 1 mM glutamate induced astrocyte swelling, quantified by the cell perimeter, but it had no effect on astrocyte viability measured by the cell counting kit-8 (CCK-8) and lactate dehydrogenase (LDH) assays. Quantitative reverse transcription polymerase chain reaction analyses revealed that AQP4, among AQP1, 4, 5, 9 and 11, was the main molecular expressed in cultured astrocytes. Glutamate-induced cell swelling was accompanied by a concentration-dependent change in AQP4 expression. Furthermore, RNAi technology revealed that AQP4 gene silencing inhibited glutamate-induced astrocyte swelling. Moreover, we found that mGluR5 expression was greatest among the mGluRs in cultured astrocytes and was co-expressed with AQP4. Activation of mGluR5 in cultured astrocytes using (S)-3,5-dihydroxyphenylglycine (DHPG), an mGluR5 agonist, mimicked the effect of glutamate. This effect was abolished by co-incubation with the mGluR5 antagonist fenobam but was not influenced by DL-threo-β-benzyloxyaspartic acid (DL-TBOA), a glutamate transporter inhibitor. Finally, experiments in a rat model of transient middle cerebral artery occlusion (tMCAO) revealed that co-expression of mGluR5 and AQP4 was increased in astrocyte endfeet around capillaries in the penumbra, and this was accompanied by brain edema. Collectively, these results suggest that glutamate induces cell swelling and alters AQP4 expression in astrocytes via mGluR5 activation, which may provide a novel approach for the treatment of edema following brain injury.

Turning down the volume: Astrocyte volume change in the generation and termination of epileptic seizures

Approximately 1% of the global population suffers from epilepsy, a class of disorders characterized by recurrent and unpredictable seizures. Of these cases roughly one-third are refractory to current antiepileptic drugs, which typically target neuronal excitability directly. The events leading to seizure generation and epileptogenesis remain largely unknown, hindering development of new treatments. Some recent experimental models of epilepsy have provided compelling evidence that glial cells, especially astrocytes, could be central to seizure development. One of the proposed mechanisms for astrocyte involvement in seizures is astrocyte swelling, which may promote pathological neuronal firing and synchrony through reduction of the extracellular space and elevated glutamate concentrations. In this review, we discuss the common conditions under which astrocytes swell, the resultant effects on neural excitability, and how seizure development may ultimately be influenced by these effects.

Enhanced interstitial fluid drainage in the hippocampus of spontaneously hypertensive rats

Hypertension is associated with cognitive decline and various forms of dementia, including Alzheimer’s disease. In animal models of hypertension, many of Alzheimer’s disease characteristics are recapitulated, including brain atrophy, cognitive decline, amyloid β accumulation and blood brain barrier dysfunction. Removal of amyloid β and other waste products depends in part on clearance via the brain interstitial fluid (ISF). Here we studied the impact of hypertension on ISF drainage, using spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). At 8 months, high (500 kD) and low (3 kD) fluorescent molecular weight tracers released passively into the hippocampus showed a drastically enhanced spreading in SHR. Tracer spreading was inhomogeneous, with accumulation at ISF-CSF borders, around arteries, and towards the stratum lacunosum moleculare. These locations stained positively for the astrocyte marker GFAP, and aquaporin 4. Despite enhanced dispersion, clearance of tracers was not affected in SHR. In conclusion, these data indicate enhanced bulk flow of ISF in the hippocampus of hypertensive rats. ISF drains along astrocytes towards the cerebrospinal fluid compartment, which leads to sieving of high molecular weight solutes. Sieving may lead to a local increase in the concentration of waste products and potentially promotes the aggregation of amyloid β.

Pathogenic implications of cerebrospinal fluid barrier pathology in neuromyelitis optica

Pathogenic autoantibodies associated with neuromyelitis optica (NMO) induce disease by targeting aquaporin-4 (AQP4) water channels enriched on astrocytic endfeet at blood–brain interfaces. AQP4 is also expressed at cerebrospinal fluid (CSF)–brain interfaces, such as the pial glia limitans and the ependyma and at the choroid plexus blood–CSF barrier. However, little is known regarding pathology at these sites in NMO. Therefore, we evaluated AQP4 expression, microglial reactivity, and complement deposition at pial and ependymal surfaces and in the fourth ventricle choroid plexus in 23 autopsy cases with clinically and/or pathologically confirmed NMO or NMO spectrum disorder. These findings were compared to five cases with multiple sclerosis, five cases of choroid plexus papilloma, and five control cases without central nervous system disease. In the NMO cases, AQP4 immunoreactivity was reduced relative to control levels in the pia (91%; 21/23), ependyma (56%; 9/16), and choroid plexus epithelium (100%; 12/12). AQP4 immunoreactivity was normal in MS cases in these regions. Compared to MS, NMO cases also showed a focal pattern of pial and ependymal complement deposition and more pronounced microglial reactivity. In addition, AQP4 loss, microglial reactivity, and complement deposition colocalized along the pia and ependyma only in NMO cases. Within the choroid plexus, AQP4 loss was coincident with C9neo immunoreactivity on epithelial cell membranes only in NMO cases. These observations demonstrate that NMO immunopathology extends beyond perivascular astrocytic foot processes to include the pia, ependyma, and choroid plexus, suggesting that NMO IgG-induced pathological alterations at CSF–brain and blood–CSF interfaces may contribute to the occurrence of ventriculitis, leptomeningitis, and hydrocephalus observed among NMO patients. Moreover, disruption of the blood–CSF barrier induced by binding of NMO IgG to AQP4 on the basolateral surface of choroid plexus epithelial cells may provide a unique portal for entry of the pathogenic antibody into the central nervous system.

Nanoagonist-mediated endothelial tight junction opening: A strategy for safely increasing brain drug delivery in mice

Even though opening endothelial tight junctions is an efficient way to up-regulate brain drug delivery, the extravasation of blood-borne components from the compromised tight junctions can result in adverse consequences such as edema and neuronal injuries. In this work, we developed a nanoagonist that temporarily opened tight junctions by signaling adenosine 2A receptor, a type of G protein-coupled receptor expressed on brain capillary endothelial cells. Magnetic resonance imaging demonstrated remarkable blood-brain barrier permeability enhancements and significantly increased brain uptakes of both small molecular and macromolecular paramagnetic agents after nanoagonist administration. Gamma ray imaging and transmission electron microscope observed tight junction opening followed by spontaneous recovery after nanoagonist treatment. Immunofluorescence staining showed the unspoiled basal membrane, pericytes and astrocyte endfeet that enwrapped the vascular endothelium. Importantly, edema, apoptosis and neuronal injuries observed after hypertonic agent mediated tight junction-opening were not observed after nanoagonist intervention. The uncompromised neurovascular units may prevent the leakage of blood-borne constituents into brain parenchyma and accelerate tight junction recovery. Considering blood-brain barrier impermeability is a major obstacle in the treatment of central nervous system diseases, nanoagonist-mediated tight junction opening provides a promising strategy to enhance brain drug delivery with minimized adverse effects.

Aquaporin 4 Forms a Macromolecular Complex with Glutamate Transporter 1 and Mu Opioid Receptor in Astrocytes and Participates in Morphine Dependence

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes and provides a mechanism by which water permeability of the plasma membrane can be regulated. Evidence suggests that AQP4 is associated with glutamate transporter-1 (GLT-1) for glutamate clearance and contributes to morphine dependence. Previous studies show that AQP4 deficiency changed the mu opioid receptor expression and opioid receptors' characteristics as well. In this study, we focused on whether AQP4 could form macromolecular complex with GLT-1 and mu opioid receptor (MOR) and participates in morphine dependence. By using immunofluorescence staining, fluorescence resonance energy transfer, and co-immunoprecipitation, we demonstrated that AQP4 forms protein complexes with GLT-1 and MOR in both brain tissue and primary cultured astrocytes. We then showed that the C-terminus of AQP4 containing the amino acid residues 252 to 323 is the site of interaction with GLT-1. Protein kinase C, activated by morphine, played an important role in regulating the expression of these proteins. These findings may help to reveal the mechanism that AQP4, GLT-1, and MOR form protein complex and participate in morphine dependence, and deeply understand the reason that AQP4 deficiency maintains extracellular glutamate homeostasis and attenuates morphine dependence, moreover emphasizes the function of astrocyte in morphine dependence.

Human and mouse cortical astrocytes differ in aquaporin-4 polarization toward microvessels

Aquaporin‐4 (AQP4), the predominant water channel in the brain, is expressed in astrocytes and ependymal cells. In rodents AQP4 is highly polarized to perivascular astrocytic endfeet and loss of AQP4 polarization is associated with disease. The present study was undertaken to compare the expression pattern of AQP4 in human and mouse cortical astrocytes. Cortical tissue specimens were sampled from 11 individuals undergoing neurosurgery wherein brain tissue was removed as part of the procedure, and compared with cortical tissue from 5 adult wild‐type mice processed similarly. The tissue samples were immersion‐fixed and prepared for AQP4 immunogold electron microscopy, allowing quantitative assessment of AQP4's subcellular distribution. In mouse we found that AQP4 water channels were prominently clustered around vessels, being 5 to 10‐fold more abundant in astrocytic endfoot membranes facing the capillary endothelium than in parenchymal astrocytic membranes. In contrast, AQP4 was markedly less polarized in human astrocytes, being only two to three‐fold enriched in astrocytic endfoot membranes adjacent to capillaries. The lower degree of AQP4 polarization in human subjects (1/3 of that in mice) was mainly due to higher AQP4 expression in parenchymal astrocytic membranes. We conclude that there are hitherto unrecognized species differences in AQP4 polarization toward microvessels in the cerebral cortex.

3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) produces edema due to BBB disruption induced by MMP-9 activation in rat hippocampus

The recreational drug of abuse, 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) disrupts blood-brain barrier (BBB) integrity in rats through an early P2X₇ receptor-mediated event which induces MMP-9 activity. Increased BBB permeability often causes plasma proteins and water to access cerebral tissue leading to vasogenic edema formation. The current study was performed to examine the effect of a single neurotoxic dose of MDMA (12.5 mg/kg, i.p.) on in vivo edema development associated with changes in the expression of the perivascular astrocytic water channel, AQP4, as well as in the expression of the tight-junction (TJ) protein, claudin-5 and Evans Blue dye extravasation in the hippocampus of adult male Dark Agouti rats. We also evaluated the ability of the MMP-9 inhibitor, SB-3CT (25 mg/kg, i.p.), to prevent these changes in order to validate the involvement of MMP-9 activation in MDMA-induced BBB disruption. The results show that MDMA produces edema of short duration temporally associated with changes in AQP4 expression and a reduction in claudin-5 expression, changes which are prevented by SB-3CT. In addition, MDMA induces a short-term increase in both tPA activity and expression, a serine-protease which is involved in BBB disruption and upregulation of MMP-9 expression. In conclusion, this study provides evidence enough to conclude that MDMA induces edema of short duration due to BBB disruption mediated by MMP-9 activation.

The role of aquaporin-4 in synaptic plasticity, memory and disease

Since the discovery of aquaporins, it has become clear that the various mammalian aquaporins play critical physiological roles in water and ion balance in multiple tissues. Aquaporin-4 (AQP4), the principal aquaporin expressed in the central nervous system (CNS, brain and spinal cord), has been shown to mediate CNS water homeostasis. In this review, we summarize new and exciting studies indicating that AQP4 also plays critical and unanticipated roles in synaptic plasticity and memory formation. Next, we consider the role of AQP4 in Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), multiple sclerosis (MS), neuromyelitis optica (NMO), epilepsy, traumatic brain injury (TBI), and stroke. Each of these conditions involves changes in AQP4 expression and/or distribution that may be functionally relevant to disease physiology. Insofar as AQP4 is exclusively expressed on astrocytes, these data provide new evidence of "astrocytopathy" in the etiology of diverse neurological diseases.

Translational readthrough generates new astrocyte AQP4 isoforms that modulate supramolecular clustering, glial endfeet localization, and water transport

Regulation of water homeostasis is a central feature of central nervous system pathophysiology. In this context, several lines of evidence suggest a crucial role for the water channel aquaporin-4 (AQP4) and its plasma membrane supramolecular organization as the key element. Here, we demonstrate the expression in tissues of additional isoforms of AQP4 characterized by a C-terminal extension generated by programmed translational readthrough. These extended isoforms (AQP4ex) display a perivascular polarization and expression in dystrophin-dependent pools. AQP4ex reduces supramolecular clustering tendency and allows AQP4 interactions with syntrophin. Furthermore, site-directed mutagenesis of two serines in the extended C-terminus of AQP4ex showed potential regulation of water permeability by phosphorylation. Finally, AQP4ex expression can be positively modulated by gentamicin treatment, demonstrating the possibility of regulating the AQP4 translational readthrough frequency. This novel regulatory mechanism could have important pathophysiological implications for conditions in which alternations have been reported in AQP4 structure.

Aquaporin-4 and Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia. Although the pathogenesis of AD remains unclear, AD is thought to result from an imbalance in the production and clearance of amyloid-β protein (Aβ). Aquaporin-4 (AQP4) is the major aquaporin in the mammalian brain, is mostly expressed on astrocytic endfeet, and functions as a water transporter. However, the distribution and expression of AQP4 are altered in both AD clinical populations and animal models. Recent studies have revealed that AQP4 is important to the clearance of Aβ in brain via lymphatic clearance, transcytotic delivery, and glial degradation, as well as to the synaptic function. Thus, AQP4 likely plays an important role in the pathogenesis of AD. Further studies would provide new targets for prevention, ultimately leading to improved treatment options for AD.

RNA interference influenced the proliferation and invasion of XWLC-05 lung cancer cells through inhibiting aquaporin 3

OBJECTIVE

The objective of this study was to construct a recombinant vector expressing siRNA targetedly inhibiting aquaporin 3 (AQP3), and to evaluate the effects of AQP3 inhibition on the proliferation and invasion of XWLC-05 human lung cancer cells.

METHODS

We obtained human AQP3 sequence from the Genbank and established the recombinant vector expressing siRNA targeting AQP3. After the transfection of the recombinant vectors, the expression of AQP3 was determined by RT-PCR and western blot. The MTS assay, flow cytometry and Transwell assay were conducted to detect the proliferation, cell cycle process, apoptosis and invasion of XWLC-05 cells. Then the activity of metal matrix proteinase (MMP) 2 was determined by gelatin zymography. Tumor formation in vivo experiments were also conducted in nude mice.

RESULTS

RNA interference (RNAi) of AQP3 substantially suppressed the XWLC-05 cell proliferation and invasion, blocked the cell cycle progressing and promoted cell apoptosis. In addition, the activity of MMP2 was remarkably attenuated in RNAi group. AQP3 RNAi did not affect the tumor formation rate in nude mice but reduced the tumor growth.

CONCLUSION

The inhibition of AQP3 retarded the growth and invasiveness of XWLC-05 lung cancer cells and decreased the activity of MMP2.

Role of Aquaporin 1 Signalling in Cancer Development and Progression

Cancer is a major health burden worldwide. Despite the advances in our understanding of its pathogenesis and continued improvement in cancer management and outcomes, there remains a strong clinical demand for more accurate and reliable biomarkers of metastatic progression and novel therapeutic targets to abrogate angiogenesis and tumour progression. Aquaporin 1 (AQP1) is a small hydrophobic integral transmembrane protein with a predominant role in trans-cellular water transport. Recently, over-expression of AQP1 has been associated with many types of cancer as a distinctive clinical prognostic factor. This has prompted researchers to evaluate the link between AQP1 and cancer biological functions. Available literature implicates the role of AQP1 in tumour cell migration, invasion and angiogenesis. This article reviews the current understanding of AQP1-facilitated tumour development and progression with a focus on regulatory mechanisms and downstream signalling pathways.

Conivaptan, a Selective Arginine Vasopressin V1a and V2 Receptor Antagonist Attenuates Global Cerebral Edema Following Experimental Cardiac Arrest via Perivascular Pool of Aquaporin-4

BACKGROUND

Cerebral edema is a major cause of mortality following cardiac arrest (CA) and cardiopulmonary resuscitation (CPR). Arginine vasopressin (AVP) and water channel aquaporin-4 (AQP4) have been implicated in the pathogenesis of CA-evoked cerebral edema. In this study, we examined if conivaptan, a V1a and V2 antagonist, attenuates cerebral edema following CA/CPR in wild type (WT) mice as well as mice with targeted disruption of the gene encoding α-syntrophin (α-syn(-/-)) that demonstrate diminished perivascular AQP4 pool.

METHODS

Isoflurane-anesthetized adult male WT C57Bl/6 and α-syn(-/-) mice were subjected to 8 min CA/CPR and treated with either bolus IV injection (0.15 or 0.3 mg/kg) followed by continuous infusion of conivaptan (0.15 mg/kg/day or 0.3 mg/kg/day), or vehicle infusion for 48 h. Serum osmolality, regional brain water content, and blood-brain barrier (BBB) disruption were determined at the end of the experiment. Sham-operated mice in both strains served as controls.

RESULTS

Treatment with conivaptan elevated serum osmolality in a dose-dependent manner. In WT mice, conivaptan at 0.3 mg dose significantly attenuated regional water content in the caudoputamen (81.0 ± 0.5 vs. 82.5 ± 0.4% in controls; mean ± SEM) and cortex (78.8 ± 0.2 vs. 79.4 ± 0.2% in controls), while conivaptan at 0.15 mg was not effective. In α-syn(-/-) mice, conivaptan at 0.3 mg dose did not attenuate water content compared with controls. Conivaptan (0.3 mg/kg/day) attenuated post-CA BBB disruption at 48 h in WT mice but not in α-syn(-/-) mice.

CONCLUSIONS

Continuous IV infusion of conivaptan attenuates cerebral edema and BBB disruption following CA. These effects of conivaptan that are dependent on the presence of perivascular pool of AQP4 appear be mediated via its dual effect on V1 and V2 receptors.

Aquaporins in Nervous System

Aquaporins (AQPs ) mediate water flux between the four distinct water compartments in the central nervous system (CNS). In the present chapter, we mainly focus on the expression and function of the 9 AQPs expressed in the CNS, which include five members of aquaporin subfamily: AQP1, AQP4, AQP5, AQP6, and AQP8; three members of aquaglyceroporin subfamily: AQP3, AQP7, and AQP9; and one member of superaquaporin subfamily: AQP11. In addition, AQP1, AQP2 and AQP4 expressed in the peripheral nervous system (PNS) are also reviewed. AQP4, the predominant water channel in the CNS, is involved both in the astrocyte swelling of cytotoxic edema and the resolution of vasogenic edema, and is of pivotal importance in the pathology of brain disorders such as neuromyelitis optica , brain tumors and Alzheimer's disease. Other AQPs are also involved in a variety of important physiological and pathological process in the brain. It has been suggested that AQPs could represent an important target in treatment of brain disorders like cerebral edema. Future investigations are necessary to elucidate the pathological significance of AQPs in the CNS.

Aquaporin Water Channels and Hydrocephalus

The aquaporins (AQPs) are a family of water-transporting proteins that are broadly expressed in mammalian cells. Two AQPs in the central nervous system, AQP1 and AQP4, might play a role in hydrocephalus and are thus potential drug targets. AQP1 is expressed in the ventricular-facing membrane of choroid plexus epithelial cells, where it facilitates the secretion of cerebrospinal fluid (CSF). AQP4 is expressed in astrocyte foot processes and ependymal cells lining ventricles, where it appears to facilitate the transport of excess water out of the brain. Altered expression of these AQPs in experimental animal models of hydrocephalus and limited human specimens suggests their involvement in the pathophysiology of hydrocephalus, as do data in knockout mice demonstrating a protective effect of AQP1 deletion and a deleterious effect of AQP4 deletion in hydrocephalus. Though significant questions remain, including the precise contribution of AQP1 to CSF secretion in humans and the mechanisms by which AQP4 facilitates clearance of excess brain water, AQP1 and AQP4 have been proposed as potential drug targets to reduce ventricular enlargement in hydrocephalus.

The study of the function of AQP4 in cerebral ischaemia-reperfusion injury using poroelastic theory

Brain oedema is thought to form and to clear through the use of water-protein channels, aquaporin-4 (AQP4), which are found in the astrocyte endfeet. The model developed here is used to study the function of AQP4 in the formation and elimination of oedema fluid in ischaemia-reperfusion injury. The cerebral space is assumed to be made of four fluid compartments: astrocyte, neuron, ECS and blood microvessels, and a solid matrix for the tissue, and this is modelled using multiple-network poroelastic theory. AQP4 allows the movement of water between astrocyte and the ECS and the microvessels. It is found that the presence of AQP4 may help in reducing vasogenic oedema shown by a decrease in brain tissue extracellular pressure. However, the astrocyte pressure will increase to compensate for this decrease, which may lead to cytotoxic oedema. In addition, the swelling will also depend on the ionic concentrations in the astrocyte and extracellular space, which may change after ischaemic stroke. Understanding the role of AQP4 in oedema may thus help the development of a treatment plan in reducing brain swelling after ischaemia-reperfusion.