Regulation of brain aquaporins

Water and brain function: effects of hydration status on neurostimulation with transcranial magnetic stimulation

Neurostimulation/neurorecording are tools to study, diagnose, and treat neurological/psychiatric conditions. Both techniques depend on volume conduction between scalp and excitable brain tissue. Here, we examine how neurostimulation with transcranial magnetic stimulation (TMS) is affected by hydration status, a physiological variable that can influence the volume of fluid spaces/cells, excitability, and cellular/global brain functioning. Normal healthy adult participants (32, 9 males) had common motor TMS measures taken in a repeated-measures design from dehydrated (12-h overnight fast/thirst) and rehydrated (identical dehydration protocol followed by rehydration with 1 L water in 1 h) testing days. The target region was left primary motor cortex hand area. Response at the target muscle was recorded with electromyography. Urinalysis confirmed hydration status. Motor hotspot shifted in half of participants. Motor threshold decreased in rehydration, indicating increased excitability. Even after redosing/relocalizing TMS to the new threshold/hotspot, rehydration still showed evidence of increased excitability: recruitment curve measures generally shifted upward and the glutamate-dependent paired-pulse protocol, short intracortical facilitation (SICF), was increased. Short intracortical inhibition (SICI), long intracortical inhibition (LICI), long intracortical facilitation (LICF), and cortical silent period (CSP) were relatively unaffected. The hydration perturbations were mild/subclinical based on the magnitude/speed and urinalysis. Motor TMS measures showed evidence of expected physiological changes of osmotic challenges. Rehydration showed signs of macroscopic and microscopic volume changes including decreased scalp-cortex distance (brain closer to stimulator) and astrocyte swelling-induced glutamate release. Hydration may be a source of variability affecting any techniques dependent on brain volumes/volume conduction. These concepts are important for researchers/clinicians using such techniques or dealing with the wide variety of disease processes involving water balance.NEW & NOTEWORTHY Hydration status can affect brain volumes and excitability, which should affect techniques dependent on electrical volume conduction, including neurostimulation/recording. We test the previously unknown effects of hydration on neurostimulation with TMS and briefly review relevant physiology of hydration. Rehydration showed lower motor threshold, shifted motor hotspot, and generally larger responses even after compensating for threshold/hotspot changes. This is important for clinical and research applications of neurostimulation/neurorecording and the many clinical disorders related to water balance.

Alterations of apparent diffusion coefficient from ultra high b-values in the bilateral thalamus and striatum in MRI-negative drug-resistant epilepsy

OBJECTIVE

Subcortical nuclei such as the thalamus and striatum have been shown to be related to seizure modulation and termination, especially in drug-resistant epilepsy. Enhance diffusion-weighted imaging (eDWI) technique and tri-component model have been used in previous studies to calculate apparent diffusion coefficient from ultra high b-values (ADCuh). This study aimed to explore the alterations of ADCuh in the bilateral thalamus and striatum in MRI-negative drug-resistant epilepsy.

METHODS

Twenty-nine patients with MRI-negative drug-resistant epilepsy and 18 healthy controls underwent eDWI scan with 15 b-values (0-5000 s/mm2). The eDWI parameters including standard ADC (ADCst), pure water diffusion (D), and ADCuh were calculated from the 15 b-values. Regions-of-interest (ROIs) analyses were conducted in the bilateral thalamus, caudate nucleus, putamen, and globus pallidus. ADCst, D, and ADCuh values were compared between the MRI-negative drug-resistant epilepsy patients and controls using multivariate generalized linear models. Inter-rater reliability was assessed using the intra-class correlation coefficient (ICC) and Bland-Altman (BA) analysis. False discovery rate (FDR) method was applied for multiple comparisons correction.

RESULTS

ADCuh values in the bilateral thalamus, caudate nucleus, putamen, and globus pallidus in MRI-negative drug-resistant epilepsy were significantly higher than those in the healthy control subjects (all p < 0.05, FDR corrected).

SIGNIFICANCE

The alterations of the ADCuh values in the bilateral thalamus and striatum in MRI-negative drug-resistant epilepsy might reflect abnormal membrane water permeability in MRI-negative drug-resistant epilepsy. ADCuh might be a sensitive measurement for evaluating subcortical nuclei-related brain damage in epilepsy patients.

PLAIN LANGUAGE SUMMARY

This study aimed to explore the alterations of apparent diffusion coefficient calculated from ultra high b-values (ADCuh) in the subcortical nuclei such as the bilateral thalamus and striatum in MRI-negative drug-resistant epilepsy. The bilateral thalamus and striatum showed higher ADCuh in epilepsy patients than healthy controls. These findings may add new evidences of subcortical nuclei abnormalities related to water and ion hemostasis in epilepsy patients, which might help to elucidate the underlying epileptic neuropathophysiological mechanisms and facilitate the exploration of therapeutic targets.

Alteration of Aquaporins 1 and 4 immunohistochemical and gene expression in the cerebellum of diabetic albino rat

Aquaporins (AQPs) are a family of transmembrane channel proteins. AQP1 and AQP4 are expressed in cerebellum amongst others. This study was designed to assess the effect of diabetes on AQP1 and AQP4 expression in cerebellum of rats. Diabetes was induced by a single intraperitoneal injection of Streptozotocin 45 mg/kg in 24 adult male Sprague Dawley rats. Six rats from control and diabetic groups were sacrificed at one, four, and eight weeks post diabetic confirmation. After eight weeks, measurement of malondialdehyde (MDA), reduced glutathione (GSH) concentrations, and cerebellar mRNA expression for AQP1 and AQP4 genes were performed. Immunohistochemical evaluation of AQP1, AQP4, and glial fibrillary acidic protein (GFAP) for cerebellar sections was performed for all groups. Diabetes caused degenerative changes in Purkinje cells with a significant increase in the cerebellar level of MDA and AQP1 immunoreactivity and a significant decrease in GSH level and AQP4 expression levels. However, the alteration in the AQP1 mRNA level was not statistically significant. GFAP immunoreactivity was increased in 8 W diabetic rats following its decrease in 1 W diabetic rats. Diabetes caused some alteration in the AQPs 1 and 4 expression in the cerebellum of diabetic rats which may contribute to diabetes-induced cerebellar complications.

Forensic application of aquaporins

Aquaporins (AQPs) are a family of water channel proteins that primarily elicit the basic functions of water transport and osmotic homeostasis. To date, at least 17 mammalian AQPs have been identified, AQP-0 to -12 have been found in higher orders including human, and AQP-13 to -16 have been described in older lineages. Moreover, these proteins have recently been shown to regulate many biological processes through unique activities, such as cell proliferation, migration, apoptosis, and mitochondrial metabolism. Several studies have focused on the involvement of AQPs in cell biology aspect, showing that they are involved in a variety of physiological processes and pathophysiological conditions. Furthermore, in the field of forensic medicine, studies on whether AQPs can be a useful marker for diagnosing various causes of death have been conducted using autopsy samples and animal experiments, which have produced interesting results. Herein, we review certain observations regarding AQPs and discuss their potential to contribute to the future practice of forensic research.

Classification and Gene Structure of Aquaporins

Aquaporins (AQPs) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, 13 AQPs, distributed widely in specific cell types in various organs and tissues, have been characterized in humans. A pair of NPA boxes forming a pore is highly conserved among all aquaporins and is also key residues for the classification of AQP superfamily into four groups according to primary sequences. AQPs may also be classified based on their transport properties. So far, chromosome localization and gene structure of 13 human AQPs have been identified, which is definitely helpful for studying phenotypes and potential targets in naturally occurring and synthetic mutations in human or cells.

Protein Structure and Modification of Aquaporins

Aquaporins (AQPs) allow water molecules and other small, neutral solutes to quickly pass through membrane. The protein structures of AQPs solved by crystallographic methods or cryo-electron microscopy technology show that AQP monomer consists of six membrane-spanning alpha-helices that form the central water-transporting pore. AQP monomers assemble to form tetramers, forming the functional units in the membrane, to transport water or other small molecules. The biological functions of AQPs are regulated by posttranslational modifications, e.g., phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation and protein interactions. Modifications of AQP combined with structural properties contribute to a better functional mechanism of AQPs. Insight into the molecular mechanisms responsible for AQP modifications as well as gating and transport properties proved to be fundamental to the development of new therapeutic targets or reliable diagnostic and prognostic biomarkers.

The Readthrough Isoform AQP4ex Is Constitutively Phosphorylated in the Perivascular Astrocyte Endfeet of Human Brain

AQP4ex is a recently discovered isoform of AQP4 generated by a translational readthrough mechanism. It is strongly expressed at the astrocyte perivascular endfeet as a component of the supramolecular membrane complex, commonly called orthogonal array of particles (OAP), together with the canonical isoforms M1 and M23 of AQP4. Previous site-directed mutagenesis experiments suggested the potential role of serine331 and serine335, located in the extended peptide of AQP4ex, in water channel activity by phosphorylation. In the present study we evaluated the effective phosphorylation of human AQP4ex. A small scale bioinformatic analysis indicated that only Ser335 is conserved in human, mouse and rat AQP4ex. The phosphorylation site of Ser335 was assessed through generation of phospho-specific antibodies in rabbits. Antibody specificity was first evaluated in binding phosphorylated peptide versus its unphosphorylated analog by ELISA, which was further confirmed by site-directed mutagenesis experiments. Western blot and immunofluorescence experiments revealed strong expression of phosphorylated AQP4ex (p-AQP4ex) in human brain and localization at the perivascular astrocyte endfeet in supramolecular assemblies identified by BN/PAGE experiments. All together, these data reveal, for the first time, the existence of a phosphorylated form of AQP4, at Ser335 in the extended sequence exclusive of AQP4ex. Therefore, we anticipate an important physiological role of p-AQP4ex in human brain water homeostasis.

A Novel Gating Mechanism of Aquaporin-4 Water Channel Mediated by Blast Shockwaves for Brain Edema

As the principal water channel in the brain, aquaporin-4 (AQP4) plays a vital role in brain edema, but its role in blast brain edema is unclear. On the basis of molecular simulations, we reveal the atomically detailed picture of AQP4 in response to blast shockwaves. The results show that the shockwave alone closes the AQP4 channel; however, shock-induced bubble collapse opens it. The jet from bubble collapse forcefully increases the distance between helices and the tilt angles of six helices relative to the membrane vertical direction in a very short time. The average channel size increases about 2.6 times, and the water flux rate is nearly 20 times higher than for normal states. It is responsible for abnormal water transport and a potential cause of acute blast brain edema. Additionally, the open AQP4 channel quickly returns to its normal state, which is in turn helpful for edema absorption. Thus, a novel gating mechanism for AQP4 related to the secondary structure change has been provided, which is different from the previous residue-mediated gating mechanism.

Uremic encephalopathy

Uremic encephalopathy encompasses a wide range of central nervous system abnormalities associated with poor kidney function occurring with either progressive chronic kidney disease or acute kidney injury. The syndrome is likely caused by retention of uremic solutes, alterations in hormonal metabolism, changes in electrolyte and acid-base homeostasis, as well as changes in vascular reactivity, blood-brain barrier transport, and inflammation. There are no defining clinical, laboratory, or imaging findings, and the diagnosis is often made retrospectively when symptoms improve after dialysis or transplantation. The diagnosis is also made difficult because of the many confounding and overlapping conditions seen in patients with chronic kidney disease and acute kidney injury. Thus, institution of kidney replacement therapy should be considered as a trial to improve symptoms in the right clinical context. Neurological symptoms that do not improve after improvement in clearance should prompt a search for other explanations. Further knowledge linking possible uremic retention solutes with neurological symptoms is needed to better understand this syndrome as well as to develop more tailored treatments that aim to improve cognitive function.

Human Cord Blood Derived Unrestricted Somatic Stem Cells Restore Aquaporin Channel Expression, Reduce Inflammation and Inhibit the Development of Hydrocephalus After Experimentally Induced Perinatal Intraventricular Hemorrhage

Intraventricular hemorrhage (IVH) is a severe complication of preterm birth associated with cerebral palsy, intellectual disability, and commonly, accumulation of cerebrospinal fluid (CSF). Histologically, IVH leads to subependymal gliosis, fibrosis, and disruption of the ependymal wall. Importantly, expression of aquaporin channels 1 and 4 (AQP1 and AQP4) regulating respectively, secretion and absorption of cerebrospinal fluids is altered with IVH and are associated with development of post hemorrhagic hydrocephalus. Human cord blood derived unrestricted somatic stem cells (USSCs), which we previously demonstrated to reduce the magnitude of hydrocephalus, as having anti-inflammatory, and beneficial behavioral effects, were injected into the cerebral ventricles of rabbit pups 18 h after glycerol-induced IVH. USSC treated IVH pups showed a reduction in ventricular size when compared to control pups at 7 and 14 days (both, P < 0.05). Histologically, USSC treatment reduced cellular infiltration and ependymal wall disruption. In the region of the choroid plexus, immuno-reactivity for AQP1 and ependymal wall AQP4 expression were suppressed after IVH but were restored following USSC administration. Effects were confirmed by analysis of mRNA from dissected choroid plexus and ependymal tissue. Transforming growth factor beta (TGF-β) isoforms, connective tissue growth factor (CTGF) and matrix metalloprotease-9 (MMP-9) mRNA, as well as protein levels, were significantly increased following IVH and restored towards normal with USSC treatment (P < 0.05). The anti-inflammatory cytokine Interleukin-10 (IL-10) mRNA was reduced in IVH, but significantly recovered after USSC injection (P < 0.05). In conclusion, USSCs exerted anti-inflammatory effects by suppressing both TGF-β specific isoforms, CTGF and MMP-9, recovered IL-10, restored aquaporins expression towards baseline, and reduced hydrocephalus. These results support the possibility of the use of USSCs to reduce IVH consequences in prematurity.

Down-regulation of Aquaporin-1 mediates a microglial phenotype switch affecting glioma growth

Aquaporin 1 (AQP1), a transmembrane protein that forms water channels, has previously been shown to facilitate growth and progression of many types of tumors by modulating tumor cell migration, proliferation and angiogenesis. Here, we determined the impact of AQP1 expression in the tumor environment on the progression of brain tumors. Primary microglia from wild type(WT) and AQP1 knockout(KO) mice were used to test AQP1 effect on microglia function by using Western blot, quantative PCR, in an experimental in vivo mouse glioma model and organotypic brain slice culture. Deletion of AQP1 in the host tissue significantly reduced the survival of the mice implanted with GL261 glioma cells. The density of glioma-associated microglia/macrophages was almost doubled in AQP1KO mice. We found that factors secreted from GL261 cells decrease microglial AQP1 expression via the MEK/ERK pathway, and that inhibition of this pathway with Trametinib reduced tumor growth and prolonged the survival of tumor bearing mice, an effect which required the presence of microglia. Deletion of AQP1 in cultured microglia resulted in an increase in migratory activity and a decrease in TLR4-dependent innate immune responses. Our study demonstrates a functional relevance of AQP1 expression in microglia and hints to AQP1 as a potential novel target for glioma therapy.

Phosphorylation-Dependent Regulation of Mammalian Aquaporins

Water homeostasis is fundamental for cell survival. Transport of water across cellular membranes is governed by aquaporins—tetrameric integral membrane channels that are highly conserved throughout the prokaryotic and eukaryotic kingdoms. In eukaryotes, specific regulation of these channels is required and is most commonly carried out by shuttling the protein between cellular compartments (trafficking) or by opening and closing the channel (gating). Structural and functional studies have revealed phosphorylation as a ubiquitous mechanism in aquaporin regulation by both regulatory processes. In this review we summarize what is currently known about the phosphorylation-dependent regulation of mammalian aquaporins. Focusing on the water-specific aquaporins (AQP0–AQP5), we discuss how gating and trafficking are controlled by phosphorylation and how phosphorylation affects the binding of aquaporins to regulatory proteins, thereby highlighting structural details and dissecting the contribution of individual phosphorylated residues when possible. Our aim is to provide an overview of the mechanisms behind how aquaporin phosphorylation controls cellular water balance and to identify key areas where further studies are needed.

AQP4 tag SNPs in patients with intracerebral hemorrhage in Greek and Polish population

BACKROUND

A relatively small number of genetic variants are implicated to pathophysiology of intracerebral hemorrhage (ICH). Aquaporin-4 (AQP4) has been reported to be implicated in the pathophysiological processes of ICH development.

OBJECTIVE

To examine the role of AQP4 gene region polymorphisms on the ICH risk.

METHODS

A total of 250 Greek and 193 Polish patients with primary ICH and 250 and 322 respective controls were enrolled, forming two independent cohorts in order to validate any significant effect. With logistic regression analyses, 7 AQP4 tag single nucleotide polymorphisms (SNPs) were examined for association with ICH risk, lobar/non-lobar ICH risk, and 6-month disability after ICH. Cox regression analysis was applied in order to the effect of AQP4 SNPs on ICH age of onset be tested. Correction for multiple comparisons was applied.

RESULTS

Multivariate logistic regression analysis showed that rs3875089 in the Greek cohort and rs3763043, rs335931 in the Polish cohort had a significant influence on the risk of ICH, lobar and non-lobar ICH. Regarding the age of onset, rs3875089 in the Greek cohort and rs3763043, rs11661256 in the Polish cohort were found to significantly alter the age of onset of ICH and its subtypes. However, all of the above associations did not survive the Bonferroni correction (p-value >0.007). Finally, AQP4 tag SNPs were not found to have any significant effect on long-term disability after ICH.

CONCLUSIONS

In conclusion, the present study provides an indication that AQP4 gene variants may affect susceptibility to primary ICH and may influence the ICH age of onset.

Specific Alterations in Astrocyte Properties via the GluA2-GAPDH Complex Associated with Multiple Sclerosis

There is strong evidence indicating neuroinflammation is an important mediator in multiple sclerosis (MS), with astrogliosis playing a significant role in this process. Surprisingly, astrocytes exert paradoxical roles during disease development, but the mechanisms remain unknown. Previously, we have reported that administering an interfering peptide (GluA2-G-Gpep) which specifically disrupts the GluA2-GAPDH interaction rescued neurological symptoms in the EAE mouse model of MS. In this study, we validated that the GluA2-GAPDH complex was elevated in LPS-induced primary reactive astrocytes, and GluA2-G-Gpep treatment significantly reduced GFAP expression levels in both EAE mice and reactive astrocytes. Further in vivo and in vitro analyses revealed that GluA2-G-Gpep administration normalized EAAT1 and EAAT2 expression, rescued compromised blood-brain barrier integrity via AQP4, promoted actin reorganization and changed mitochondrial dynamics. These alterations may partially be explained by changes in the nuclear GAPDH and p53 transcription pathways. Our findings provide critical implications for understanding the astrocyte properties regulated by GluA2-GAPDH associated with MS, and insights for novel treatment options targeting at astrocytes.

The potential roles of aquaporin 4 in malignant gliomas

Aquaporin 4 (AQP4) is the major water channel expressed in the central nervous system and is primarily expressed in astrocytes. Recently, accumulated evidence has pointed to AQP4 as a key molecule that could play a critical role in glioma development. Discoveries of the role of AQP4 in cell migration suggest that AQP4 could be a significant factor regarding glioma malignancies. However, the AQP4 expression levels in glioma have not been fully elucidated; furthermore, the correlation of AQP4 expression with glioma malignancy remains controversial. Here, we review the expression pattern and predictive significance of AQP4 in malignant glioma. The molecular mechanism of AQP4 as it pertains to the migration and invasion of human glioma cells has been summarized. In addition, the important roles of AQP4 in combating drug resistance as well as potential pharmacological blockers of AQP4 have been systematically discussed. More research should be conducted to elucidate the potential roles of AQP4 in malignant glioma for identifying the tumor type, progression stages and optimal treatment strategies. The observed experimental results strongly emphasize the importance of this topic for future investigations.

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.

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.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

Glucocorticoids in the management of peritumoral brain edema: a review of molecular mechanisms

Peritumoral brain edema (PTBE) is mediated by blood-brain barrier breakdown. PTBE results from interstitial vasogenic brain edema due to vascular endothelial growth factor and other inflammatory products of brain tumors. Glucocorticoids (GCs) are the mainstay for treatment of PTBE despite significant systemic side effects. GCs are thought to affect multiple cell types in the edematous brain. Here, we review preclinical studies of GC effects on edematous brain and review mechanisms underlying GC action on tumor cells, endothelial cells, and astrocytes. GCs may reduce tumor cell viability and suppress vascular endothelial growth factor (VEGF) production in tumor cells. Modulation of expression and distribution of tight junction proteins occludin, claudin-5, and ZO-1 in endothelial cells likely plays a central role in GC action on endothelial cells. GCs may also have an effect on astrocyte angiopoietin production and limited effect on astrocyte aquaporin. A better understanding of these molecular mechanisms may lead to the development of novel therapeutics for management of PTBE with a better side effect profile.

Fragment Screening of Human Aquaporin 1

Aquaporins (AQPs) are membrane proteins that enable water transport across cellular plasma membranes in response to osmotic gradients. Phenotypic analyses have revealed important physiological roles for AQPs, and the potential for AQP water channel modulators in various disease states has been proposed. For example, AQP1 is overexpressed in tumor microvessels, and this correlates with higher metastatic potential and aggressiveness of the malignancy. Chemical modulators would help in identifying the precise contribution of water channel activity in these disease states. These inhibitors would also be important therapeutically, e.g., in anti-cancer treatment. This perceived importance contrasts with the lack of success of high-throughput screens (HTS) to identify effective and specific inhibitors of aquaporins. In this paper, we have screened a library of 1500 "fragments", i.e., smaller than molecules used in HTS, against human aquaporin (hAQP1) using a thermal shift assay and surface plasmon resonance. Although these fragments may not inhibit their protein target, they bound to and stabilized hAQP1 (sub mM binding affinities (KD), with an temperature of aggregation shift ΔTagg of +4 to +50 °C) in a concentration-dependent fashion. Chemically expanded versions of these fragments should follow the determination of their binding site on the aquaporin surface.

Enabling dual cellular destinations of polymeric nanoparticles for treatment following partial injury to the central nervous system

Following neurotrauma, oxidative stress is spread via the astrocytic syncytium and is associated with increased aquaporin 4 (AQP4), inflammatory cell infiltration, loss of neurons and glia and functional deficits. Herein we evaluate multimodal polymeric nanoparticles functionalized with an antibody to an extracellular epitope of AQP4, for targeted delivery of an anti-oxidant as a therapeutic strategy following partial optic nerve transection. Using fluorescence microscopy, spectrophotometry, correlative nanoscale secondary ion mass spectrometry (NanoSIMS) and transmission electron microscopy, in vitro and in vivo, we demonstrate that functionalized nanoparticles are coated with serum proteins such as albumin and enter both macrophages and astrocytes when administered to the site of a partial optic nerve transection in rat. Antibody functionalized nanoparticles synthesized to deliver the antioxidant resveratrol are effective in reducing oxidative damage to DNA, AQP4 immunoreactivity and preserving visual function. Non-functionalized nanoparticles evade macrophages more effectively and are found more diffusely, including in astrocytes, however they do not preserve the optic nerve from oxidative damage or functional loss following injury. Our study highlights the need to comprehensively investigate nanoparticle location, interactions and effects, both in vitro and in vivo, in order to fully understand functional outcomes.

Effects of propofol and sevoflurane on aquaporin-4 and aquaporin-9 expression in patients performed gliomas resection

Post-operative cerebral edema is a threat for patients performed gliomas resection. Some studies have shown that general anesthesia drugs, such as, propofol had neuroprotective effect. Aquaporin-4 (AQP4) and Aquaporin-9 (AQP9) play an important role in maintaining brain water homeostasis under various conditions. The aim of this study was to compare the effect of propofol or sevoflurane on expression of AQP4 and AQP9 in patients performed gliomas resection. 30 patients performed gliomas resection were included in this study. The patients were randomly divided into two groups: propofol group and sevoflurane group. Fresh human gliomas specimens were obtained and hematoxylin eosin (HE) staining, immunohistochemical staining and Western blot analysis were used for observation of the expression of AQP4 and AQP9. The immunohistochemical staining of the sections showed that the percentage of AQP4 positive cells in the propofol group (14.3±4.61%) was significantly lower than that in sevoflurane group (37.3±10.01%) (n=15, P<0.05). There was no significant difference in the percentage of AQP9 positive cells in propofol group and sevoflurane group (25.8±2.67 versus 28.1±7.81%, n=15, P>0.05). Western blot analysis confirmed the immunohistochemistry results. AQP4 protein level in propofol group was significantly lower than that in sevoflurane group (1.4±0.13 versus 1.7±0.12, P<0.05). Western blot analysis did not show any difference of expression of AQP9 protein between the propofol group and sevoflurane group (2.0±0.13 versus 2.1±0.13, P>0.05, n=6). AQP4 expression was lower in patients of propofol group than that in sevoflurane group. Our results suggested that propofol could inhibit the expression of AQP4.

Ischemic Postconditioning Alleviates Brain Edema After Focal Cerebral Ischemia Reperfusion in Rats Through Down-Regulation of Aquaporin-4

Cerebral edema is a serious complication associated with cerebral ischemia/reperfusion (I/R). Aquaporin-4 (AQP4) plays a role in generating postischemic edema after reperfusion. Recently, ischemic postconditioning (Postcond) has been shown to produce neuroprotective effects and reduce brain edema in rats after cerebral I/R. It is unclear if ischemic Postcond alleviates brain edema injury through regulation of AQP4. In this study, middle cerebral artery occlusion (MCAO) was induced in rats by filament insertion for 2 h following 24-h reperfusion: ischemic Postcond treatment was performed before reperfusion in the experimental group. We used the wet-dry weight ratio and transmission electron microscopy to evaluate brain edema after 24 h of reperfusion. We used immunohistochemistry and Western blot analyses to evaluate the distribution and expression of AQP4. Ischemic Postcond significantly reduced the water content of the brain tissue and swelling of the astrocytic foot processes. AQP4 expression increased in the I/R and Postcond groups compared to the sham group, but it decreased in the Postcond group compared to the I/R group. The results of our study suggest that ischemic Postcond effectively reduces brain edema after reperfusion by inhibiting AQP4 expression. The data in this study support the use of ischemic Postcond for alleviating brain edema after cerebral I/R.

Loss of AQP4 polarized localization with loss of β-dystroglycan immunoreactivity may induce brain edema following intracerebral hemorrhage

The aquaporin-4 (AQP4) water channel contributes to brain water homeostasis in perivascular and subpial membrane domains of astrocytes where it is concentrated. These membranes form the interface between the neuropil and the extracellular liquid spaces. The brain-selective deletion of the dystroglycan (DG) gene causes a disorganization of AQP4 on the astroglial endfeet. First, we analyzed the expression of AQP4, β-DG in the brain following intracerebral hemorrhage (ICH) and correlated AQP4 expression with the expression pattern of the β-DG, which is a component of dystrophin-dystroglycan complex (DDC). Besides, the vessels ultrastructure and brain water content were investigated at different time points post-ICH (day 1, day 3, day 7). We found that AQP4 polarity was disturbed in parallel with the loss of β-DG in the perihematomal area post-ICH. At day 1 post-ICH, brain edema was obvious and the damage of vascular ultrastructure was the most severe. These results suggest a role for β-DG in targeting and stabilizing AQP4 channel in astrocytic cells, which may be critical for water homeostasis in brain.

γ-Aminobutyric A receptor (GABA(A)R) regulates aquaporin 4 expression in the subependymal zone: relevance to neural precursors and water exchange

Activation of γ-aminobutyric A receptors (GABA(A)Rs) in the subependymal zone (SEZ) induces hyperpolarization and osmotic swelling in precursors, thereby promoting surface expression of the epidermal growth factor receptor (EGFR) and cell cycle entry. However, the mechanisms underlying the GABAergic modulation of cell swelling are unclear. Here, we show that GABA(A)Rs colocalize with the water channel aquaporin (AQP) 4 in prominin-1 immunopositive (P(+)) precursors in the postnatal SEZ, which include neural stem cells. GABA(A)R signaling promotes AQP4 expression by decreasing serine phosphorylation associated with the water channel. The modulation of AQP4 expression by GABA(A)R signaling is key to its effect on cell swelling and EGFR expression. In addition, GABA(A)R function also affects the ability of neural precursors to swell in response to an osmotic challenge in vitro and in vivo. Thus, the regulation of AQP4 by GABA(A)Rs is involved in controlling activation of neural stem cells and water exchange dynamics in the SEZ.

Short-term functional adaptation of aquaporin-1 surface expression in the proximal tubule, a component of glomerulotubular balance

Transepithelial water flow across the renal proximal tubule is mediated predominantly by aquaporin-1 (AQP1). Along this nephron segment, luminal delivery and transepithelial reabsorption are directly coupled, a phenomenon called glomerulotubular balance. We hypothesized that the surface expression of AQP1 is regulated by fluid shear stress, contributing to this effect. Consistent with this finding, we found that the abundance of AQP1 in brush border apical and basolateral membranes was augmented >2-fold by increasing luminal perfusion rates in isolated, microperfused proximal tubules for 15 minutes. Mouse kidneys with diminished endocytosis caused by a conditional deletion of megalin or the chloride channel ClC-5 had constitutively enhanced AQP1 abundance in the proximal tubule brush border membrane. In AQP1-transfected, cultured proximal tubule cells, fluid shear stress or the addition of cyclic nucleotides enhanced AQP1 surface expression and concomitantly diminished its ubiquitination. These effects were also associated with an elevated osmotic water permeability. In sum, we have shown that luminal surface expression of AQP1 in the proximal tubule brush border membrane is regulated in response to flow. Cellular trafficking, endocytosis, an intact endosomal compartment, and controlled protein stability are the likely prerequisites for AQP1 activation by enhanced tubular fluid shear stress, serving to maintain glomerulotubular balance.

AQP4 plasma membrane trafficking or channel gating is not significantly modulated by phosphorylation at COOH-terminal serine residues

Aquaporin 4 (AQP4) is the predominant water channel in the mammalian brain and is mainly expressed in the perivascular glial endfeet at the brain-blood interface. AQP4 serves as a water entry site during brain edema formation, and regulation of AQP4 may therefore be of therapeutic interest. Phosphorylation of aquaporins can regulate plasma membrane localization and, possibly, the unit water permeability via gating of the AQP channel itself. In vivo phosphorylation of six serine residues in the COOH terminus of AQP4 has been detected by mass spectrometry: Ser(276), Ser(285), Ser(315), Ser(316), Ser(321), and Ser(322). To address the role of these phosphorylation sites for AQP4 function, serine-to-alanine mutants were created to abolish the phosphorylation sites. All mutants were detected at the plasma membrane of transfected C6 cells, with the fraction of the total cellular AQP4 expressed at the plasma membrane of transfected C6 cells being similar between the wild-type (WT) and mutant forms of AQP4. Activation of protein kinases A, C, and G in primary astrocytic cultures did not affect the plasma membrane abundance of AQP4. The unit water permeability was determined for the mutant AQP4s upon heterologous expression in Xenopus laevis oocytes (along with serine-to-aspartate mutants of the same residues to mimic a phosphorylation). None of the mutant AQP4 constructs displayed alterations in the unit water permeability. Thus phosphorylation of six different serine residues in the COOH terminus of AQP4 appears not to be required for proper plasma membrane localization of AQP4 or to act as a molecular switch to gate the water channel.

Overactivation of corticotropin-releasing factor receptor type 1 and aquaporin-4 by hypoxia induces cerebral edema

Cerebral edema is a potentially life-threatening illness, but knowledge of its underlying mechanisms is limited. Here we report that hypobaric hypoxia induces rat cerebral edema and neuronal apoptosis and increases the expression of corticotrophin releasing factor (CRF), CRF receptor type 1 (CRFR1), aquaporin-4 (AQP4), and endothelin-1 (ET-1) in the cortex. These effects, except for the increased expression of CRF itself, could all be blocked by pretreatment with an antagonist of the CRF receptor CRFR1. We also show that, in cultured primary astrocytes: (i) both CRFR1 and AQP4 are expressed; (ii) exogenous CRF, acting through CRFR1, triggers signaling of cAMP/PKA, intracellular Ca(2+), and PKCε; and (iii) the up-regulated cAMP/PKA signaling contributes to the phosphorylation and expression of AQP4 to enhance water influx into astrocytes and produces an up-regulation of ET-1 expression. Finally, using CHO cells transfected with CRFR1(+) and AQP4(+), we show that transfected CRFR1(+) contributes to edema via transfected AQP4(+). In conclusion, hypoxia triggers cortical release of CRF, which acts on CRFR1 to trigger signaling of cAMP/PKA in cortical astrocytes, leading to activation of AQP4 and cerebral edema.

Structure and function of the ependymal barrier and diseases associated with ependyma disruption

The neuroepithelium is a germinal epithelium containing progenitor cells that produce almost all of the central nervous system cells, including the ependyma. The neuroepithelium and ependyma constitute barriers containing polarized cells covering the embryonic or mature brain ventricles, respectively; therefore, they separate the cerebrospinal fluid that fills cavities from the developing or mature brain parenchyma. As barriers, the neuroepithelium and ependyma play key roles in the central nervous system development processes and physiology. These roles depend on mechanisms related to cell polarity, sensory primary cilia, motile cilia, tight junctions, adherens junctions and gap junctions, machinery for endocytosis and molecule secretion, and water channels. Here, the role of both barriers related to the development of diseases, such as neural tube defects, ciliary dyskinesia, and hydrocephalus, is reviewed.

Melatonin renders neuroprotection by protein kinase C mediated aquaporin-4 inhibition in animal model of focal cerebral ischemia

AIM

Aquaporin-4(AQP4) expression in the brain with relation to edema formation following focal cerebral ischemia was investigated. Studies have shown that brain edema is one of the significant factors in worsening stroke outcomes. While many mechanisms may aggravate brain injury, one such potential system may involve AQP4 up regulation in stroke patients that could result in increased edema formation. Post administration of melatonin following ischemic stroke reduces AQP4 mediated brain edema and confers neuroprotection.

MATERIALS AND METHODS

An in-silico approach was undertaken to confirm effective melatonin-AQP4 binding. Rats were treated with 5mg/kg, i.p. melatonin or placebo at 30 min prior, 60 min post and 120 min post 60 min of middle cerebral artery occlusion (MCAO) followed by 24h reperfusion. Rats were evaluated for battery of neurological and motor function tests just before sacrifice. Brains were harvested for infarct size estimation, water content measurement, biochemical analysis, apoptosis study and western blot experiments.

KEY FINDINGS

Melatonin at 60 min post ischemia rendered neuroprotection as evident by reduction in cerebral infarct volume, improvement in motor and neurological deficit and reduction in brain edema. Furthermore, ischemia induced surge in levels of nitrite and malondialdehyde (MDA) were also found to be significantly reduced in ischemic brain regions in treated animals. Melatonin potentiated intrinsic antioxidant status, inhibited acid mediated rise in intracellular calcium levels, decreased apoptotic cell death and also markedly inhibited protein kinase C (PKC) influenced AQP4 expression in the cerebral cortex and dorsal striatum.

SIGNIFICANCE

Melatonin confers neuroprotection by protein kinase C mediated AQP4 inhibition in ischemic stroke.

Unraveling aquaporin interaction partners

BACKGROUND

Insight into protein-protein interactions (PPIs) is highly desirable in order to understand the physiology of cellular events. This understanding is one of the challenges in biochemistry and molecular biology today, especially for eukaryotic membrane proteins where hurdles of production, purification and structural determination must be passed.

SCOPE OF REVIEW

We have explored the common strategies used to find medically relevant interaction partners of aquaporins (AQPs). The most frequently used methods to detect direct contact, yeast two-hybrid interaction assay and co-precipitation, are described together with interactions specifically found for the selected targets AQP0, AQP2, AQP4 and AQP5.

MAJOR CONCLUSIONS

The vast majority of interactions involve the aquaporin C-terminus and the characteristics of the interaction partners are strikingly diverse. While the well-established methods for PPIs are robust, a novel approach like bimolecular fluorescence complementation (BiFC) is attractive for screening many conditions as well as transient interactions. The ultimate goal is structural evaluation of protein complexes in order to get mechanistic insight into how proteins communicate at a molecular level.

GENERAL SIGNIFICANCE

What we learn from the human aquaporin field in terms of method development and communication between proteins can be of major use for any integral membrane protein of eukaryotic origin. This article is part of a Special Issue entitled Aquaporins.

Updated physiology and pathophysiology of CSF circulation--the pulsatile vector theory

INTRODUCTION

Hydrocephalus is still a not well-understood diagnostic and a therapeutic dilemma because of the lack of sufficient and comprehensive model of cerebrospinal fluid circulation and pathological alterations.

CONCLUSIONS

Based on current studies, reviews, and knowledge of cerebrospinal fluid dynamics, brain water dynamics, intracranial pressure, and cerebral perfusion physiology, a new concept is deducted that can describe normal and pathological changes of cerebrospinal fluid circulation and pathophysiology of idiopathic intracranial hypertension.

Human aquaporins: regulators of transcellular water flow

BACKGROUND

Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes.

SCOPE OF REVIEW

AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.

MAJOR CONCLUSIONS

AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.

GENERAL SIGNIFICANCE

Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.

Aquaporin-4 inhibition mediates piroxicam-induced neuroprotection against focal cerebral ischemia/reperfusion injury in rodents

BACKGROUND AND PURPOSE

Aquaporin-4(AQP4) is an abundant water channel protein in brain that regulates water transport to maintain homeostasis. Cerebral edema resulting from AQP4 over expression is considered to be one of the major determinants for progressive neuronal insult during cerebral ischemia. Although, both upregulation and downregulation of AQP4 expression is associated with brain pathology, over expression of AQP4 is one of the chief contributors of water imbalance in brain during ischemic pathology. We have found that Piroxicam binds to AQP4 with optimal binding energy value. Thus, we hypothesized that Piroxicam is neuroprotective in the rodent cerebral ischemic model by mitigating cerebral edema via AQP4 regulation.

METHODS

Rats were treated with Piroxicam OR placebo at 30 min prior, 2 h post and 4 h post 60 minutes of MCAO followed by 24 hour reperfusion. Rats were evaluated for neurological deficits and motor function just before sacrifice. Brains were harvested for infarct size estimation, water content measurement, biochemical analysis, RT-PCR and western blot experiments.

RESULTS

Piroxicam pretreatment thirty minutes prior to ischemia and four hour post reperfusion afforded neuroprotection as evident through significant reduction in cerebral infarct volume, improvement in motor behavior, neurological deficit and reduction in brain edema. Furthermore, ischemia induced surge in levels of nitrite and malondialdehyde were also found to be significantly reduced in ischemic brain regions in treated animals. This neuroprotection was found to be associated with inhibition of acid mediated rise in intracellular calcium levels and also downregulated AQP4 expression.

CONCLUSIONS

Findings of the present study provide significant evidence that Piroxicam acts as a potent AQP4 regulator and renders neuroprotection in focal cerebral ischemia. Piroxicam could be clinically exploited for the treatment of brain stroke along with other anti-stroke therapeutics in future.

Water permeability of aquaporin-4 channel depends on bilayer composition, thickness, and elasticity

Aquaporin-4 (AQP4) is the primary water channel in the mammalian brain, particularly abundant in astrocytes, whose plasma membranes normally contain high concentrations of cholesterol. Here we test the hypothesis that the water permeabilities of two naturally occurring isoforms (AQP4-M1 and AQP4-M23) depend on bilayer mechanical/structural properties modulated by cholesterol and phospholipid composition. Osmotic stress measurements were performed with proteoliposomes containing AQP4 and three different lipid mixtures: 1), phosphatidylcholine (PC) and phosphatidylglycerol (PG); 2), PC, PG, with 40 mol % cholesterol; and 3), sphingomyelin (SM), PG, with 40 mol % cholesterol. The unit permeabilities of AQP4-M1 were 3.3 ± 0.4 × 10(-13) cm(3)/s (mean ± SE), 1.2 ± 0.1 × 10(-13) cm(3)/s, and 0.4 ± 0.1 × 10(-13) cm(3)/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. The unit permeabilities of AQP4-M23 were 2.1 ± 0.2 × 10(-13) cm(3)/s, 0.8 ± 0.1 × 10(-13) cm(3)/s, and 0.3 ± 0.1 × 10(-13) cm(3)/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. Thus, for each isoform the unit permeabilities strongly depended on bilayer composition and systematically decreased with increasing bilayer compressibility modulus and bilayer thickness. These observations suggest that altering lipid environment provides a means of regulating water channel permeability. Such permeability changes could have physiological consequences, because AQP4 water permeability would be reduced by its sequestration into SM:cholesterol-enriched raft microdomains. Conversely, under ischemic conditions astrocyte membrane cholesterol content decreases, which could increase AQP4 permeability.

Hyperglycemia exacerbates intracerebral hemorrhage via the downregulation of aquaporin-4: temporal assessment with magnetic resonance imaging

BACKGROUND AND PURPOSE

Intracerebral hemorrhage (ICH) is associated with high mortality and neurological deficits, and concurrent hyperglycemia usually worsens clinical outcomes. Aquaporin-4 (AQP-4) is important in cerebral water movement. Our aim was to investigate the role of AQP-4 in hyperglycemic ICH.

METHODS

Hyperglycemia was induced by intraperitoneal injection of streptozotocin (STZ; 60 mg/kg) in adult Sprague-Dawley male rats. ICH was induced by stereotaxic infusion of collagenase/heparin into the right striatum. One set of rats was repeatedly monitored by MRI at 1, 4, and 7 days after ICH induction so as to acquire information on the formation of hematoma and edema. Another set of rats was killed and brains were examined for differences in the degree of hemorrhage and edema, water content, blood-brain barrier destruction, and AQP-4 expression.

RESULTS

Hyperglycemia ICH rats exhibited increased brain water content, more severe blood-brain barrier destruction, and greater vasogenic edema as seen on diffusion-weighted MRI. Significant downregulation of AQP-4 was observed in STZ-treated rats after ICH as compared with non-STZ-treated rats. Apoptosis was greater on day 1 after ICH in STZ-treated rats.

CONCLUSIONS

The expression of AQP-4 in the brain is downregulated in hyperglycemic rats as compared with normoglycemic rats after ICH. This change is accompanied by increased vasogenic brain edema and more severe blood-brain barrier destruction.

Diclofenac enhances proinflammatory cytokine-induced aquaporin-4 expression in cultured astrocyte

Acute encephalopathy is a generic term for acute brain dysfunction occurring after infection. Acute encephalopathy induced by influenza virus results in high mortality, and most cases of influenza-associated encephalopathy (IAE) result in brain edema. Administration of diclofenac sodium (DCF), a non-steroidal anti-inflammatory drug (NSAID), is associated with a significant increased mortality rate of IAE. These previous clinical findings proposed further investigation of DCF administration and brain edema to clarify how DCF aggravates IAE. Aquaporin-4 (AQP4) is the predominant water channel protein in the mammalian brain, and is mainly expressed in astrocytes. AQP4 plays an important role in brain water homeostasis. Therefore, we investigated a possible association between DCF and AQP4 production in astrocytes. We stimulated cultured rat astrocytes with three cytokines, interleukin-1β, tumor necrosis factor α, and interferon γ, and then treated with DCF. DCF enhanced proinflammatory cytokine-induced AQP4 gene and protein expression in astrocytes, whereas DCF alone did not change the AQP4 gene expression. The addition of nuclear factor-kappa B (NF-κB) inhibitor abrogated AQP4 gene and protein expression completely in astrocytes treated with cytokines alone and in those also treated with DCF. In conclusion, this study demonstrated that AQP4 is upregulated in astrocyte by proinflammatory cytokines, and that the addition of DCF further augments AQP4 production. This effect is mediated via NF-κB signaling. The enhancement of AQP4 production by DCF may explain the significantly increased mortality rates in IAE patients treated with DCF.

Effect of repetitive daily ethanol intoxication on adult rat brain: significant changes in phospholipase A2 enzyme levels in association with increased PARP-1 indicate neuroinflammatory pathway activation

Collaborating on studies of subchronic daily intoxication in juvenile and adult rats, we examined whether the repetitive ethanol treatments at these two life stages altered levels of key neuroinflammation-associated proteins-aquaporin-4 (AQP4), certain phospholipase A2 (PLA2) enzymes, PARP-1 and caspase-3-in hippocampus (HC) and entorhinal cortex (EC). Significant changes in the proteins could implicate activation of specific neuroinflammatory signaling pathways in these rats as well as in severely binge-intoxicated adult animals that are reported to incur degeneration of vulnerable neurons in HC and EC. Male Wistar rats, ethanol-intoxicated (3 g/kg i.p.) once daily for 6 days over an 8-day interval beginning at 37 days old and repeated at age 68-75 days, were sacrificed 1 h after the day 75 dose (blood ethanol, 200- 230 mg/dl). Analysis of HC with an immunoblot technique showed that AQP4, Ca(+2)-dependent PLA2 (cPLA2 IVA), phosphorylated (activated) p-cPLA2, cleaved (89 kD) PARP (c-PARP), and caspase-3 levels were significantly elevated over controls, whereas Ca(+2)-independent PLA2 (iPLA2 VIA) was reduced ∼70%; however, cleaved caspase-3 was undetectable. In the EC, AQP4 was unchanged, but cPLA2 and p-cPLA2 were significantly increased while iPLA2 levels were diminished (∼40%) similar to HC, although just outside statistical significance (p = 0.06). In addition, EC levels of PARP-1 and c-PARP were significantly increased. The ethanol-induced activation of cPLA2 in association with reduced iPLA2 mirrors PLA2 changes in reports of neurotrauma and also of dietary omega-3 fatty acid depletion. Furthermore, the robust PARP-1 elevations accompanied by negligible caspase-3 activation indicate that repetitive ethanol intoxication may be potentiating non-apoptotic neurodegenerative processes such as parthanatos. Overall, the repetitive ethanol treatments appeared to instigate previously unappreciated neuroinflammatory pathways in vivo. The data provide insights into mechanisms of binge ethanol abuse that might suggest new therapeutic approaches to counter neurodegeneration and dementia.

Protective role of brain water channel AQP4 in murine cerebral malaria

Tragically common among children in sub-Saharan Africa, cerebral malaria is characterized by rapid progression to coma and death. In this study, we used a model of cerebral malaria appearing in C57BL/6 WT mice after infection with the rodent malaria parasite Plasmodium berghei ANKA. Expression and cellular localization of the brain water channel aquaporin-4 (AQP4) was investigated during the neurological syndrome. Semiquantitative real-time PCR comparing uninfected and infected mice showed a reduction of brain AQP4 transcript in cerebral malaria, and immunoblots revealed reduction of brain AQP4 protein. Reduction of brain AQP4 protein was confirmed in cerebral malaria by quantitative immunogold EM; however, polarized distribution of AQP4 at the perivascular and subpial astrocyte membranes was not altered. To further examine the role of AQP4 in cerebral malaria, WT mice and littermates genetically deficient in AQP4 were infected with P. berghei. Upon development of cerebral malaria, WT and AQP4-null mice exhibited similar increases in width of perivascular astroglial end-feet in brain. Nevertheless, the AQP4-null mice exhibited more severe signs of cerebral malaria with greater brain edema, although disruption of the blood-brain barrier was similar in both groups. In longitudinal studies, cerebral malaria appeared nearly 1 d earlier in the AQP4-null mice, and reduced survival was noted when chloroquine rescue was attempted. We conclude that the water channel AQP4 confers partial protection against cerebral malaria.

mGlu3 receptor and astrocytes: partners in neuroprotection

Astrocytes are currently studied intensively because of their now highlighted relevance as key players with neurons that modulate a wide range of central functions, from synaptic plasticity and synaptogenesis to regulation of metabolic and neuroinflammatory processes. Since the discovery of mGlu3 receptors on astrocytes, accumulating evidence supports a role of these receptors not only in maintaining synaptic homeostasis and treating psychiatric disorders but also in promoting astrocyte survival in several pathologic conditions. This review focuses on providing up-to-date knowledge regarding effects of activating astroglial mGlu3 receptors on psychiatric disorders, astrocyte and neuronal survival, and neurodegenerative diseases. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Rapid aquaporin translocation regulates cellular water flow: mechanism of hypotonicity-induced subcellular localization of aquaporin 1 water channel

The control of cellular water flow is mediated by the aquaporin (AQP) family of membrane proteins. The structural features of the family and the mechanism of selective water passage through the AQP pore are established, but there remains a gap in our knowledge of how water transport is regulated. Two broad possibilities exist. One is controlling the passage of water through the AQP pore, but this only has been observed as a phenomenon in some plant and microbial AQPs. An alternative is controlling the number of AQPs in the cell membrane. Here, we describe a novel pathway in mammalian cells whereby a hypotonic stimulus directly induces intracellular calcium elevations through transient receptor potential channels, which trigger AQP1 translocation. This translocation, which has a direct role in cell volume regulation, occurs within 30 s and is dependent on calmodulin activation and phosphorylation of AQP1 at two threonine residues by protein kinase C. This direct mechanism provides a rationale for the changes in water transport that are required in response to constantly changing local cellular water availability. Moreover, because calcium is a pluripotent and ubiquitous second messenger in biological systems, the discovery of its role in the regulation of AQP translocation has ramifications for diverse physiological and pathophysiological processes, as well as providing an explanation for the rapid regulation of water flow that is necessary for cell homeostasis.

The role of aquaporin 4 in the brain

Emerging evidence suggests that aquaporin (AQP) 4 water channels play an important role in water homeostasis in the brain. These water channels are most abundant in the cell membrane of astrocytes, but are also present within ependymal cell membranes and in osmosensory areas of the hypothalamus. Water transport through AQP4 depends on concentration gradients across the membrane, but the rate of transport is determined by the capacity of astrocytes to up- and down-regulate AQP4 numbers, their location within the membrane, and the overall permeability of the channel. Other functions of brain AQP4 involve potassium uptake and release by astrocytes, migration of glial cells, glial scarring, and astrocyte-to-astrocyte cell communication. AQP water channels are involved in formation and control of edema in the brain and in multiple disease processes in the brain, such as seizures and tumors. There is abundant scientific literature on AQP4 describing its structure, function, location, and role in water homeostasis and edema in the brain. Investigation of AQP expression in the canine and feline brain should be pursued so that clinically relevant comparisons between findings in mice, rats, and people and animal patients can be made.