Increased seizure duration in mice lacking aquaporin-4 water channels

The Water Transport System in Astrocytes–Aquaporins

Highlights

(AQPs) are transmembrane proteins responsible for fast water movement across cell membranes, including those of astrocytes.

The expression and subcellular localization of AQPs in astrocytes are highly dynamic under physiological and pathological conditions.

Besides their primary function in water homeostasis, AQPs participate in many ancillary functions including glutamate clearance in tripartite synapses and cell migration.

Abstract

Astrocytes have distinctive morphological and functional characteristics, and are found throughout the central nervous system. Astrocytes are now known to be far more than just housekeeping cells in the brain. Their functions include contributing to the formation of the blood–brain barrier, physically and metabolically supporting and communicating with neurons, regulating the formation and functions of synapses, and maintaining water homeostasis and the microenvironment in the brain. Aquaporins (AQPs) are transmembrane proteins responsible for fast water movement across cell membranes. Various subtypes of AQPs (AQP1, AQP3, AQP4, AQP5, AQP8 and AQP9) have been reported to be expressed in astrocytes, and the expressions and subcellular localizations of AQPs in astrocytes are highly correlated with both their physiological and pathophysiological functions. This review describes and summarizes the recent advances in our understanding of astrocytes and AQPs in regard to controlling water homeostasis in the brain. Findings regarding the features of different AQP subtypes, such as their expression, subcellular localization, physiological functions, and the pathophysiological roles of astrocytes are presented, with brain edema and glioma serving as two representative AQP-associated pathological conditions. The aim is to provide a better insight into the elaborate “water distribution” system in cells, exemplified by astrocytes, under normal and pathological conditions.

Streptococcal meningitis reveals the presence of residual streptococci and down-regulated aquaporin 4 in the brain

The pathology of streptococcal meningitis is poorly understood, even though streptococcal infection induces meningitis. The aim of this study was to clarify the relationship between streptococcal meningitis and aquaporin 4 (AQP4) in the mouse brain. After Streptococcus suis infection, the streptococcal number was calculated, and AQP4 mRNA expression in the brain was quantified at 2 and 7 days after infection. At 7-day post-infection, mice with neurological symptoms showed significantly higher S. suis levels in the brain than mice without neurological symptoms. AQP4 expression was significantly decreased in mice with neurological symptoms than in mice without neurological symptoms. Image analysis demonstrated that S. suis progressed to invade the white matter. Pathological analysis revealed that infected mouse brains had higher inflammation and neurological damage scores than uninfected mouse brains. Therefore, mice with neurological symptoms caused by streptococcal meningitis had high S. suis levels in the brain and reduced AQP4 expression.

Neuronal Aquaporin 1 Inhibits Amyloidogenesis by Suppressing the Interaction Between Beta-Secretase and Amyloid Precursor Protein

The accumulation of amyloid-β (Aβ) is a characteristic event in the pathogenesis of Alzheimer's disease (AD). Aquaporin 1 (AQP1) is a membrane water channel protein belonging to the AQP family. AQP1 levels are elevated in the cerebral cortex during the early stages of AD, but the role of AQP1 in AD pathogenesis is unclear. We first determined the expression and distribution of AQP1 in brain tissue samples of AD patients and two AD mouse models (3xTg-AD and 5xFAD). AQP1 accumulation was observed in vulnerable neurons in the cerebral cortex of AD patients, and in neurons affected by the Aβ or tau pathology in the 3xTg-AD and 5xFAD mice. AQP1 levels increased in neurons as aging progressed in the AD mouse models. Stress stimuli increased AQP1 in primary cortical neurons. In response to cellular stress, AQP1 appeared to translocate to endocytic compartments of β- and γ-secretase activities. Ectopic expression of AQP1 in human neuroblastoma cells overexpressing amyloid precussir protein (APP) with the Swedish mutations reduced β-secretase (BACE1)-mediated cleavage of APP and reduced Aβ production without altering the nonamyloidogenic pathway. Conversely, knockdown of AQP1 enhanced BACE1 activity and Aβ production. Immunoprecipitation experiments showed that AQP1 decreased the association of BACE1 with APP. Analysis of a human database showed that the amount of Aβ decreases as the expression of AQP1 increases. These results suggest that the upregulation of AQP1 is an adaptive response of neurons to stress that reduces Aβ production by inhibiting the binding between BACE1 and APP.

Molecular mechanisms of brain water transport

Our brains consist of 80% water, which is continuously shifted between different compartments and cell types during physiological and pathophysiological processes. Disturbances in brain water homeostasis occur with pathologies such as brain oedema and hydrocephalus, in which fluid accumulation leads to elevated intracranial pressure. Targeted pharmacological treatments do not exist for these conditions owing to our incomplete understanding of the molecular mechanisms governing brain water transport. Historically, the transmembrane movement of brain water was assumed to occur as passive movement of water along the osmotic gradient, greatly accelerated by water channels termed aquaporins. Although aquaporins govern the majority of fluid handling in the kidney, they do not suffice to explain the overall brain water movement: either they are not present in the membranes across which water flows or they appear not to be required for the observed flow of water. Notably, brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow may not describe all transmembrane fluid transport in the brain. The cotransport of water is an unconventional molecular mechanism that is introduced in this Review as a missing link to bridge the gap in our understanding of cellular and barrier brain water transport.

Molecular mechanisms of K+ clearance and extracellular space shrinkage-Glia cells as the stars

Neuronal signaling in the central nervous system (CNS) associates with release of K⁺ into the extracellular space resulting in transient increases in [K⁺ ]_o . This elevated K⁺ is swiftly removed, in part, via uptake by neighboring glia cells. This process occurs in parallel to the [K⁺ ]_o elevation and glia cells thus act as K⁺ sinks during the neuronal activity, while releasing it at the termination of the pulse. The molecular transport mechanisms governing this glial K⁺ absorption remain a point of debate. Passive distribution of K⁺ via Kir4.1-mediated spatial buffering of K⁺ has become a favorite within the glial field, although evidence for a quantitatively significant contribution from this ion channel to K⁺ clearance from the extracellular space is sparse. The Na⁺ /K⁺ -ATPase, but not the Na⁺ /K⁺ /Cl^- cotransporter, NKCC1, shapes the activity-evoked K⁺ transient. The different isoform combinations of the Na⁺ /K⁺ -ATPase expressed in glia cells and neurons display different kinetic characteristics and are thereby distinctly geared toward their temporal and quantitative contribution to K⁺ clearance. The glia cell swelling occurring with the K⁺ transient was long assumed to be directly associated with K⁺ uptake and/or AQP4, although accumulating evidence suggests that they are not. Rather, activation of bicarbonate- and lactate transporters appear to lead to glial cell swelling via the activity-evoked alkaline transient, K⁺ -mediated glial depolarization, and metabolic demand. This review covers evidence, or lack thereof, accumulated over the last half century on the molecular mechanisms supporting activity-evoked K⁺ and extracellular space dynamics.

Genome wide association study of incomplete hippocampal inversion in adolescents

Incomplete hippocampal inversion (IHI), also called hippocampal malrotation, is an atypical presentation of the hippocampus present in about 20% of healthy individuals. Here we conducted the first genome-wide association study (GWAS) in IHI to elucidate the genetic underpinnings that may contribute to the incomplete inversion during brain development. A total of 1381 subjects contributed to the discovery cohort obtained from the IMAGEN database. The incidence rate of IHI was 26.1%. Loci with P<1e-5 were followed up in a validation cohort comprising 161 subjects from the PING study. Summary statistics from the discovery cohort were used to compute IHI heritability as well as genetic correlations with other traits. A locus on 18q11.2 (rs9952569; OR = 1.999; Z = 5.502; P = 3.755e-8) showed a significant association with the presence of IHI. A functional annotation of the locus implicated genes AQP4 and KCTD1. However, neither this locus nor the other 16 suggestive loci reached a significant p-value in the validation cohort. The h² estimate was 0.54 (sd: 0.30) and was significant (Z = 1.8; P = 0.036). The top three genetic correlations of IHI were with traits representing either intelligence or education attainment and reached nominal P< = 0.013.

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.

Early Gabapentin Treatment during the Latency Period Increases Convulsive Threshold, Reduces Microglial Activation and Macrophage Infiltration in the Lithium-Pilocarpine Model of Epilepsy

The lithium-pilocarpine model of epilepsy reproduces several features of temporal lobe epilepsy in humans, including the chronological timeline of an initial latency period followed by the development of spontaneous seizures. Epilepsy therapies in humans are implemented, as a rule, after the onset of the spontaneous seizures. We here studied the potential effect on epileptogenesis of starting an early treatment during the latency period, in order to prevent the development of spontaneous seizures. Adult male Wistar rats were treated with 3 mEq/kg LiCl, and 20 h later 30 mg/kg pilocarpine. Once status epilepticus (SE) was achieved, it was allowed to last for 20 min, and then motor seizures were controlled with the administration of 20 mg/kg diazepam. At 1DPSE (DPSE, days post-status epilepticus), animals started to receive 400 mg/kg/day gabapentin or saline for 4 days. At 5DPSE, we observed that SE induced an early profuse microglial and astroglial reactivity, increased synaptogenic trombospondin-1 expression and reduced AQP4 expression in astroglial ending feet. Blood brain barrier (BBB) integrity seemed to be compromised, as infiltrating NG2+ macrophages and facilitated access to the CNS was observed by transplanting eGFP+ blood cells and bone marrow-derived progenitors in the SE animals. The early 4-day gabapentin treatment successfully reduced microglial cell reactivity and blood-borne cell infiltration, without significantly altering the mRNA of proinflammatory cytokines IL-1β and TNFα immediately after the treatment. After 21DSPE, another group of animals that developed SE and received 4 days of gabapentin treatment, were re-exposed to subconvulsive accumulative doses of pilocarpine (10 mg/kg/30 min) and were followed by recording the Racine scale reached. Early 4-day gabapentin treatment reduced the Racine scale reached by the animals, reduced animal mortality, and reduced the number of animals that achieved SE (34% vs. 72%). We conclude that early gabapentin treatment following SE, during the latency period, is able to reduce neuroinflammation and produces a persistent effect that limits seizures and increases convulsive threshold, probably by restricting microglial reactivity and spurious synaptogenesis.

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.

The Potential Roles of Aquaporin 4 in Alzheimer's Disease

Aquaporin 4 (AQP4) is the major water channel expressed in the central nervous system (CNS), and it is primarily expressed in astrocytes. It has been studied in various brain pathological conditions. However, the potential for AQP4 to influence Alzheimer's disease (AD) is still unclear. Research regarding AQP4 functions related to AD can be traced back several years and has gradually progressed toward a better understanding of the potential mechanisms. Currently, it has been suggested that AQP4 influences synaptic plasticity, and AQP4 deficiency may impair learning and memory, in part, through glutamate transporter-1 (GLT-1). AQP4 may mediate the clearance of amyloid beta peptides (Aβ). In addition, AQP4 may influence potassium (K(+)) and calcium (Ca(2+)) ion transport, which could play decisive roles in the pathogenesis of AD. Furthermore, AQP4 knockout is involved in neuroinflammation and interferes with AD. To date, no specific therapeutic agents have been developed to inhibit or enhance AQP4. However, experimental results strongly emphasize the importance of this topic for future investigations.

Challenges and achievements in the therapeutic modulation of aquaporin functionality

Aquaporin (AQP) water and solute channels have basic physiological functions throughout the human body. AQP-facilitated water permeability across cell membranes is required for rapid reabsorption of water from pre-urine in the kidneys and for sustained near isosmolar water fluxes e.g. in the brain, eyes, inner ear, and lungs. Cellular water permeability is further connected to cell motility. AQPs of the aquaglyceroporin subfamily are necessary for lipid degradation in adipocytes and glycerol uptake into the liver, as well as for skin moistening. Modulation of AQP function is desirable in several pathophysiological situations, such as nephrogenic diabetes insipidus, Sjögren's syndrome, Menière's disease, heart failure, or tumors to name a few. Attempts to design or to find effective small molecule AQP inhibitors have yielded only a few hits. Challenges reside in the high copy number of AQP proteins in the cell membranes, and spatial restrictions in the protein structure. This review gives an overview on selected physiological and pathophysiological conditions in which modulation of AQP functions appears beneficial and discusses first achievements in the search of drug-like AQP inhibitors.

An aquaporin 3-notch1 axis in keratinocyte differentiation and inflammation

Aquaporin 3 (AQP3) is an aquaglyceroporin which transports water, glycerol and small solutes across the plasma membrane. Its functions are not limited to fluid transport but also involve the regulation of cell proliferation, migration, skin hydration, wound healing and tumorigenesis. While AQP3 has been reported to play an important role in keratinocyte proliferation, its role in differentiation remains controversial. Our study demonstrated that the expression of AQP3 was regulated during differentiation and that it participated in keratinocyte differentiation control. We further revealed that AQP3 was a transcriptional target of Notch signaling, a critical pathway regulating keratinocyte differentiation and tumor suppression, and it regulated differentiation through a reciprocal negative feedback loop with Notch1. When the expression level of AQP3 was elevated, impaired barrier integrity and increased pro-inflammatory cytokine production ensued, mimicking the pathological conditions in Notch deficient mice and in atopic dermatitis. Dysregulation of AQP3 and Notch receptors has been reported in several skin diseases, including skin cancer. Our discovery of the novel AQP3-Notch1 axis may provide insight into epidermal homeostasis control and possible translational applications, including its potential use as a biomarker for molecular diagnosis in environmental studies.

Intraperitoneal administration of Shiga toxin 2 induced neuronal alterations and reduced the expression levels of aquaporin 1 and aquaporin 4 in rat brain

Shiga toxin-producing Escherichia coli produces watery and hemorrhagic diarrhea, and hemolytic uremic syndrome (HUS) characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure. Central nervous system (CNS) complications are observed in around 30% of infant population with HUS. Common signs of severe CNS involvement leading to death include seizures, alteration of consciousness, hemiparesis, visual disturbances, and brain stem symptoms. The purpose of the present work was to study the effects of Shiga toxin 2 (Stx2) in the brain of rats intraperitoneally (i.p.) injected with a supernatant from recombinant E. coli expressing Stx2 (sStx2). Neurological alterations such as postural and motor abnormalities including lethargy, abnormal walking, and paralysis of hind legs, were observed in this experimental model of HUS in rats. Neuronal damage, as well as significant decrease in aquaporin 1 (AQP1) and aquaporin 4 (AQP4) expression levels were observed in the brain of rats, 2 days after sStx2 injection, compared to controls. Downregulation of aquaporin protein levels, and neuronal alterations, observed in brain of rats injected with sStx2, may be involved in edema formation and in neurological manifestations characteristic of HUS.

Aquaporin-4 expression is not elevated in mild hydrocephalus

BACKGROUND

Aquaporin-4 (aqp-4) is a member of water channel family proteins primarily expressed in the central nervous system. Physiologically it is the main channel providing water transport into the nervous system water compartments and across the blood-brain barrier. Several studies demonstrated its compensatory role in severe hydrocephalus. However, its role is not clear during the initial stages of hydrocephalus.

OBJECTIVE

This study was designed to investigate aqp-4 expression in less severe forms of hydrocephalus and to determine its role in disease progression.

METHODS

Twenty-five male Wistar-Hannover rats, were distributed into experimental (n = 20) and control (n = 5) groups. Hydrocephalus was induced in the experimental group by injection of 5 μl 25% kaolin suspension into the cisterna magna. Control animals received an injection of 5 μl normal saline. Eight weeks later, the animals were killed by the perfusion-fixation method. Immunohistochemical and Western blot analysis were performed.

RESULTS

Ventricular dilatations were noted in all experimental animals. Both groups demonstrated positive immunoreactive signals to aqp-4. Immunohistochemically there were no changes in aqp-4 pattern and expression intensity between experimental and control animals. Similarly, Western blot analysis revealed mean aqp-4 values in experimental and control groups as 0.3436 and 0.3917, respectively, and the difference did not reach statistical significance (p > 0.05).

CONCLUSION

Our results indicate that aqp-4 is not up-regulated during the initial stages of hydrocephalus. This implies that aqp-4 may not play a significant role in hydrocephalus compensation until severe ventricular dilatation occurs.

Seizures in Women with Preeclampsia: Mechanisms and Management

Eclampsia is defined in the obstetrical literature as the occurrence of unexplained seizure during pregnancy in a woman with preeclampsia. In the Western world, the incidence of eclampsia is ~1 per 2000 to 1 per 3000 pregnancies, but the incidence is 10-fold higher in tertiary referral centers and undeveloped countries where there is poor prenatal care, and in multi-fetal gestations. Nearly 1 in 50 women with eclampsia die as do 1 in 14 of their offspring, and mortality rates are considerably higher in undeveloped countries. Eclampsia is also associated with significant life-threatening complications, including neurological events. Seizure acutely can cause stroke, haemorrhage, oedema and brain herniation and thus lead to epilepsy and cognitive impairment later in life.

Binding affinity and specificity of neuromyelitis optica autoantibodies to aquaporin-4 M1/M23 isoforms and orthogonal arrays

Autoantibodies against astrocyte water channel aquaporin-4 (AQP4) are highly specific for the neuroinflammatory disease neuromyelitis optica (NMO). We measured the binding of NMO autoantibodies to AQP4 in human astrocyte-derived U87MG cells expressing M1 and/or M23 AQP4, or M23 mutants that do not form orthogonal array of particles (OAPs). Binding affinity was quantified by two-color fluorescence ratio imaging of cells stained with NMO serum or a recombinant monoclonal NMO autoantibody (NMO-rAb), together with a C terminus anti-AQP4 antibody. NMO-rAb titrations showed binding with dissociation constants down to 44 ± 7 nm. Different NMO-rAbs and NMO patient sera showed a wide variation in NMO-IgG binding to M1 versus M23 AQP4. Differences in binding affinity rather than stoichiometry accounted for M1 versus M23 binding specificity, with consistently greater affinity of NMO-IgG binding to M23 than M1 AQP4. Binding and OAP measurements in cells expressing different M1:M23 ratios or AQP4 mutants indicated that the differential binding of NMO-IgG to M1 versus M23 was due to OAP assembly rather than to differences in the M1 versus M23 N termini. Purified Fab fragments of NMO-IgG showed similar patterns of AQP4 isoform binding, indicating that structural changes in the AQP4 epitope upon array assembly, and not bivalent cross-linking of whole IgG, result in the greater binding affinity to OAPs. Our study establishes a quantitative assay of NMO-IgG binding to AQP4 and indicates remarkable, OAP-dependent heterogeneity in NMO autoantibody binding specificity.

Hydrocephalus induces dynamic spatiotemporal regulation of aquaporin-4 expression in the rat brain

BACKGROUND

The water channel protein aquaporin-4 (AQP4) is reported to be of possible major importance for accessory cerebrospinal fluid (CSF) circulation pathways. We hypothesized that changes in AQP4 expression in specific brain regions correspond to the severity and duration of hydrocephalus.

METHODS

Hydrocephalus was induced in adult rats (~8 weeks) by intracisternal kaolin injection and evaluated after two days, one week and two weeks. Using magnetic resonance imaging (MRI) we quantified lateral ventricular volume, water diffusion and blood-brain barrier properties in hydrocephalic and control animals. The brains were analysed for AQP4 density by western blotting and localisation by immunohistochemistry. Double fluorescence labelling was used to study cell specific origin of AQP4.

RESULTS

Lateral ventricular volume was significantly increased over control at all time points after induction and the periventricular apparent diffusion coefficient (ADC) value significantly increased after one and two weeks of hydrocephalus. Relative AQP4 density was significantly decreased in both cortex and periventricular region after two days and normalized after one week. After two weeks, periventricular AQP4 expression was significantly increased. Relative periventricular AQP4 density was significantly correlated to lateral ventricular volume. AQP4 immunohistochemical analysis demonstrated the morphological expression pattern of AQP4 in hydrocephalus in astrocytes and ventricular ependyma. AQP4 co-localized with astrocytic glial fibrillary acidic protein (GFAP) in glia limitans. In vascular structures, AQP4 co-localized to astroglia but not to microglia or endothelial cells.

CONCLUSIONS

AQP4 levels are significantly altered in a time and region dependent manner in kaolin-induced hydrocephalus. The presented data suggest that AQP4 could play an important neurodefensive role, and may be a promising future pharmaceutical target in hydrocephalus and CSF disorders.

Actin cytoskeleton remodeling governs aquaporin-4 localization in astrocytes

Aquaporin-4 (AQP4) is constitutively concentrated in the plasma membrane of the perivascular glial processes, and its expression is altered in certain pathological conditions associated with brain edema or altered glial migration. When astrocytes are grown in culture, they lose their characteristic star-like shape and AQP4 continuous plasma membrane localization observed in vivo. In this study, we differentiated primary astrocyte cultures with cAMP and lovastatin, both able to induce glial stellation through a reorganization of F-actin cytoskeleton, and obtained AQP4 selectively localized on the cell plasma membrane associated with an increase in the plasma membrane water transport level, but only cAMP induced an increase in AQP4 total protein expression. Phosphorylation experiments indicated that AQP4 in astrocytes is neither phosphorylated nor a substrate of PKA. Depolymerization of F-actin cytoskeleton performed by cytochalasin-D suggested that F-actin cytoskeleton plays a primary role for AQP4 plasma membrane localization and during cell adhesion. Finally, AQP4 knockdown does not compromise the ability of astrocytes to stellate in the presence of cAMP, indicating that astrocyte stellation is independent of AQP4.

Potassium dynamics in the epileptic cortex: new insights on an old topic

The role of changes in the extracellular potassium concentration [K(+)](o) in epilepsy has remained unclear. Historically, it was hypothesized that [K(+)]( o) is the causal factor for epileptic seizures. This so-called potassium accumulation hypothesis led to substantial debate but subsequently failed to find wide acceptance. However, recent studies on the pathophysiology of tissue from epileptic human patients and animal epilepsy models revealed aberrations in [K(+)](o) regulation. Computational models of cortical circuits that include ion concentration dynamics have catalyzed a renewed interest in the role of [K(+)](o) in epilepsy. The authors here connect classical and more recent insights on [K(+)]( o) dynamics in the cortex with the goal of providing starting points for a next generation of [K(+)](o) research. Such research may ultimately lead to an entirely new class of antiepileptic drugs that act on the [K(+)](o) regulation system.