Delayed K+ clearance associated with aquaporin-4 mislocalization: phenotypic defects in brains of alpha-syntrophin-null mice

Sulphoraphane enhances aquaporin-4 expression and decreases spinal cord oedema following spinal cord injury

BACKGROUND

Aquaporin-4 (AQP4) is a water channel protein and it is an important determinant of outcome after brain injury. Sulphoraphane (SFN) increases AQP4 levels with reduction of brain oedema at 3 days post-traumatic brain injury. However, little is known about the effect of SFN on AQP4 expression and oedema after spinal cord injury (SCI).

METHODS AND PROCEDURES

The present study used a murine SCI model induced by compression injury. AQP4 protein level and mRNA level were detected by Western blot and by RT-PCR at 48 hours after SCI, respectively. In addition, immunohistochemical study was used to show AQP4 expression in the spinal cord segments and water content of the spinal cord segments were measured by wet?:?dry weight ratio.

MAIN OUTCOMES AND RESULTS

This study shows that AQP4 level was decreased in the injured spinal cord segments at 48 hours following SCI. Post-injury administration of SFN increased AQP4 levels, which was accompanied by a significant reduction in spinal cord segment oedema at 48 hours post-injury.

CONCLUSION

These findings suggest that the reduction of spinal cord oedema in response to SFN administration could be due, in part, to water clearance by AQP4 from the injured spinal cord segments.

Astroglial loss and edema formation in the rat piriform cortex and hippocampus following pilocarpine-induced status epilepticus

In the present study we analyzed aquaporin-4 (AQP4) immunoreactivity in the piriform cortex (PC) and the hippocampus of pilocarpine-induced rat epilepsy model to elucidate the roles of AQP4 in brain edema following status epilepticus (SE). In non-SE-induced animals, AQP4 immunoreactivity was diffusely detected in the PC and the hippocampus. AQP4 immunoreactivity was mainly observed in the endfeet of astrocytes. Following SE the AQP4-deleted area was clearly detected in the PC, not in the hippocampus. Decreases in dystrophin and α-syntrophin immunoreactivities were followed by reduction in AQP4 immunoreactivity. These alterations were accompanied by the development of vasogenic edema and the astroglial loss in the PC. In addition, acetazolamide (an AQP4 inhibitor) treatment exacerbated vasogenic edema and astroglial loss both in the PC and in the hippocampus. These findings suggest that SE may induce impairments of astroglial AQP4 functions via disruption of the dystrophin/α-syntrophin complex that worsen vasogenic edema. Subsequently, vasogenic edema results in extensive astroglial loss that may aggravate vasogenic edema.

Laminar-specific and developmental expression of aquaporin-4 in the mouse hippocampus

Mice deficient in the water channel aquaporin-4 (AQP4) demonstrate increased seizure duration in response to hippocampal stimulation as well as impaired extracellular K+ clearance. However, the expression of AQP4 in the hippocampus is not well described. In this study, we investigated (i) the developmental, laminar and cell-type specificity of AQP4 expression in the hippocampus; (ii) the effect of Kir4.1 deletion on AQP4 expression; and (iii) performed Western blot and RT-PCR analyses. AQP4 immunohistochemistry on coronal sections from wild-type (WT) or Kir4.1-/- mice revealed a developmentally-regulated and laminar-specific pattern, with highest expression in the CA1 stratum lacunosum-moleculare (SLM) and the molecular layer (ML) of the dentate gyrus (DG). AQP4 was colocalized with the glial markers glial fibrillary acidic protein (GFAP) and S100β in the hippocampus, and was also ubiquitously expressed on astrocytic endfeet around blood vessels. No difference in AQP4 immunoreactivity was observed in Kir4.1-/- mice. Electrophysiological and postrecording RT-PCR analyses of individual cells revealed that AQP4 and Kir4.1 were co-expressed in nearly all CA1 astrocytes. In NG2 cells, AQP4 was also expressed at the transcript level. This study is the first to examine subregional AQP4 expression during development of the hippocampus. The strikingly high expression of AQP4 in the CA1 SLM and DG ML identifies these regions as potential sites of astrocytic K+ and H2O regulation. These results begin to delineate the functional capabilities of hippocampal subregions and cell types for K+ and H2O homeostasis, which is critical to excitability and serves as a potential target for modulation in diverse diseases.

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

Deletion of aquaporin-4 renders retinal glial cells more susceptible to osmotic stress

The glial water channel aquaporin-4 (AQP4) is implicated in the control of ion and osmohomeostasis in the sensory retina. Using retinal slices from AQP4-deficient and wild-type mice, we investigated whether AQP4 is involved in the regulation of glial cell volume under altered osmotic conditions. Superfusion of retinal slices with a hypoosmolar solution induced a rapid swelling of glial somata in tissues from AQP4 null mice but not from wild-type mice. The swelling was mediated by oxidative stress, inflammatory lipid mediators, and sodium influx into the cells and was prevented by activation of glutamatergic and purinergic receptors. Distinct inflammatory proteins, including interleukin-1 beta, interleukin-6, and inducible nitric oxide synthase, were up-regulated in the retina of AQP4 null mice compared with control, whereas cyclooxygenase-2 was down-regulated. The data suggest that water flux through AQP4 is involved in the rapid volume regulation of retinal glial (Müller) cells in response to osmotic stress and that deletion of AQP4 results in an inflammatory response of the retinal tissue. Possible implications of the data for understanding the pathophysiology of neuromyelitis optica, a human disease that has been suggested to involve serum antibodies to AQP4, are discussed.

Glial dystrophin-associated proteins, laminin and agrin, are downregulated in the brain of mdx mouse

In this study, we investigated the involvement of dystrophin-associated proteins (DAPs) and their relationship with the perivascular basement membrane in the brains of mdx mice and controls at the age of 2 months. We analyzed (1) the expression of glial DAPs α-β-dystroglycan (DG), α-syntrophin, aquaporin-4 (AQP4) water channel, Kir 4.1 and dystrophin isoform (Dp71) by immunocytochemistry, laser confocal microscopy, immunogold electron microscopy, immunoblotting and RT-PCR; (2) the ultrastructure of the basement membrane and expression of laminin and agrin; and (3) the dual immunofluorescence colocalization of AQP4/α-β-DG, and of Kir 4.1/agrin. The following results were observed in mdx brain as compared with controls: (1) a significant reduction in protein content and mRNA expression of DAPs; (2) ultrastructurally, a thickened and discontinuous appearance of the basement membrane and a significant reduction in laminin and agrin; and (3) a molecular rearrangment of α-β-DG, coupled with a parallel loss of agrin and Kir 4.1 on basement membrane and glial endfeet. These data indicate that in mdx brain the deficiency in dystrophin and dystrophin isoform (Dp71) is coupled with a reduction of DAP components, coupled with an altered anchoring to the basement membrane.

Aquaporins: relevance to cerebrospinal fluid physiology and therapeutic potential in hydrocephalus

The discovery of a family of membrane water channel proteins called aquaporins, and the finding that aquaporin 1 was located in the choroid plexus, has prompted interest in the role of aquaporins in cerebrospinal fluid (CSF) production and consequently hydrocephalus. While the role of aquaporin 1 in choroidal CSF production has been demonstrated, the relevance of aquaporin 1 to the pathophysiology of hydrocephalus remains debated. This has been further hampered by the lack of a non-toxic specific pharmacological blocking agent for aquaporin 1. In recent times aquaporin 4, the most abundant aquaporin within the brain itself, which has also been shown to have a role in brain water physiology and relevance to brain oedema in trauma and tumours, has become an alternative focus of attention for hydrocephalus research. This review summarises current knowledge and concepts in relation to aquaporins, specifically aquaporin 1 and 4, and hydrocephalus. It also examines the relevance of aquaporins as potential therapeutic targets in hydrocephalus and other CSF circulation disorders.

The alpha-syntrophin PH and PDZ domains scaffold acetylcholine receptors, utrophin, and neuronal nitric oxide synthase at the neuromuscular junction

At the neuromuscular junction (NMJ), the dystrophin protein complex provides a scaffold that functions to stabilize acetylcholine receptor (AChR) clusters. Syntrophin, a key component of that scaffold, is a multidomain adapter protein that links a variety of signaling proteins and ion channels to the dystrophin protein complex. Without syntrophin, utrophin and neuronal nitric oxide synthase mu (nNOSmu) fail to localize to the NMJ and the AChRs are distributed abnormally. Here we investigate the contribution of syntrophin domains to AChR distribution and to localization of utrophin and nNOSmu at the NMJ. Transgenic mice expressing alpha-syntrophin lacking portions of the first pleckstrin homology (PH) domain (DeltaPH1a or DeltaPH1b) or the entire PDZ domain (DeltaPDZ) were bred onto the alpha-syntrophin null background. As expected the DeltaPDZ transgene did not restore the NMJ localization of nNOS. The DeltaPH1a transgene did restore postsynaptic nNOS but surprisingly did not restore sarcolemmal nNOS (although sarcolemmal aquaporin-4 was restored). Mice lacking the alpha-syntrophin PDZ domain or either half of the PH1 domain were able to restore utrophin to the NMJ but did not correct the aberrant AChR distribution of the alpha-syntrophin knock-out mice. However, mice expressing both the transgenic DeltaPDZ and the transgenic DeltaPH1a constructs did restore normal AChR distribution, demonstrating that both domains are required but need not be confined within the same protein to function. We conclude that the PH1 and PDZ domains of alpha-syntrophin work in concert to facilitate the localization of AChRs and nNOS at the NMJ.

Spatial and temporal dissociation of AQP4 and Kir4.1 expression during induction of refractive errors

PURPOSE

Spatial co-localization of aquaporin water channels (AQP4) and inwardly rectifying potassium ion channels (Kir4.1) on the endfeet regions of glial cells has been suggested as the basis of functionally interrelated mechanisms of osmoregulation in brain edema. The aim of this study was to investigate the spatial and temporal changes in the expression of AQP4 and Kir4.1 channels in an avascular retina during the first week of the optical induction of refractive errors.

METHODS

Three-day-old hatchling chicks were randomly assigned to three groups and either did not wear lenses or were monocularly goggled with +/-10D lenses for varying times up to 7 days before biometric assessment. Retinal tissue was prepared either for western blot analysis to show the presence of the AQP4 and Kir4.1 protein in the chick retina or for immunolocalization using AQP4 and Kir4.1 antibodies to determine the regional distribution and intensity of labeling during the induction of refractive errors.

RESULTS

As expected, ultrasonography demonstrated that all eyes showed rapid elongation post hatching. Negative lens-wearing eyes elongated faster than fellow eyes or normal non goggled eyes and became progressively more myopic with time post lensing. Positive lens-wearing eyes showed reduced ocular growth compared to normal controls and developed a hyperopic refraction. Quantitative immunohistochemistry revealed the upregulation of AQP4 channel expression on Müller cells in the retinal nerve fiber layer during the first 2 days of negative lens wear. Kir4.1 channel upregulation in the inner plexiform layer was only found on day 4 of positive lens wear during the development of refractive hyperopia.

CONCLUSIONS

These results indicate that the expression of AQP4 and Kir4.1 channels on Müller cells is associated with the changes in ocular volume seen during the induction of refractive errors. However, the sites of greatest expression and the temporal pattern of the upregulation of AQP4 and Kir4.1 were dissimilar, indicating a dissociation of AQP4 and Kir4.1 function during refractive error development. Increased AQP4 expression in the nerve fiber layer is suggested to contribute to the rapid axial elongation and movement of fluid into the vitreous cavity in the presence of minus lenses; whereas, upregulation of Kir4.1 channels appears to play a role in limiting axial elongation in the presence of plus lenses.

Role of aquaporin-4 in the development of brain oedema in liver failure

BACKGROUND & AIMS

Liver failure is associated with progressive cytotoxic brain oedema (astrocyte swelling), which underlies hepatic encephalopathy (HE). Ammonia and superimposed inflammation are key synergistic factors in HE, but the mechanism(s) involved remain unknown. We aimed to determine whether aquaporin-4 (AQP4), an astrocyte endfeet bi-directional water channel, is associated with the brain oedema of HE.

METHOD

Rats (n=60) received sham-operation (sham), 5 days hyperammonaemia-inducing diet (HD), galactosamine (GALN) induced acute liver failure (ALF), 4 weeks bile duct-ligation (BDL) induced cirrhosis, or caecal ligation and puncture (CLP), a 24h model of bacterial peritonitis. Rats from every group (except CLP) were randomised to receive intraperitoneal injections of lipopolysaccharide (LPS; 1mg/kg) or saline, prior to termination 3h later. Brain water, AQP4 protein expression (western blot) and AQP4 localisation by immunogold electron microscopy were investigated.

RESULTS

Significant hyperammonaemia was observed in saline-injected BDL (p<0.05), GALN (p<0.01), and HD (p<0.01), compared to sham rats. LPS injection did not affect arterial ammonia or plasma biochemistry in any of the treatment groups. Increased brain water was observed in saline-injected GALN (p<0.05), HD (p<0.01), and CLP (p<0.001) compared to sham rats. Brain water was numerically increased in BDL rats, but this failed to reach significance (p=0.09). LPS treatment further increased oedema significantly in all treatment groups (p<0.05, respectively). AQP4 expression was significantly increased in saline-injected BDL (p<0.05), but not other treatment groups, compared to sham rats. Membrane polarisation was maintained in BDL rats.

CONCLUSION

The results suggest that AQP4 is not directly associated with the development of brain oedema in liver failure, hyperammonaemia, or sepsis. In cirrhosis, there is increased AQP4 protein expression, but membrane polarisation, is maintained, possibly in a compensatory attempt to limit severe brain oedema.

Regulation of brain water: is there a role for aquaporins in epilepsy?

Aquaporin-4 Is Increased in the Sclerotic Hippocampus in Human Temporal Lobe Epilepsy

Lee TS, Eid T, Mane S, Kim JH, Spencer DD, Ottersen OP, de Lanerolle NC

Acta Neuropathol (Berl) 2004;108:493–502

The hippocampus of patients with mesial temporal lobe epilepsy is often hardened and shrunken, a condition known as sclerosis. MRI reveals an increase in the T2-weighted signal, whereas diffusion-weighted imaging shows a higher apparent diffusion coefficient in sclerotic hippocampi, indicating increased water content. As water transport appears to be coupled to K+ clearance and neuronal excitability, the molecular basis of the perturbed water homeostasis in the sclerotic hippocampus was explored. The expression of aquaporin-4 (AQP-4), the predominant water channel in the brain, was studied with quantitative real-time polymerase chain reaction analysis, light-microscopic immunohistochemistry, and high-resolution immunogold labeling. A significant increase in AQP-4 was observed in sclerotic, but not in nonsclerotic, hippocampi obtained from patients with medically intractable temporal lobe epilepsy. This increase was positively correlated with an increase in the astrocyte marker glial fibrillary acidic protein. AQP-4 was localized to the plasma membranes of astrocytes including the perivascular endfeet. Gene expression associated with increased AQP-4 was evaluated by high-throughput gene-expression analysis with Affymetrix GeneChip U133A, and related gene networks were investigated with Ingenuity Pathways Analysis. AQP-4 expression was associated with a decrease in expression of the dystrophin gene, a protein implicated in the anchoring of AQP-4 in perivascular endfeet. The decreased expression of dystrophin may indicate a loss of polarity in the distribution of AQP-4 in astrocytes. We conclude that the perturbed expression of AQP-4 and dystrophin may be one factor underlying the loss of ion and water homeostasis in the sclerotic hippocampus and hypothesize that the reported changes may contribute to the epileptogenic properties of the sclerotic tissue.

Components of the basal lamina and dystrophin-dystroglycan complex in the neurointermediate lobe of rat pituitary gland: different localizations of beta-dystroglycan, dystrobrevins, alpha1-syntrophin, and aquaporin-4

The so-called neurointermediate lobe is composed of the intermediate and neural lobes of the pituitary. The present immunohistochemical study investigated components of the basal lamina (laminin, agrin, and perlecan), the dystrophin-dystroglycan complex (dystrophin, beta-dystroglycan, alpha1-dystrobrevin, beta-dystrobrevin, utrophin, and alpha1-syntrophin), and the aquaporins (aquaporin-4 and -9). Glia markers (GFAP, S100, and glutamine synthetase) and components of connective tissue (collagen type I and fibronectin) were also labeled. In the neurohypophysis, immunostaining of basal lamina delineated meningeal invaginations. In these invaginations, vessels were seen to penetrate the organ without submerging into its parenchyma. On the parenchymal side of the invaginations, beta-dystroglycan was detected, whereas utrophin was detected in the walls of vessels. Immunostaining of alpha1-dystrobrevin and alpha1-syntrophin did not delineate the vessels. The cells of the intermediate lobe were fully immunoreactive to alpha1-dystrobrevin and alpha1-syntrophin, whereas components of the basal lamina delineated the contours of the cells. GFAP-immunoreactive processes surrounded them. Aquaporin-4 localized at the periphery of the neurohypophysis, mainly adjacent to the intermediate lobe but not along the vessels. It colocalized only partially with GFAP and not at all with alpha1-syntrophin. Aquaporin-9 was not detected. These results emphasize the possibility that the components of the dystrophin-dystroglycan complex localize differently and raise the question about the roles of dystrobrevins, alpha1-syntrophin, and aquaporin-4 in the functions of the intermediate and neural lobes, respectively.

Changes in ocular aquaporin expression following optic nerve crush

PURPOSE

Changes in the expression of water channels (aquaporins; AQP) have been reported in several diseases. However, such changes and mechanisms remain to be evaluated for retinal injury after optic nerve crush (ONC). This study was designed to analyze changes in the expression of AQP4 (water selective channel) and AQP9 (water and lactate channel) following ONC in the rat.

METHODS

Rat retinal ganglion cells (RGCs) were retrogradely labeled by applying FluoroGold onto the left superior colliculus 1 week before ONC. Retinal injuries were induced by ONC unilaterally. Real-time PCR was used to measure changes in AQP4, AQP9, thy-1, Kir4.1 (K(+) channel), and beta-actin messages. Changes in AQP4, AQP9, Kir4.1, B cell lymphoma-x (bcl-xl), and glial fibrillary acidic protein (GFAP) expression were measured in total retinal extracts using western blotting.

RESULTS

The number of RGCs labeled retrogradely from the superior colliculus was 2,090+/-85 cells/mm(2) in rats without any treatment, which decreased to 1,091+/-78 (47% loss) and 497+/-87 cells/mm(2) (76% loss) on days 7 and 14, respectively. AQP4, Kir4.1, and thy-1 protein levels decreased at days 2, 7, and 14, which paralleled a similar reduction in mRNA levels, with the exception of Kir4.1 mRNA at day 2 showing an apparent upregulation. In contrast, AQP9 mRNA and protein levels showed opposite changes to those observed for the latter targets. Whereas AQP9 mRNA increased at days 2 and 14, protein levels decreased at both time points. AQP9 mRNA decreased at day 7, while protein levels increased. GFAP (a marker of astrogliosis) remained upregulated at days 2, 7, and 14, while bcl-xl (anti-apoptotic) decreased.

CONCLUSIONS

The reduced expression of AQP4 and Kir4.1 suggests dysfunctional ion coupling in retina following ONC and likely impaired retinal function. The sustained increase in GFAP indicates astrogliosis, while the decreased bcl-xl protein level suggests a commitment to cellular death, as clearly shown by the reduction in the RGC population and decreased thy-1 expression. Changes in AQP9 expression suggest a contribution of the channel to retinal ganglion cell death and response of distinct amacrine cells known to express AQP9 following traumatic injuries.

Differential water permeability and regulation of three aquaporin 4 isoforms

Aquaporin 4 (AQP4) is expressed in the perivascular glial endfeet and is an important pathway for water during formation and resolution of brain edema. In this study, we examined the functional properties and relative unit water permeability of three functional isoforms of AQP4 expressed in the brain (M1, M23, Mz). The M23 isoform gave rise to square arrays when expressed in Xenopus laevis oocytes. The relative unit water permeability differed significantly between the isoforms in the order of M1 > Mz > M23. None of the three isoforms were permeable to small osmolytes nor were they affected by changes in external K(+) concentration. Upon protein kinase C (PKC) activation, oocytes expressing the three isoforms demonstrated rapid reduction of water permeability, which correlated with AQP4 internalization. The M23 isoform was more sensitive to PKC regulation than the longer isoforms and was internalized significantly faster. Our results suggest a specific role for square array formation.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

MAPK induces AQP1 expression in astrocytes following injury

Aquaporin-4 (AQP4) is the principle water channel and the primary route for water transport across astrocytic membranes. AQP4 co-localizes with Kir4.1 channels at astrocytic endfeet, and it has been suggested that these channels cooperate in K(+) and water homeostasis. In response to injury, two additional aquaporins, AQP1 and AQP9, can be detected in astrocytes, yet neither is found in cultured astrocytes, and therefore their contribution to astrocyte water uptake and biology is poorly investigated. In this study, we used a cortical stab wound assay to demonstrate an upregulation of AQP1 following injury in reactive glia. We were able to mimic such injury in astrocytic cultures and show that AQP1 expression is induced within 16 h following injury in vitro. This induction could be blocked by inhibition of MEK1/2 using U0126, and suggests that AQP1 is specifically induced in reactive astrocytes via the mitogen-activated protein kinases signaling pathway.

Water transport between CNS compartments: functional and molecular interactions between aquaporins and ion channels

The physiological ability of the mammalian CNS to integrate peripheral stimuli and to convey information to the body is tightly regulated by its capacity to preserve the ion composition and volume of the perineuronal milieu. It is well known that astroglial syncytium plays a crucial role in such process by controlling the homeostasis of ions and water through the selective transmembrane movement of inorganic and organic molecules and the equilibration of osmotic gradients. Astrocytes, in fact, by contacting neurons and cells lining the fluid-filled compartments, are in a strategic position to fulfill this role. They are endowed with ion and water channel proteins that are localized in specific plasma membrane domains facing diverse liquid spaces. Recent data in rodents have demonstrated that the precise dynamics of the astroglia-mediated homeostatic regulation of the CNS is dependent on the interactions between water channels and ion channels, and their anchoring with proteins that allow the formation of macromolecular complexes in specific cellular domains. Interplay can occur with or without direct molecular interactions suggesting the existence of different regulatory mechanisms. The importance of molecular and functional interactions is pinpointed by the numerous observations that as consequence of pathological insults leading to the derangement of ion and volume homeostasis the cell surface expression and/or polarized localization of these proteins is perturbed. Here, we critically discuss the experimental evidence concerning: (1) molecular and functional interplay of aquaporin 4, the major aquaporin protein in astroglial cells, with potassium and gap-junctional channels that are involved in extracellular potassium buffering. (2) the interactions of aquaporin 4 with chloride and calcium channels regulating cell volume homeostasis. The relevance of the crosstalk between water channels and ion channels in the pathogenesis of astroglia-related acute and chronic diseases of the CNS is also briefly discussed.

Functional and molecular interactions between aquaporins and Na,K-ATPase

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. Astrocytes play a key role in the clearance of both potassium (K(+)) and glutamate released during neuronal activity. Emerging evidence suggests that AQP4 facilitates K(+) clearance by astrocytes and contributes to recovery of neuronal excitability. Here we report that AQP4 can assemble with its regulator metabotropic glutamate receptor 5 (mGluR5) and with Na,K-ATPase; the enzyme responsible for active K(+) transport and for establishing the electrochemical gradient across the cell plasma membrane. We have, by use of pull down assays in rat brain tissue, identified the segment in the AQP4 NH(2)-terminus containing the amino acid residues 23-32 as the site for interaction with Na,K-ATPase catalytic subunit and with mGluR5. Mutagenesis studies revealed that the AQP4 amino acids K27 and W30 are of key importance for interaction with both Na,K-ATPase and mGluR5. To confirm that interaction also occurs within intact cells, we have performed fluorescence resonance energy transfer (FRET) studies in primary astrocytes derived from rat striatum. The results indicate close proximity of wild type AQP4 and Na,K-ATPase in the plasma membrane of rat astrocytes. FRET efficiencies observed with the mutants AQP4 K27A and AQP4 W30A were significantly lower, highlighting the importance of these residues for the interaction between AQP4 and Na,K-ATPase. We conclude that AQP4/Na,K-ATPase/mGluR5 can form a macromolecular complex/transporting microdomain in astrocytes. This complex may be of functional importance for the regulation of water and K(+) homeostasis in the brain, as well as for neuron-astrocyte metabolic crosstalk.

Variants of the genes encoding AQP4 and Kir4.1 are associated with subgroups of patients with temporal lobe epilepsy

OBJECTIVE

The etiopathogenesis of temporal lobe epilepsy (TLE) and its subgroups - mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) and TLE with antecedent febrile seizures (TLE-FS) - is poorly understood. It has been proposed that the water channel aquaporin-4 (AQP4) and the potassium channel Kir4.1 (KCNJ10 gene) act in concert to regulate extracellular K(+) homeostasis and that functional alterations of these channels influence neuronal excitability. The current study was designed to identify variants of the AQP4 and KCNJ10 genes associated with TLE and subgroups of this condition.

MATERIAL AND METHODS

We included 218 Norwegian patients with TLE and 181 ethnically matched healthy controls. An association study was established in which all TLE patients were compared with healthy controls. Additionally, subgroups of 56 MTLE-HS patients were compared with 162 TLE patients without HS, and 102 TLE-FS patients were compared with 105 TLE without FS.

RESULTS

We found eight single SNPs, seven in KCNJ10 and one between KCNJ10 and KCNJ9, associated with TLE-FS (nominal p-values from 0.009 to 0.041). Seven of the SNPs segregate into one large haplotype block expanding from KCNJ10 to KCNJ9, including the region interposed those genes. One haplotype was overrepresented in the TLE-FS cases (nominal p-value 0.014). These results were confirmed by explorative multivariate analysis indicating that a combination of SNPs from KCNJ10, the region between KCNJ10 and KCNJ9, and the AQP4 gene is associated with TLE-FS. For the TLE cohort as a whole, explorative multivariate analysis indicated a combination of SNPs from the KCNJ10 and AQP4 genes in association with TLE.

CONCLUSION

Variations in the AQP4 and the KCNJ10/KCNJ9 region are likely to be associated with TLE, particularly TLE-FS, supporting the suggestion that perturbations of water and K(+) transport are involved in the etiopathogenesis of TLE.

Cellular elements of the blood-brain barrier

The Blood-brain-barrier (BBB) provides both anatomical and physiological protection for the central nervous system (CNS), shielding the brain for toxic substances in the blood, supplying brain tissues with nutrients and filtering harmful compounds from the brain back to the bloodstream. The BBB is composed of four main cellular elements: endothelial cells (ECs), astrocyte end-feet, microglial cells, and pericytes. Transport across the BBB is limited by both physical and metabolic barriers (enzymes, and different transport systems). Tight junctions (TJs) present between ECs form an important barrier against diffusion, excluding most blood-borne substances for entering the brain.

Functional implication of Dp71 in osmoregulation and vascular permeability of the retina

Functional alterations of Müller cells, the principal glia of the retina, are an early hallmark of most retina diseases and contribute to their further progression. The molecular mechanisms of these reactive Müller cell alterations, resulting in disturbed retinal homeostasis, remain largely unknown. Here we show that experimental detachment of mouse retina induces mislocation of the inwardly rectifying potassium channels (Kir4.1) and a downregulation of the water channel protein (AQP4) in Müller cells. These alterations are associated with a strong decrease of Dp71, a cytoskeleton protein responsible for the localization and the clustering of Kir4.1 and AQP4. Partial (in detached retinas) or total depletion of Dp71 in Müller cells (in Dp71-null mice) impairs the capability of volume regulation of Müller cells under osmotic stress. The abnormal swelling of Müller cells In Dp71-null mice involves the action of inflammatory mediators. Moreover, we investigated whether the alterations in Müller cells of Dp71-null mice may interfere with their regulatory effect on the blood-retina barrier. In the absence of Dp71, the retinal vascular permeability was increased as compared to the controls. Our results reveal that Dp71 is crucially implicated in the maintenance of potassium homeostasis, in transmembraneous water transport, and in the Müller cell-mediated regulation of retinal vascular permeability. Furthermore, our data provide novel insights into the mechanisms of retinal homeostasis provided by Müller cells under normal and pathological conditions.

Phenotypic characteristics of temporal lobe epilepsy: the impact of hippocampal sclerosis

OBJECTIVES

Whether mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) is a condition with a unique biological background that can be delineated from other TLE, is unresolved. Here we performed a comparative analysis of two TLE patient cohorts - one cohort with HS and one without HS - in order to identify phenotypic characteristics specifically associated with MTLE-HS.

METHODS

Epidemiological data and clinical and diagnostic features were compared between patients with MTLE-HS and TLE patients without HS. When appropriate, data were compared with healthy controls.

RESULTS

Fifty-six (26%) patients were diagnosed with MTLE-HS and 162 (74%) with other TLE. Age at epilepsy onset was lower in patients with MTLE-HS (P = 0.003) than in TLE patients without HS. Incidence of simple partial seizures was higher in the MTLE-HS group (P = 0.006), as were complex partial seizures (P = 0.001), ictal psychiatric symptoms (P = 0.015), and autonomic symptoms (P < 0.001). Interictal psychiatric symptoms, including depression, were less frequent in MTLE-HS (P = 0.043). MTLE-HS patients had a higher incidence of childhood febrile seizures (FS; P = 0.043) than TLE patients without HS. In contrast, the former group had the lower frequency of first-grade family members with childhood FS (P = 0.019).

CONCLUSIONS

We identified phenotypic characteristics that distinguish MTLE-HS from other types of TLE. These characteristics will be important in diagnostics, treatment, and determination of prognosis, and provide a basis for future phenotype-genotype studies.

Aquaporin-4 in brain and spinal cord oedema

Brain oedema is a major clinical problem produced by CNS diseases (e.g. stroke, brain tumour, brain abscess) and systemic diseases that secondarily affect the CNS (e.g. hyponatraemia, liver failure). The swollen brain is compressed against the surrounding dura and skull, which causes the intracranial pressure to rise, leading to brain ischaemia, herniation, and ultimately death. A water channel protein, aquaporin-4 (AQP4), is found in astrocyte foot processes (blood-brain border), the glia limitans (subarachnoid cerebrospinal fluid-brain border) and ependyma (ventricular cerebrospinal fluid-brain border). Experiments using mice lacking AQP4 or alpha syntrophin (which secondarily downregulate AQP4) showed that AQP4 facilitates oedema formation in diseases causing cytotoxic (cell swelling) oedema such as cerebral ischaemia, hyponatraemia and meningitis. In contrast, AQP4 facilitates oedema elimination in diseases causing vasogenic (vessel leak) oedema and therefore AQP4 deletion aggravates brain oedema produced by brain tumour and brain abscess. AQP4 is also important in spinal cord oedema. AQP4 deletion was associated with less cord oedema and improved outcome after compression spinal cord injury in mice. Here we consider the possible routes of oedema formation and elimination in the injured cord and speculate about the role of AQP4. Finally we discuss the role of AQP4 in neuromyelitis optica (NMO), an inflammatory demyelinating disease that produces oedema in the spinal cord and optic nerves. NMO patients have circulating AQP4 IgG autoantibody, which is now used for diagnosing NMO. We speculate how NMO-IgG might produce CNS inflammation, demyelination and oedema. Since AQP4 plays a key role in the pathogenesis of CNS oedema, we conclude that AQP4 inhibitors and activators may reduce CNS oedema in many diseases.

Modulation of Kir4.1 and Kir4.1-Kir5.1 channels by small changes in cell volume

The K+ channels Kir4.1 and Kir4.1-Kir5.1 are expressed in the glial cells of the CNS and are involved in regulation of the K+ homeostasis. Several studies have shown that Kir4.1 channels are co-localized with aquaporins (AQP4) in the glial endfeet, and a putative functional coupling between the Kir channels and aquaporins is therefore debated. To test a possible volume-sensitivity of the Kir channels, the Kir4.1 or Kir4.1-Kir5.1 channels were expressed in Xenopus oocytes with or without co-expression of aquaporins and subsequently exposed to cell volume alterations. Our results show an increase in Kir4.1 and Kir4.1-Kir5.1 currents upon swelling of the oocytes and a reduction in the current when the oocytes were shrunk. The volume-dependent changes in channel activity were not due to changes in the kinetics of the channels. These findings implicate a putative functional interaction between the Kir channels and aquaporins via small, fast cell volume changes in the glial cells.

Immunotherapy, vascular pathology, and microhemorrhages in transgenic mice

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder that results in severe cognitive decline. Amyloid plaques are a principal pathology found in AD and are composed of aggregated amyloid-beta (Abeta) peptides. According to the amyloid hypothesis, Abeta peptides initiate the other pathologies characteristic for AD including cognitive deficits. Immunotherapy against Abeta is a potential therapeutic for the treatment of humans with AD. While anti-Abeta immunotherapy has been shown to reduce amyloid burden in both mouse models and in humans, immunotherapy also exacerbates vascular pathologies. Cerebral amyloid angiopathy (CAA), that is, the accumulation of amyloid in the cerebrovasculature, is increased with immunotherapy in humans with AD and in mouse models of amyloid deposition. CAA persists in the brains of clinical trial patients that show removal of parenchymal amyloid. Mouse model studies also show that immunotherapy results in multiple small bleeds in the brain, termed microhemorrhages. The neurovascular unit is a term used to describe the cerebrovasculature and its associated cells-astrocytes, neurons, pericytes and microglia. CAA affects brain perfusion and there is now evidence that the neurovascular unit is affected in AD when CAA is present. Understanding the type of damage to the neurovascular unit caused by CAA in AD and the underlying cause of microhemorrhage after immunotherapy is essential to the success of therapeutic vaccines as a treatment for AD.

Differential expression of utrophin-A and -B promoters in the central nervous system (CNS) of normal and dystrophic mdx mice

Utrophin (Utrn) is the autosomal homolog of dystrophin, the Duchene Muscular Dystrophy (DMD) locus product and of therapeutic interest, as its overexpression can compensate dystrophin's absence. Utrn is transcribed by Utrn-A and -B promoters with mRNAs differing at their 5' ends. However, previous central nervous system (CNS) studies used C-terminal antibodies recognizing both isoforms. As this distinction may impact upregulation strategies, we generated Utrn-A and -B promoter-specific antibodies, Taqman Polymerase chain reaction (PCR)-based absolute copy number assays, and luciferase-reporter constructs to study CNS of normal and dystrophic mdx mice. Differential expression of Utrn-A and -B was noted in microdissected and capillary-enriched fractions. At the protein level, Utrn-B was predominantly expressed in vasculature and ependymal lining, whereas Utrn-A was expressed in neurons, astrocytes, choroid plexus and pia mater. mRNA quantification demonstrated matching patterns of differential expression; however, transcription-translation mismatch was noted for Utrn-B in caudal brain regions. Utrn-A and Utrn-B proteins were significantly upregulated in olfactory bulb and cerebellum of mdx brain. Differential promoter activity, mRNA and protein expressions were studied in cultured C2C12, bEnd3, neurons and astrocytes. Promoter activity ranking for Utrn-A and -B was neurons > astrocytes > C2C12 > bEnd3 and bEnd3 > astrocytes > neurons > C2C12, respectively. Our results identify promoter usage patterns for therapeutic targeting and define promoter-specific differential distribution of Utrn isoforms in normal and dystrophic CNS.

Interdependence of laminin-mediated clustering of lipid rafts and the dystrophin complex in astrocytes

Astrocyte endfeet surrounding blood vessels are active domains involved in water and potassium ion transport crucial to the maintenance of water and potassium ion homeostasis in brain. A growing body of evidence points to a role for dystroglycan and its interaction with perivascular laminin in the targeting of the dystrophin complex and the water-permeable channel, aquaporin 4 (AQP4), at astrocyte endfeet. However, the mechanisms underlying such compartmentalization remain poorly understood. In the present study we found that AQP4 resided in Triton X-100-insoluble fraction, whereas dystroglycan was recovered in the soluble fraction in astrocytes. Cholesterol depletion resulted in the translocation of a pool of AQP4 to the soluble fraction indicating that its distribution is indeed associated with cholesterol-rich membrane domains. Upon laminin treatment AQP4 and the dystrophin complex, including dystroglycan, reorganized into laminin-associated clusters enriched for the lipid raft markers GM1 and flotillin-1 but not caveolin-1. Reduced diffusion rates of GM1 in the laminin-induced clusters were indicative of the reorganization of raft components in these domains. In addition, both cholesterol depletion and dystroglycan silencing reduced the number and area of laminin-induced clusters of GM1, AQP4, and dystroglycan. These findings demonstrate the interdependence between laminin binding to dystroglycan and GM1-containing lipid raft reorganization and provide novel insight into the dystrophin complex regulation of AQP4 polarization in astrocytes.

AQP4 gene deletion in mice does not alter blood-brain barrier integrity or brain morphology

The glial cell water channel aquaporin-4 (AQP4) plays an important role in brain edema, astrocyte migration, and neuronal excitability. Zhou et al. [Zhou J, Kong H, Hua X, Xiao M, Ding J, Hu G (2008) Altered blood-brain barrier integrity in adult aquaporin-4 knockout mice. Neuroreport 19:1-5] recently reported that AQP4 deletion significantly altered blood-brain barrier integrity and glial fibrillary acidic protein (GFAP) immunoreactivity in their AQP4 null mice. Here we describe a detailed characterization of baseline brain properties in our AQP4 null mice, including gross appearance, neuronal, astrocyte and oligodendrocyte characteristics, and blood-brain barrier integrity. Gross anatomical measurements included estimates of brain and ventricle size. Neurons, astrocytes and oligodendrocytes were assessed using the neuronal nuclear marker NeuN, the astrocyte marker GFAP, and the myelin stain Luxol Fast Blue. The blood-brain barrier was studied by electron microscopy and the horseradish peroxidase extravasation technique. There were no differences in brain and ventricle sizes between wild type and AQP4 null mice, nor were there differences in the cerebral cortical density of NeuN positive nuclei, perimicrovessel and glia limitans GFAP immunoreactivity, or the thickness and myelination of the corpus callosum. The ultrastructure of microvessels in the frontal cortex and caudate nucleus of wild type vs. AQP4 null mice was indistinguishable, with features including intact endothelial tight junctions, absence of perimicrovessel astrocyte foot process edema, and absence of horseradish peroxidase extravasation. In contrast to the report by Zhou et al. (2008), our data show that AQP4 deletion in mice does not produce major structural abnormalities in the brain.

Activity-dependent glial swelling is impaired in aquaporin-4 knockout mice

We investigated the role of aquaporin-4 (AQP4), a water channel expressed in glial cells, in neural activity mediated morphological changes observed in brain slice preparation. Changes in flavoprotein fluorescence (FF) and infrared light scattering (LS) signals were measured before and after repetitive stimulation of layer VI in rostral somatosensory cortical slices taken from AQP4 knockout (KO) and wild-type (WT) mice. Changes in FF, which reflect neural aerobic activities, were comparable for the two groups in all cortical layers. However, changes in LS signals, which are indicative of cell swelling, were significantly decreased in layer I of AQP4 KO mice compared to that of WT mice. We conclude that AQP4 likely plays a significant role in neural activity-dependent glial swelling.

Astrocytic plasticity and patterned oxytocin neuronal activity: dynamic interactions

Astroglial-neuronal interactions are important in brain functions. However, roles of glial fibrillary acidic protein (GFAP) in this interaction remain unclear in acute physiological processes. We explored this issue using the supraoptic nucleus (SON) in lactating rats. At first, we identified the essential role of astrocytes in the milk-ejection reflex (MER) by disabling astrocytic functions via intracerebroventricular application of l-aminoadipic acid (l-AAA). l-AAA blocked the MER and reduced GFAP levels in the SON. In brain slices, l-AAA suppressed oxytocin (OT) neuronal activity and EPSCs. Suckling reduced GFAP in immunocytochemical images and in Western blots, reductions that were partially reversed after the MER. OT, the dominant hormone mediating the MER, reduced GFAP expression in brain slices. Tetanus toxin suppressed EPSCs but did not influence OT-reduced GFAP. Protease inhibitors did not influence OT-reduced GFAP images but blocked the degradation of GFAP molecules. In the presence of OT, transient 12 mm K(+) exposure, simulating effects of synchronized bursts before the MER, reversed OT-reduced GFAP expression. Consistently, suckling first reduced and then increased the expression of aquaporin 4, astrocytic water channels coupled to K(+) channels. Moreover, GFAP molecules were associated with astrocytic proteins, including aquaporin 4, actin, and glutamine synthetase and serine racemase. GFAP-aquaporin 4 association decreased during initial suckling and increased after the MER, whereas opposite changes occurred between GFAP and actin. MER also decreased the association between GFAP and glutamine synthetase. These results indicate that suckling elicits dynamic glial neuronal interactions in the SON; GFAP plasticity dynamically reflects OT neuronal activity.

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.

Vascular amyloid alters astrocytic water and potassium channels in mouse models and humans with Alzheimer's disease

The neurovascular unit (NVU) comprises cerebral blood vessels and surrounding astrocytes, neurons, perivascular microglia and pericytes. Astrocytes associated with the NVU are responsible for maintaining cerebral blood flow and ionic and osmotic balances in the brain. A significant proportion of individuals with Alzheimer's disease (AD) have vascular amyloid deposits (cerebral amyloid angiopathy, CAA) that contribute to the heterogeneous nature of the disease. To determine whether NVU astrocytes are affected by the accumulation of amyloid at cerebral blood vessels we examined astrocytic markers in four transgenic mouse models of amyloid deposition. These mouse models represent mild CAA, moderate CAA with disease progression to tau pathology and neuron loss, severe CAA and severe CAA with disease progression to tau pathology and neuron loss. We found that CAA and disease progression both resulted in distinct NVU astrocytic changes. CAA causes a loss of apparent glial fibrillary acidic protein (GFAP)-positive astrocytic end-feet and loss of water channels (aquaporin 4) localized to astrocytic end feet. The potassium channels Kir4.1, an inward rectifying potassium channel, and BK, a calcium-sensitive large-conductance potassium channel, were also lost. The anchoring protein, dystrophin 1, is common to these channels and was reduced in association with CAA. Disease progression was associated with a phenotypic switch in astrocytes indicated by a loss of GFAP-positive cells and a gain of S100 beta-positive cells. Aquaporin 4, Kir4.1 and dystrophin 1 were also reduced in autopsied brain tissue from individuals with AD that also display moderate and severe CAA. Together, these data suggest that damage to the neurovascular unit may be a factor in the pathogenesis of Alzheimer's disease.

Discovery of the aquaporins and development of the field

The study of water transport began long before the molecular identification of water channels with studies of water-permeable tissues. The discovery of the first aquaporin, AQP1, occurred during experiments focused on the identity of the Rh blood group antigens. Since then the field has expanded dramatically to study aquaporins in all types of organisms. In mammals, some of the aquaporins transport only water. However, there are some family members that collectively transport a diverse set of solutes. The aquaporins can be regulated by factors that affect channel permeability or subcellular localization. An extensive set of studies examines the physiological role of many of the mammalian aquaporins. However, much is still to be discovered about the physiological role of this membrane protein family.

Knock-out models reveal new aquaporin functions

Knockout mice have been informative in the discovery of unexpected biological functions of aquaporins. Knockout mice have confirmed the predicted roles of aquaporins in transepithelial fluid transport, as in the urinary concentrating mechanism and glandular fluid secretion. A less obvious, though predictable role of aquaporins is in tissue swelling under stress, as in the brain in stroke, tumor and infection. Phenotype analysis of aquaporin knockout mice has revealed several unexpected cellular roles of aquaporins whose mechanisms are being elucidated. Aquaporins facilitate cell migration, as seen in aquaporin-dependent tumor angiogenesis and tumor metastasis, by a mechanism that may involve facilitated water transport in lamellipodia of migrating cells. The ' aquaglyceroporins', aquaporins that transport both glycerol and water, regulate glycerol content in epidermis, fat and other tissues, and lead to a multiplicity of interesting consequences of gene disruption including dry skin, resistance to skin carcinogenesis, impaired cell proliferation and altered fat metabolism. An even more surprising role of a mammalian aquaporin is in neural signal transduction in the central nervous system. The many roles of aquaporins might be exploited for clinical benefit by modulation of aquaporin expression/function - as diuretics, and in the treatment of brain swelling, glaucoma, epilepsy, obesity and cancer.

Role of aquaporin-4 in cerebral edema and stroke

Cerebral edema plays a central role in the pathophysiology of many diseases of the central nervous system (CNS) including ischemia, trauma, tumors, inflammation, and metabolic disturbances. The formation of cerebral edema results in an increase in tissue water content and brain swelling which, if unchecked, can lead to elevated intracranial pressure (ICP), reduced cerebral blood flow, and ultimately cerebral herniation and death. Despite the clinical significance of cerebral edema, the mechanism of brain water transport and edema formation remain poorly understood. As a result, current therapeutic tools for managing cerebral edema have changed little in the past 90 years. "Malignant ischemic stroke" is characterized by high mortality (80%) and represents a major clinical problem in cerebrovascular disease. Widespread ischemic injury in these patients causes progressive cerebral edema, increased ICP, and rapid clinical decline. In response to these observations, a series of recent studies have begun to target cerebral edema in the management of large ischemic strokes. During cerebral edema formation, the glial water channel aquaporin-4 (AQP4) has been show to facilitate astrocyte swelling ("cytotoxic swelling"). AQP4 has also been seen to be responsible for the reabsorption of extracellular edema fluid ("vasogenic edema"). In the present review, the role of AQP4 in the development of cerebral edema is discussed with emphasis on its contribution to ischemic edema. We also examine the potential of AQP4 as a therapeutic target in edema associated with stroke.

Lack of sex-linked differences in cerebral edema and aquaporin-4 expression after experimental stroke

Aquaporin-4 (AQP4) has been shown to be important in the evolution of stroke-associated cerebral edema. However, the role of AQP4 in stroke-associated cerebral edema as it pertains to sex has not been previously studied. The perivascular pool of AQP4 is important in the influx and efflux of water during focal cerebral ischemia. We used mice with targeted disruption of the gene encoding alpha-syntrophin (alpha-Syn(-/-)) that lack the perivascular AQP4 pool but retain the endothelial pool of this protein. Infarct volume at 72 h after transient focal ischemia (90 mins) in isoflurane-anesthetized mice was attenuated in both sexes with alpha-Syn deletion as compared with their wild-type (WT) counterparts. There were no sex differences in hemispheric water content in WT and alpha-Syn(-/-) mice or regional AQP4 expression in WT mice. In neither sex did alpha-Syn deletion lead to alterations in end-ischemic regional cerebral blood flow (rCBF). These data suggest that after experimental stroke: (1) there is no difference in stroke-associated cerebral edema based on sex, (2) AQP4 does not involve in sex-based differences in stroke volume, and (3) perivascular pool of AQP4 has no significant role in end-ischemic rCBF.

Functional implications for Kir4.1 channels in glial biology: from K+ buffering to cell differentiation

Astrocytes and oligodendrocytes are characterized by a very negative resting potential and a high resting permeability for K(+) ions. Early pharmacological and biophysical studies suggested that the resting potential is established by the activity of inwardly rectifying, Ba(2+) sensitive, weakly rectifying Kir channels. Molecular cloning has identified 16 Kir channels genes of which several mRNA transcripts and protein products have been identified in glial cells. However, genetic deletion and siRNA knock-down studies suggest that the resting conductance of astrocytes and oligodendrocytes is largely due to Kir4.1. Loss of Kir4.1 causes membrane depolarization, and a break-down of K(+) and glutamate homeostasis which results in seizures and wide-spread white matter pathology. Kir channels have also been shown to act as critical regulators of cell division whereby Kir function is correlated with an exit from the cell cycle. Conversely, loss of functional Kir channels is associated with re-entry of cells into the cell cycle and gliosis. A loss of functional Kir channels has been shown in a number of neurological diseases including temporal lobe epilepsy, amyotrophic lateral sclerosis, retinal degeneration and malignant gliomas. In the latter, expression of Kir4.1 is sufficient to arrest the aberrant growth of these glial derived tumor cells. Kir4.1 therefore represents a potential therapeutic target in a wide variety of neurological conditions.

The perivascular pool of aquaporin-4 mediates the effect of osmotherapy in postischemic cerebral edema

OBJECTIVE

Osmotherapy with hypertonic saline ameliorates cerebral edema associated with experimental ischemic stroke. We tested the hypothesis that hypertonic saline exerts its antiedema effect by promoting an efflux of water from brain via the perivascular aquaporin-4 pool. We used mice with targeted disruption of the gene encoding alpha-syntrophin (alpha-Syn(-/-)) that lack the perivascular aquaporin-4 pool but retain the endothelial pool of this protein.

DESIGN

Prospective laboratory animal study.

SETTING

Research laboratory in a university teaching hospital.

MEASUREMENTS AND MAIN RESULTS

Halothane-anesthetized adult male wildtype C57B/6 and alpha-Syn(-/-) mice were subjected to 90 min of transient middle cerebral artery occlusion and treated with either a continuous intravenous infusion of 0.9% saline or 3% hypertonic saline (1.5 mL/kg/hr) for 48 hr. In the first series of experiments (n = 59), increased brain water content analyzed by wet-to-dry ratios in the ischemic hemisphere of wildtype mice was attenuated after hypertonic saline (79.9% +/- 0.5%; mean +/- SEM) but not after 0.9% saline (82.3% +/- 1.0%) treatment. In contrast in alpha-Syn(-/-) mice, hypertonic saline had no effect on the postischemic edema (hypertonic saline: 80.3% +/- 0.7%; 0.9% saline: 80.3% +/- 0.4%). In the second series of experiments (n = 32), treatment with hypertonic saline attenuated postischemic blood-brain barrier disruption at 48 hr in wildtype mice but not in alpha-Syn(-/-) mice; alpha-Syn(-/-) deletion alone had no effect on blood-brain barrier integrity. In the third series of experiments (n = 34), alpha-Syn(-/-) mice treated with either hypertonic saline or 0.9% saline had smaller infarct volume as compared with their wildtype counterparts.

CONCLUSIONS

These data demonstrate that 1) osmotherapy with hypertonic saline exerts antiedema effects via the perivascular pool of aquaporin-4, 2) hypertonic saline attenuates blood-brain barrier disruption depending on the presence of perivascular aquaporin-4, and 3) deletion of the perivascular pool of aquaporin-4 alleviates tissue damage after stroke, in mice subjected to osmotherapy and in nontreated mice.

Changes in ocular aquaporin-4 (AQP4) expression following retinal injury

PURPOSE

Changes in the expression of water channels or aquaporins (AQP) have been reported in several diseases. However, such changes and mechanisms remain to be evaluated for retinal injury. This study was designed to analyze changes in the expression of AQP4 following elevation of intraocular pressure (IOP) and after intravitreal endothelin-1 injection and the potential involvement of the ubiquitin-dependent proteasome.

METHODS

Retinal injuries were induced by the elevation of intraocular pressure in rat eyes using the Morrison model or following endothelin-1 intravitreal injection. Immunohistochemistry using a combination of glial fibrillary acidic protein (GFAP) and aquaporin-4 antibodies were employed to follow changes in the optic nerve head astrocytes. Real-time quantitative PCR (Q-PCR) was used for measuring changes in AQP4, ubiquitin hydrolase L1 (UCH-L1), and beta-actin messages. Changes in AQP4, caspase-3, thy-1, ubiquitination, and GFAP expression were also followed in total retinal extracts using western blotting. An S5a column was used to purify ubiquitinated proteins.

RESULTS

In retinas of both injury models, there was an upregulation of GFAP (a marker of astrogliosis), caspase-3, and downregulation of thy-1, a marker for retinal ganglion cell stress, and decreased retinal AQP4 mRNA and protein levels as determined by Q-PCR, and western blotting, respectively. By contrast, IOP enhanced expression and co-localization of GFAP and AQP4 in optic nerve astrocytes. AQP4 was detected in affinity-purified ubiquitinated proteins using S5a column, suggesting that AQP4 is a target for degradation by the ubiquitin-dependent proteasome. While elevation of IOP induced an increase in ubiquitination in retinal extracts, it decreased ubiquitination in optic nerve extracts as detected by western blotting. Enhanced ubiquitination and decreased ubiquitination appear to correlate with AQP4 expression. IOP decreased UCH-L1 (or protein gene protein [PGP9.5]) in retinal extracts as judged by Q-PCR.

CONCLUSIONS

The enhanced expression of AQP4 in optic nerve astrocytes following elevation of IOP may explain the astrocytic hypertrophy normally seen in glaucoma patients and may involve alteration in the activity of ubiquitin-dependent proteasomal degradation system. The decreased ubiquitination in the optic nerve may lead to increased levels of proapoptotic proteins known to be degraded by the proteasome, and thus to axonal degeneration in glaucoma.

Recurrent seizures and brain pathology after inhibition of glutamine synthetase in the hippocampus in rats

An excess of extracellular glutamate in the hippocampus has been linked to the generation of recurrent seizures and brain pathology in patients with medically intractable mesial temporal lobe epilepsy (MTLE). However, the mechanism which results in glutamate excess in MTLE remains unknown. We recently reported that the glutamate-metabolizing enzyme glutamine synthetase is deficient in the hippocampus in patients with MTLE, and we postulated that this deficiency is critically involved in the pathophysiology of the disease. To further explore the role of glutamine synthetase in MTLE we created a novel animal model of hippocampal glutamine synthetase deficiency by continuous (approximately 28 days) microinfusion of methionine sulfoximine (MSO: 0.625 to 2.5 microg/h) unilaterally into the hippocampus in rats. This treatment led to a deficiency in hippocampal glutamine synthetase activity by 82-97% versus saline. The majority (>95%) of the MSO-treated animals exhibited recurrent seizures that continued for several weeks. Some of the MSO-treated animals exhibited neuropathological features that were similar to mesial temporal sclerosis, such as hippocampal atrophy and patterned loss of hippocampal neurons. However, many MSO-treated animals displayed only minimal injury to the hippocampus, with no clear evidence of mesial temporal sclerosis. These findings support the hypothesis that a deficiency in hippocampal glutamine synthetase causes recurrent seizures, even in the absence of classical mesial temporal sclerosis, and that restoration of glutamine synthetase may represent a novel approach to therapeutic intervention in this disease.