Greatly impaired migration of implanted aquaporin-4-deficient astroglial cells in mouse brain toward a site of injury

Aquaporin water channels in the nervous system

The aquaporins (AQPs) are plasma membrane water-transporting proteins. AQP4 is the principal member of this protein family in the CNS, where it is expressed in astrocytes and is involved in water movement, cell migration and neuroexcitation. AQP1 is expressed in the choroid plexus, where it facilitates cerebrospinal fluid secretion, and in dorsal root ganglion neurons, where it tunes pain perception. The AQPs are potential drug targets for several neurological conditions. Astrocytoma cells strongly express AQP4, which may facilitate their infiltration into the brain, and the neuroinflammatory disease neuromyelitis optica is caused by AQP4-specific autoantibodies that produce complement-mediated astrocytic damage.

Nuclear envelope composition determines the ability of neutrophil-type cells to passage through micron-scale constrictions

Neutrophils are characterized by their distinct nuclear shape, which is thought to facilitate the transit of these cells through pore spaces less than one-fifth of their diameter. We used human promyelocytic leukemia (HL-60) cells as a model system to investigate the effect of nuclear shape in whole cell deformability. We probed neutrophil-differentiated HL-60 cells lacking expression of lamin B receptor, which fail to develop lobulated nuclei during granulopoiesis and present an in vitro model for Pelger-Huët anomaly; despite the circular morphology of their nuclei, the cells passed through micron-scale constrictions on similar timescales as scrambled controls. We then investigated the unique nuclear envelope composition of neutrophil-differentiated HL-60 cells, which may also impact their deformability; although lamin A is typically down-regulated during granulopoiesis, we genetically modified HL-60 cells to generate a subpopulation of cells with well defined levels of ectopic lamin A. The lamin A-overexpressing neutrophil-type cells showed similar functional characteristics as the mock controls, but they had an impaired ability to pass through micron-scale constrictions. Our results suggest that levels of lamin A have a marked effect on the ability of neutrophils to passage through micron-scale constrictions, whereas the unusual multilobed shape of the neutrophil nucleus is less essential.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Neuroinflammatory imaging biomarkers: relevance to multiple sclerosis and its therapy

Magnetic resonance imaging is an established tool in the management of multiple sclerosis (MS). Loss of blood brain barrier integrity assessed by gadolinium (Gd) enhancement is the current standard marker of MS activity. To explore the complex cascade of the inflammatory events, other magnetic resonance imaging, but also positron emission tomographic markers reviewed in this article are being developed to address active neuroinflammation with increased sensitivity and specificity. Alternative magnetic resonance contrast agents, positron emission tomographic tracers and imaging techniques could be more sensitive than Gd to early blood brain barrier alteration, and they could assess the inflammatory cell recruitment and/or the associated edema accumulation. These markers of active neuroinflammation, although some of them are limited to experimental studies, could find great relevance to complete Gd information and thereby increase our understanding of acute lesion pathophysiology and its noninvasive follow-up, especially to monitor treatment efficacy. Furthermore, such accurate markers of inflammation combined with those of neurodegeneration hold promise to provide a more complete picture of MS, which will be of great benefit for future therapeutic strategies.

Aquaporin 4: a player in cerebral edema and neuroinflammation

Neuroinflammation is a common pathological event observed in many different brain diseases, frequently associated with blood brain barrier (BBB) dysfunction and followed by cerebral edema. Neuroinflammation is characterized with microglia activation and astrogliosis, which is a hypertrophy of the astrocytes. Astrocytes express aquaporin 4, the water channel protein, involved in water homeostasis and edema formation. Aside from its function in water homeostasis, recent studies started to show possible interrelations between aquaporin 4 and neuroinflammation. In this review the roles of aquaporin 4 in neuroinflammation associated with BBB disruption and cerebral edema will be discussed with recent studies in the field.

Aggravated inflammation and increased expression of cysteinyl leukotriene receptors in the brain after focal cerebral ischemia in AQP4-deficient mice

OBJECTIVE

Aquaporin-4 (AQP4), the main water channel protein in the brain, plays a critical role in water homeostasis and brain edema. Here, we investigated its role in the inflammatory responses after focal cerebral ischemia.

METHODS

In AQP4-knockout (KO) and wild-type mice, focal cerebral ischemia was induced by 30 min of middle cerebral arterial occlusion (MCAO). Ischemic neuronal injury and cellular inflammatory responses, as well as the expression and localization of cysteinyl leukotriene CysLT(2) and CysLT(1) receptors, were determined at 24 and 72 h after MCAO.

RESULTS

AQP4-KO mice showed more neuronal loss, more severe microglial activation and neutrophil infiltration, but less astrocyte proliferation in the brain after MCAO than wild-type mice. In addition, the protein levels of both CysLT(1) and CysLT(2) receptors were up-regulated in the ischemic brain, and the up-regulation was more pronounced in AQP4-KO mice. The CysLT(1) and CysLT(2) receptors were primarily localized in neurons, microglia and neutrophils; those localized in microglia and neutrophils were enhanced in AQP4-KO mice.

CONCLUSION

AQP4 may play an inhibitory role in postischemic inflammation.

Characteristics of aquaporin expression surrounding senile plaques and cerebral amyloid angiopathy in Alzheimer disease

Senile plaques (SPs) containing amyloid β peptide (Aβ) 1-42 are the major species present in Alzheimer disease (AD), whereas Aβ1-40 is the major constituent of arteriolar walls affected by cerebral amyloid angiopathy. The water channel proteins astrocytic aquaporin 1 (AQP1) and aquaporin 4 (AQP4) are known to be abnormally expressed in AD brains, but the expression of AQPs surrounding SPs and cerebral amyloid angiopathy has not been described in detail. Here, we investigated whether AQP expression is associated with each species of Aβ deposited in human brains affected by either sporadic or familial AD. Immunohistochemical analysis demonstrated more numerous AQP1-positive reactive astrocytes in the AD cerebral cortex than in controls, located close to Aβ42- or Aβ40-positive SPs. In AD cases, however, AQP1-positive astrocytes were not often observed in Aβ-rich areas, and there was a significant negative correlation between the levels of AQP1 and Aβ42 assessed semiquantitatively. We also found that Aβ plaque-like AQP4 was distributed in association with Aβ42- or Aβ40-positive SPs and that the degree of AQP4 expression around Aβ40-positive vessels was variable. These findings suggest that a defined population of AQP1-positive reactive astrocytes may modify Aβ deposition in the AD brain, whereas the Aβ deposition process might alter astrocytic expression of AQP4.

Neuromyelitis optica IgG does not alter aquaporin-4 water permeability, plasma membrane M1/M23 isoform content, or supramolecular assembly

Neuromyelitis optica (NMO) is thought to be caused by immunoglobulin G autoantibodies (NMO-IgG) against astrocyte water channel aquaporin-4 (AQP4). A recent study (Hinson et al. (2012) Proc Natl Acad Sci USA 109:1245-1250) reported that NMO-IgG inhibits AQP4 water permeability directly and causes rapid cellular internalization of the M1 but not M23 isoform of AQP4, resulting in AQP4 clustering, enhanced complement-dependent cytotoxicity, and tissue swelling. Here, we report evidence challenging this proposed mechanism of NMO-IgG-mediated pathology. We measured osmotic water permeability by stopped-flow light scattering on plasma membrane vesicles isolated from AQP4-expressing CHO cells, an approach that can detect changes in water permeability as small as 5% and is not confounded by internalization effects. We found similar single-molecule water permeability for M1-AQP4 tetramers and M23-AQP4 clusters (orthogonal arrays of particles, OAPs). Exposure of AQP4 to high concentrations of NMO-IgG from six seropositive NMO patients, and to high-affinity recombinant monoclonal NMO antibodies, did not reduce AQP4 water permeability. Also, NMO-IgG did not reduce water permeability in AQP4-reconstituted proteoliposomes. In transfected cells expressing M1- or M23-AQP4 individually, NMO-IgG caused more rapid internalization of M23- than M1-AQP4. In cells coexpressing both isoforms, M1- and M23-AQP4 comingled in OAPs that were internalized together in response to NMO-IgG. Super-resolution imaging and native gel electrophoresis showed that the size of AQP4 OAPs was not altered by NMO sera or recombinant NMO antibodies. We conclude that NMO-IgG does not: (i) inhibit AQP4 water permeability, (ii) cause preferential internalization of M1-AQP4, or (iii) cause intramembrane AQP4 clustering.

AQP4 knockout aggravates ischemia/reperfusion injury in mice

BACKGROUND AND PURPOSE

The glial water channel aquaporin-4 (AQP4) has been shown to be involved in a wide range of brain disorders. Although its important role in stroke has already been documented, the underlying mechanism was not clarified yet. Therefore, this study was designed to investigate the impacts of AQP4 deletion in ischemia/reperfusion (I/R).

METHODS AND RESULTS

Herein we found a higher mortality and more severe neurological deficits in AQP4 knockout (AQP4(-/-)) mice after transient middle cerebral artery occlusion while no difference was observed in water content variation during I/R between two genotypes except a higher basal water content developed in AQP4(-/-) mouse brain, implying the same increment of water content over a higher basal level may provoke an even more elevated intracranial pressure, which might be an important cause of increased mortality in AQP4(-/-) mice. Moreover, AQP4 knockout aggravated I/R injury with enlarged infarct size and a more serious loss of CA1 neurons accompanied by a striking hypertrophy of astrocytes, suggesting an involvement of AQP4 in astrocytic dysfunction.

CONCLUSIONS

Our findings provide direct evidence that AQP4 plays a crucial role in the pathogenesis of I/R injury, which may confer a new option for stroke treatment.

Plasminogen activator inhibitor type 1 regulates microglial motility and phagocytic activity

Background

Plasminogen activator inhibitor type 1 (PAI-1) is the primary inhibitor of urokinase type plasminogen activators (uPA) and tissue type plasminogen activators (tPA), which mediate fibrinolysis. PAI-1 is also involved in the innate immunity by regulating cell migration and phagocytosis. However, little is known about the role of PAI-1 in the central nervous system.

Methods

In this study, we identified PAI-1 in the culture medium of mouse mixed glial cells by liquid chromatography and tandem mass spectrometry. Secretion of PAI-1 from glial cultures was detected by ELISA and western blotting analysis. Cell migration was evaluated by in vitro scratch-wound healing assay or Boyden chamber assay and an in vivo stab wound injury model. Phagocytic activity was measured by uptake of zymosan particles.

Results

The levels of PAI-1 mRNA and protein expression were increased by lipopolysaccharide and interferon-γ stimulation in both microglia and astrocytes. PAI-1 promoted the migration of microglial cells in culture via the low-density lipoprotein receptor-related protein (LRP) 1/Janus kinase (JAK)/signal transducer and activator of transcription (STAT)1 axis. PAI-1 also increased microglial migration in vivo when injected into mouse brain. PAI-1-mediated microglial migration was independent of protease inhibition, because an R346A mutant of PAI-1 with impaired PA inhibitory activity also promoted microglial migration. Moreover, PAI-1 was able to modulate microglial phagocytic activity. PAI-1 inhibited microglial engulfment of zymosan particles in a vitronectin- and Toll-like receptor 2/6-dependent manner.

Conclusion

Our results indicate that glia-derived PAI-1 may regulate microglial migration and phagocytosis in an autocrine or paracrine manner. This may have important implications in the regulation of brain microglial activities in health and disease.

Delayed increase of astrocytic aquaporin 4 after juvenile traumatic brain injury: possible role in edema resolution?

Traumatic brain injury (TBI) is one of the leading causes of death and disability in children and adolescents. The neuropathological sequelae that result from TBI are a complex cascade of events including edema formation, which occurs more frequently in the pediatric than the adult population. This developmental difference in the response to injury may be related to higher water content in the young brain and also to molecular mechanisms regulating water homeostasis. Aquaporins (AQPs) provide a unique opportunity to examine the mechanisms underlying water mobility, which remain poorly understood in the juvenile post-traumatic edema process. We examined the spatiotemporal expression pattern of principal brain AQPs (AQP1, AQP4, and AQP9) after juvenile TBI (jTBI) related to edema formation and resolution observed using magnetic resonance imaging (MRI). Using a controlled cortical impact in post-natal 17 day-old rats as a model of jTBI, neuroimaging analysis showed a global decrease in water mobility (apparent diffusion coefficient, ADC) and an increase in edema (T2-values) at 1 day post-injury, which normalized by 3 days. Immunohistochemical analysis of AQP4 in perivascular astrocyte endfeet was increased in the lesion at 3 and 7days post-injury as edema resolved. In contrast, AQP1 levels distant from the injury site were increased at 7, 30, and 60 days within septal neurons but did not correlate with changes in edema formation. Group differences were not observed for AQP9. Overall, our observations confirm that astrocyticAQP4 plays a more central role than AQP1 or AQP9 during the edema process in the young brain.

Neuromyelitis optica: aquaporin-4 based pathogenesis mechanisms and new therapies

Neuromyelitis optica (NMO) is an autoimmune 'aquaporinopathy' of the central nervous system that causes inflammatory demyelinating lesions primarily in spinal cord and optic nerve, leading to paralysis and blindness. NMO lesions show loss of aquaporin-4 (AQP4), GFAP and myelin, infiltration of granulocytes and macrophages, and perivascular deposition of activated complement. Most patients with NMO are seropositive for immunoglobulin autoantibodies (AQP4-IgG) against AQP4, the principal water channel of astrocytes. There is strong evidence that AQP4-IgG is pathogenic in NMO, probably by a mechanism involving complement-dependent astrocyte cytotoxicity, causing leukocyte infiltration, cytokine release and blood-brain barrier disruption, which leads to oligodendrocyte death, myelin loss and neuron death. Here, we review the evidence for this and alternative proposed NMO pathogenesis mechanisms, such as AQP4-IgG-induced internalization of AQP4 and glutamate transporters, complement-independent cell-mediated cytotoxicity, and AQP4-IgG inhibition of AQP4 water transport function. Based on the initiating pathogenic role of AQP4-IgG binding to astrocyte AQP4 in NMO, selective blocker therapies are under development in which AQP4-targeted monoclonal antibodies or small molecules block binding of AQP4-IgG to astrocytes and consequent downstream pathology.

Aggravated chronic brain injury after focal cerebral ischemia in aquaporin-4-deficient mice

The water channel aquaporin-4 (AQP4) is important in brain water homeostasis, and is also involved in astrocyte growth and glial scar formation. It has been reported that AQP4 deficiency attenuates acute ischemic brain injury as a result of reducing cytotoxic edema. Here, we determined whether AQP4 deficiency influences chronic brain injury after focal cerebral ischemia induced by 30 min of middle cerebral artery occlusion (MCAO). AQP4(-/-) mice exhibited a lower survival rate and less body weight gain than wild-type mice, but their neurological deficits were similar to wild-type mice during 35 days after MCAO. At 35 days after MCAO, AQP4(-/-) mice showed more severe brain atrophy and cavity formation in the ischemic hemisphere as well as more neuronal loss in the hippocampus. Furthermore, astrocyte proliferation and glial scar formation were impaired in AQP4(-/-) mice. Therefore, AQP4 deficiency complicated by astrocyte dysfunction aggravates chronic brain injury after focal cerebral ischemia, suggesting that AQP4 may be important in the chronic phase of the post-ischemic recovery process.

Aquaporin 4 and neuromyelitis optica

Neuromyelitis optica is an inflammatory demyelinating disorder of the CNS. The discovery of circulating IgG1 antibodies against the astrocyte water channel protein aquaporin 4 (AQP4) and the evidence that AQP4-IgG is involved in the development of neuromyelitis optica revolutionised our understanding of the disease. However, important unanswered questions remain--for example, we do not know the cause of AQP4-IgG-negative disease, how astrocyte damage causes demyelination, the role of T cells, why peripheral AQP4-expressing organs are undamaged, and how circulating AQP4-IgG enters neuromyelitis optica lesions. New drug candidates have emerged, such as aquaporumab (non-pathogenic antibody blocker of AQP4-IgG binding), sivelestat (neutrophil elastase inhibitor), and eculizumab (complement inhibitor). Despite rapid progress, randomised clinical trials to test new drugs will be challenging because of the small number of individuals with the disorder.

Aquaporins in clinical medicine

The aquaporins are a family of membrane water channels, some of which also transport glycerol. They are involved in a wide range of physiological functions (including water/salt homeostasis, exocrine fluid secretion, and epidermal hydration) and human diseases (including glaucoma, cancer, epilepsy, and obesity). At the cellular level, aquaporin-mediated osmotic water transport across cell plasma membranes facilitates transepithelial fluid transport, cell migration, and neuroexcitation; aquaporin-mediated glycerol transport regulates cell proliferation, adipocyte metabolism, and epidermal water retention. Genetic diseases caused by loss-of-function mutations in aquaporins include nephrogenic diabetes insipidus and congenital cataracts. The neuroinflammatory demyelinating disease neuromyelitis optica is marked by pathogenic autoantibodies against astrocyte water channel aquaporin-4. There remain broad opportunities for the development of aquaporin-based diagnostics and therapeutics. Disease-relevant aquaporin polymorphisms are beginning to be explored. There is great promise in the development of small-molecule aquaporin modulators for therapy of some types of refractory edema, brain swelling, neuroinflammation, glaucoma, epilepsy, cancer, pain, and obesity.

Aquaporin-4 deficiency attenuates acute lesions but aggravates delayed lesions and microgliosis after cryoinjury to mouse brain

OBJECTIVE

To determine whether aquaporin-4 (AQP4) regulates acute lesions, delayed lesions, and the associated microglial activation after cryoinjury to the brain.

METHODS

Brain cryoinjury was applied to AQP4 knockout (KO) and wild-type mice. At 24 h and on days 7 and 14 after cryoinjury, lesion volume, neuronal loss, and densities of microglia and astrocytes were determined, and their changes were compared between AQP4 KO and wild-type mice.

RESULTS

Lesion volume and neuronal loss in AQP4 KO mice were milder at 24 h following cryoinjury, but worsened on days 7 and 14, compared to those in wild-type mice. Besides, microglial density increased more, and astrocyte proliferation and glial scar formation were attenuated on days 7 and 14 in AQP4 KO mice.

CONCLUSION

AQP4 deficiency ameliorates acute lesions, but worsens delayed lesions, perhaps due to the microgliosis in the late phase.

Small-molecule inhibitors of NMO-IgG binding to aquaporin-4 reduce astrocyte cytotoxicity in neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of spinal cord and optic nerve caused by pathogenic autoantibodies (NMO-IgG) against astrocyte aquaporin-4 (AQP4). We developed a high-throughput screen to identify blockers of NMO-IgG binding to human AQP4 using a human recombinant monoclonal NMO-IgG and transfected Fisher rat thyroid cells stably expressing human M23-AQP4. Screening of ∼60,000 compounds yielded the antiviral arbidol, the flavonoid tamarixetin, and several plant-derived berbamine alkaloids, each of which blocked NMO-IgG binding to AQP4 without affecting AQP4 expression, array assembly, or water permeability. The compounds inhibited NMO-IgG binding to AQP4 in NMO patient sera and blocked NMO-IgG-dependent complement- and cell-mediated cytotoxicity with IC(50) down to ∼5 μM. Docking computations identified putative sites of blocker binding at the extracellular surface of AQP4. The blockers did not affect complement-dependent cytotoxicity caused by anti-GD3 antibody binding to ganglioside GD3. The blockers reduced by >80% the severity of NMO lesions in an ex vivo spinal cord slice culture model of NMO and in mice in vivo. Our results provide proof of concept for a small-molecule blocker strategy to reduce NMO pathology. Small-molecule blockers may also be useful for other autoimmune diseases caused by binding of pathogenic autoantibodies to defined targets.

Anti-aquaporin-4 monoclonal antibody blocker therapy for neuromyelitis optica

OBJECTIVE

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system. Circulating autoantibodies (NMO-immunoglobulin [Ig]G) against astrocyte water channel aquaporin-4 (AQP4) cause complement- and cell-mediated astrocyte damage with consequent neuroinflammation and demyelination. Current NMO therapies, which have limited efficacy, include immunosuppression and plasma exchange. The objective of this study was to develop a potential new NMO therapy based on blocking of pathogenic NMO-IgG binding to its target, AQP4.

METHODS

We generated nonpathogenic recombinant monoclonal anti-AQP4 antibodies that selectively block NMO-IgG binding to AQP4. These antibodies comprise a tight-binding anti-AQP4 Fab and a mutated Fc that lacks functionality for complement- and cell-mediated cytotoxicity. The efficacy of the blocking antibodies was studied using cell culture, spinal cord slice, and in vivo mouse models of NMO.

RESULTS

In AQP4-expressing cell cultures, the nonpathogenic competing antibodies blocked binding of NMO-IgG in human sera, reducing to near zero complement- and cell-mediated cytotoxicity. The antibodies prevented the development of NMO lesions in an ex vivo spinal cord slice model of NMO and in an in vivo mouse model, without causing cytotoxicity.

INTERPRETATION

Our results provide proof of concept for a therapy of NMO with blocking antibodies. The broad efficacy of antibody inhibition is likely due to steric competition because of its large physical size compared to AQP4. Blocker therapy to prevent binding of pathogenic autoantibodies to their targets may be useful for treatment of other autoimmune diseases as well.

Aquaporin-4 mediates astrocyte response to β-amyloid

It has been demonstrated that the water channel protein aquaporin-4 (AQP4) plays an important role in astrocyte plasticity in response to a variety of injuries or stimuli. However, the potential role of AQP4 in astrocyte response to β-amyloid (Aβ) has not been studied. The purpose of this study was to investigate this issue. Compared to media control, the lower concentrations of Aβ(1-42) (0.1-1 μM) increased AQP4 expression in cultured mouse cortical astrocytes, while the higher concentrations of Aβ(1-42) (10 μM) decreased AQP4 expression. AQP4 gene knockout reduced Aβ(1-42)-induced astrocyte activation and apoptosis, which was associated with a reduction in the uptake of Aβ via decreased upregulation of low-density lipoprotein receptor related protein-1. Moreover, time-course and levels of Aβ(1-42)-induced mitogen-activated protein kinase phosphorylation were altered in AQP4 null astrocytes compared with wild-type controls. Our data reveal a novel role of AQP4 in the uptake of Aβ by astrocytes, indicating that AQP4 is a molecular target for Alzheimer's disease.

Aquaporin-4 deficiency exacerbates brain oxidative damage and memory deficits induced by long-term ovarian hormone deprivation and D-galactose injection

Astrocyte dysfunction is implicated in pathogenesis of certain neurological disorders including Alzheimer's disease (AD). A growing body of evidence indicates that water channel aquaporin-4 (AQP4) is a potential molecular target for the regulation astrocyte function. Recently, we reported that AQP4 expression was increased in the hippocampus of an AD mouse model established by long-term ovarian hormone deprivation combined with D-galactose (D-gal) exposure. However, pathophysiological roles and mechanisms of AQP4 up-regulation remain unclear. To address this issue, age-matched female wild-type and AQP4 null mice underwent ovariectomy, followed by D-gal administration for 8 wk. AQP4 null mice showed more severe brain oxidative stress, spatial learning and memory deficits, and basal forebrain cholinergic impairment than the wild-type controls. Notably, AQP4 null hippocampus contained more prominent amyloid-β production and loss of synapse-related proteins. These results suggested that ovariectomy and D-gal injection induced oxidative damage results in compensatory increases of AQP4 expression, and deficiency of AQP4 exacerbates brain oxidative stress and memory deficits. Therefore, regulation of astrocyte function by AQP4 may attenuate oxidative damage, offering a promising therapeutic strategy for AD.

The role of aquaporin 4 in the brain

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

Aqp 9 and brain tumour stem cells

Several studies have implicated the aquaporins (aqp) 1, 4, and 9 in the pathogenesis of malignant brain tumours, suggesting that they contribute to motility, invasiveness, and oedema formation and facilitate metabolism in tumour cells under hypoxic conditions. We have studied the expression of aqp1, 4, and 9 in biopsies from glioblastomas, isolated tumour stem cells grown in a tumoursphere assay and analyzed the progenitor and differentiated cells from these cultures. We have compared these to the situation in normal rat brain, its stem cells, and differentiated cells derived thereof. In short, qPCR in tumour tissue showed presence of aqp1, 4, and 9. In the tumour progenitor population, aqp9 was markedly more highly expressed, whilst in tumour-derived differentiated cells, aqp4 was downregulated. However, immunostaining did not reveal increased protein expression of aqp9 in the tumourspheres containing progenitor cells; in contrast, its expression (both mRNA and protein) was high in differentiated cultures. We, therefore, propose that aquaporin 9 may have a central role in the tumorigenesis of glioblastoma.

Live-cell imaging of aquaporin-4 supramolecular assembly and diffusion

Aquaporin-4 (AQP4) is a water channel expressed in astrocytes throughout the central nervous system, as well as in epithelial cells in various peripheral organs. AQP4 is involved in brain water balance, neuroexcitation, astrocyte migration, and neuroinflammation and is the target of pathogenic autoantibodies in neuromyelitis optica. Two AQP4 isoforms produced by alternative splicing, M1 and M23 AQP4, form heterotetramers that assemble in cell plasma membranes in supramolecular aggregates called orthogonal arrays of particles (OAPs). OAPs have been studied morphologically, by freeze-fracture electron microscopy, and biochemically, by native gel electrophoresis. We have applied single-molecule and high-resolution fluorescence microscopy methods to visualize AQP4 and OAPs in live cells. Quantum dot single particle tracking of fluorescently labeled AQP4 has quantified AQP4 diffusion in membranes, and has elucidated the molecular determinants and regulation of OAP formation. The composition, structure, and kinetics of OAPs containing fluorescent protein-AQP4 chimeras have been studied utilizing total internal reflection fluorescence microscopy, single-molecule photobleaching, and super-resolution imaging methods. The biophysical data afforded by live-cell imaging of AQP4 and OAPs has provided new insights in the roles of AQP4 in organ physiology and neurological disease.

Genetic deletion of cdc42 reveals a crucial role for astrocyte recruitment to the injury site in vitro and in vivo

It is generally suggested that astrocytes play important restorative functions after brain injury, yet little is known regarding their recruitment to sites of injury, despite numerous in vitro experiments investigating astrocyte polarity. Here, we genetically manipulated one of the proposed key signals, the small RhoGTPase Cdc42, selectively in mouse astrocytes in vitro and in vivo. We used an in vitro scratch assay as a minimal wounding model and found that astrocytes lacking Cdc42 (Cdc42Δ) were still able to form protrusions, although in a nonoriented way. Consequently, they failed to migrate in a directed manner toward the scratch. When animals were injured in vivo through a stab wound, Cdc42Δ astrocytes developed protrusions properly oriented toward the lesion, but the number of astrocytes recruited to the lesion site was significantly reduced. Surprisingly, however, lesions in Cdc42Δ animals, harboring fewer astrocytes contained significantly higher numbers of microglial cells than controls. These data suggest that impaired recruitment of astrocytes to sites of injury has a profound and unexpected effect on microglia recruitment.

Differential aquaporin 4 expression during edema build-up and resolution phases of brain inflammation

Background

Vasogenic edema dynamically accumulates in many brain disorders associated with brain inflammation, with the critical step of edema exacerbation feared in patient care. Water entrance through blood-brain barrier (BBB) opening is thought to have a role in edema formation. Nevertheless, the mechanisms of edema resolution remain poorly understood. Because the water channel aquaporin 4 (AQP4) provides an important route for vasogenic edema resolution, we studied the time course of AQP4 expression to better understand its potential effect in countering the exacerbation of vasogenic edema.

Methods

Focal inflammation was induced in the rat brain by a lysolecithin injection and was evaluated at 1, 3, 7, 14 and 20 days using a combination of in vivo MRI with apparent diffusion coefficient (ADC) measurements used as a marker of water content, and molecular and histological approaches for the quantification of AQP4 expression. Markers of active inflammation (macrophages, BBB permeability, and interleukin-1β) and markers of scarring (gliosis) were also quantified.

Results

This animal model of brain inflammation demonstrated two phases of edema development: an initial edema build-up phase during active inflammation that peaked after 3 days (ADC increase) was followed by an edema resolution phase that lasted from 7 to 20 days post injection (ADC decrease) and was accompanied by glial scar formation. A moderate upregulation in AQP4 was observed during the build-up phase, but a much stronger transcriptional and translational level of AQP4 expression was observed during the secondary edema resolution phase.

Conclusions

We conclude that a time lag in AQP4 expression occurs such that the more significant upregulation was achieved only after a delay period. This change in AQP4 expression appears to act as an important determinant in the exacerbation of edema, considering that AQP4 expression is insufficient to counter the water influx during the build-up phase, while the second more pronounced but delayed upregulation is involved in the resolution phase. A better pathophysiological understanding of edema exacerbation, which is observed in many clinical situations, is crucial in pursuing new therapeutic strategies.

Role of aquaporins in cell migration and edema formation in human brain tumors

The aquaporins (AQPs) are a family of transmembrane water channel proteins widely distributed and play a major role in transcellular and transepithelial water movement. Moreover, recent evidence indicates that AQPs may be involved in cell migration, angiogenesis, and tumor growth. This review article summarizes literature data concerning the involvement of AQP-1 and -4 in human brain tumor growth and edema formation and suggests a potential therapeutic approach by antagonizing their biological activity.

Aquaporin-4: orthogonal array assembly, CNS functions, and role in neuromyelitis optica

Aquaporin-4 (AQP4) is a water-selective transporter expressed in astrocytes throughout the central nervous system, as well as in kidney, lung, stomach and skeletal muscle. The two AQP4 isoforms produced by alternative spicing, M1 and M23 AQP4, form heterotetramers that assemble in cell plasma membranes in supramolecular structures called orthogonal arrays of particles (OAPs). Phenotype analysis of AQP4-null mice indicates the involvement of AQP4 in brain and spinal cord water balance, astrocyte migration, neural signal transduction and neuroinflammation. AQP4-null mice manifest reduced brain swelling in cytotoxic cerebral edema, but increased brain swelling in vasogenic edema and hydrocephalus. AQP4 deficiency also increases seizure duration, impairs glial scarring, and reduces the severity of autoimmune neuroinflammation. Each of these phenotypes is likely explicable on the basis of reduced astrocyte water permeability in AQP4 deficiency. AQP4 is also involved in the neuroinflammatory demyelinating disease neuromyelitis optica (NMO), where autoantibodies (NMO-IgG) targeting AQP4 produce astrocyte damage and inflammation. Mice administered NMO-IgG and human complement by intracerebral injection develop characteristic NMO lesions with neuroinflammation, demyelination, perivascular complement deposition and loss of glial fibrillary acidic protein and AQP4 immunoreactivity. Our findings suggest the potential utility of AQP4-based therapeutics, including small-molecule modulators of AQP4 water transport function for therapy of brain swelling, injury and epilepsy, as well as small-molecule or monoclonal antibody blockers of NMO-IgG binding to AQP4 for therapy of NMO.

Aquaporin-4 Mz isoform: brain expression, supramolecular assembly and neuromyelitis optica antibody binding

Water channel aquaporin-4 (AQP4) is expressed in astrocytes throughout brain and spinal cord. Two major AQP4 isoforms are expressed, M1 and M23, having different translation initiation sites. A longer isoform (Mz) has been reported in rat with translation initiation 126-bp upstream from that of M1. By immunoblot analysis of SDS and native gels probed with a C-terminus anti-AQP4 antibody, Mz was detected in rat brain as a distinct band of size ∼39 kDa. Mz was absent in human and mouse brain because of in-frame stop codons. The ability of rat Mz to form orthogonal arrays of particles (OAPs) was investigated by single particle tracking and native gel electrophoresis. We found that Mz, like M1, diffused rapidly in the cell plasma membrane and did not form OAPs. However, when co-expressed with M23, Mz associated in OAPs by forming heterotetramers with M23. Unexpectedly, Mz-expressing cells bound neuromyelitis optica autoantibodies (NMO-IgG) poorly, <5-fold compared with M1-expressing cells. Truncation analysis suggested that the poor NMO-IgG binding to Mz involves residues 31-41 upstream of Met-1. We conclude that Mz AQP4 is (a) present at low level in rat but not human or mouse brain, (b) unable to form OAPs on its own but able to associate with M23 AQP4 in heterotetramers, and (c) largely unable to bind NMO-IgG because of N-terminus effects on the structure of the AQP4/NMO-IgG binding site.

Magnetic stimulation influences injury-induced migration of white matter astrocytes

This study investigates the effects and underlying mechanism of magnetic stimulation on injury-induced migration of white matter astrocytes. Twenty-four adult healthy SD rats were selected to inject 0.5 ml of 1% ethidium bromide (EB) in PBS into the dorsal spinal cord funiculus on the left side at the T10-11 level to make located spinal cord injury models. Then they were randomly divided into four groups (A, B, C, and D). Groups A, B, C, and D were exposed to 1 Hz pulsed magnetic stimulation underwent 5-min sessions on 14 consecutive days at the following levels: 0T (Group A) 1.9x40% T (Group B); 1.9x80% T (Group C); 1.9x100% T (Group D). On day 14 after stimulation, the rats were killed and the expression of glial fibrillary acidic protein (GFAP), microtubule associated protein-2 (MAP-2), extracellular signal-regulated kinase1/2 (ERK1/2), and the volume of holes were detected with immunohistochemistry. Quantitative analysis of the expression of GFAP, MAP-2, and ERK1/2 were performed with the image analysis system. With the increase of magnetic stimulation intensity, the volume of hole decreased at day 14 (P<0.05). In lesion areas, the expression of GFAP and ERK1/2 could be seen, while that of MAP-2 did not change before and after magnetic stimulation. Significant difference was revealed in the expression of GFAP, ERK1/2 among the four groups. It was significantly higher in the magnetic stimulation groups than that in the control group (P<0.05). After magnetic stimulation, astrocytes migrated into the hole. U0126, a potent and selective MEK1/2 inhibitor, inhibited up-regulation of pERK1/2 which was stimulated by magnetic stimulation. These data indicate that magnetic stimulation increases the migratory capacity of reactive white matter astrocytes in the injured center nervous system, which may be associated with activation of MEK1,2/ERK mitogenic pathway.

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.

A high throughput screen identifies chemical modulators of the laminin-induced clustering of dystroglycan and aquaporin-4 in primary astrocytes

BACKGROUND

Aquaporin-4 (AQP4) constitutes the principal water channel in the brain and is clustered at the perivascular astrocyte endfeet. This specific distribution of AQP4 plays a major role in maintaining water homeostasis in the brain. A growing body of evidence points to a role of the dystroglycan complex and its interaction with perivascular laminin in the clustering of AQP4 at perivascular astrocyte endfeet. Indeed, mice lacking components of this complex or in which laminin-dystroglycan interaction is disrupted show a delayed onset of brain edema due to a redistribution of AQP4 away from astrocyte endfeet. It is therefore important to identify inhibitory drugs of laminin-dependent AQP4 clustering which may prevent or reduce brain edema.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study we used primary rat astrocyte cultures to screen a library of >3,500 chemicals and identified 6 drugs that inhibit the laminin-induced clustering of dystroglycan and AQP4. Detailed analysis of the inhibitory drug, chloranil, revealed that its inhibition of the clustering is due to the metalloproteinase-2-mediated ß-dystroglycan shedding and subsequent loss of laminin interaction with dystroglycan. Furthermore, chemical variants of chloranil induced a similar effect on ß-dystroglycan and this was prevented by the antioxidant N-acetylcysteine.

CONCLUSION/SIGNIFICANCE

These findings reveal the mechanism of action of chloranil in preventing the laminin-induced clustering of dystroglycan and AQP4 and validate the use of high-throughput screening as a tool to identify drugs that modulate AQP4 clustering and that could be tested in models of brain edema.

Brain water mobility decreases after astrocytic aquaporin-4 inhibition using RNA interference

Neuroimaging with diffusion-weighted imaging is routinely used for clinical diagnosis/prognosis. Its quantitative parameter, the apparent diffusion coefficient (ADC), is thought to reflect water mobility in brain tissues. After injury, reduced ADC values are thought to be secondary to decreases in the extracellular space caused by cell swelling. However, the physiological mechanisms associated with such changes remain uncertain. Aquaporins (AQPs) facilitate water diffusion through the plasma membrane and provide a unique opportunity to examine the molecular mechanisms underlying water mobility. Because of this critical role and the recognition that brain AQP4 is distributed within astrocytic cell membranes, we hypothesized that AQP4 contributes to the regulation of water diffusion and variations in its expression would alter ADC values in normal brain. Using RNA interference in the rodent brain, we acutely knocked down AQP4 expression and observed that a 27% AQP4-specific silencing induced a 50% decrease in ADC values, without modification of tissue histology. Our results demonstrate that ADC values in normal brain are modulated by astrocytic AQP4. These findings have major clinical relevance as they suggest that imaging changes seen in acute neurologic disorders such as stroke and trauma are in part due to changes in tissue AQP4 levels.

A developmental defect in astrocytes inhibits programmed regression of the hyaloid vasculature in the mammalian eye

Previously we reported the novel observation that astrocytes ensheath the persistent hyaloid artery, both in the Nuc1 spontaneous mutant rat, and in human PFV (persistent fetal vasculature) disease (Developmental Dynamics 234:36-47, 2005). We now show that astrocytes isolated from both the optic nerve and retina of Nuc1 rats migrate faster than wild type astrocytes. Aquaporin 4 (AQP4), the major water channel in astrocytes, has been shown to be important in astrocyte migration. We demonstrate that AQP4 expression is elevated in the astrocytes in PFV conditions, and we hypothesize that this causes the cells to migrate abnormally into the vitreous where they ensheath the hyaloid artery. This abnormal association of astrocytes with the hyaloid artery may impede the normal macrophage-mediated remodeling and regression of the hyaloid system.

Persistent directional cell migration requires ion transport proteins as direction sensors and membrane potential differences in order to maintain directedness

BACKGROUND

Ion transport proteins generate small electric fields that can induce directional cell motility; however, little is known about their mechanisms that lead to directedness. We investigated Na, K-ATPase (NaKA) and Na+/H+ exchanger isoforms (NHE1 and 3) in SaOS-2 and Calvarial osteoblasts, which present anode- and cathode- directed motility, during electrotaxis.

RESULTS

Significant colocalizations of NaKA with vinculin and pNHE3 with ß-actin were observed to occur at the leading edges of cells. The directedness were attenuated when NaKA or NHE3 was inhibited, confirming their implication in directional sensing. Depending on the perceived direction, a divergent regulation in PIP2 levels as a function of NHE3 and NaKA levels was observed, suggesting that PIP2 may act as a spatiotemporal regulator of the cell membrane during electrotaxis. Moreover, at the same places where pNHE3 accumulates, bubble-shaped H+ clouds were observed, suggesting a physio-mechanical role for NHE3. The cell membrane becomes hyperpolarized at the front and depolarized at the back, which confirms NaKA activity at the leading edge.

CONCLUSION

We suggest a novel role for both NaKA and NHE3 that extends beyond ion translocation and conclude that they can act as directional sensors and Vmem as a regulatory cue which maintain the persistent direction in electrotaxis.

Proinflammatory role of aquaporin-4 in autoimmune neuroinflammation

Aquaporin-4 (AQP4) deficiency in mice reduces neuroinflammation in experimental autoimmune encephalomyelitis (EAE) produced by active immunization with myelin oligodendrocyte glycoprotein peptide (MOG). Potential mechanisms for the protective effect of AQP4 deficiency were investigated, including AQP4-dependent leukocyte and microglia cell function, immune cell entry in the central nervous system (CNS), intrinsic neuroinflammation, and humoral immune response. As we found with active-immunization EAE, neuroinflammation was greatly reduced in AQP4-knockout mice in adoptive-transfer EAE. AQP4 was absent in immune cells, including activated T lymphocytes. The CNS migration of fluorescently labeled, MOG-sensitized T lymphocytes was comparable in wild-type and AQP4-knockout mice. Microglia did not express AQP4. Serum anti-AQP4 antibodies were absent in EAE. Remarkably, intracerebral injection of LPS produced much greater neuroinflammation in wild-type than in AQP4-knockout mice, and cytokine (TNF-α and IL-6) secretion was reduced in astrocyte cultures from AQP4-knockout mice. Adenovirus-mediated expression of AQP4, or of an unrelated aquaporin, AQP1, increased cytokine secretion in astrocyte and nonastrocyte cell cultures, supporting the involvement of aquaporin water permeability in cytokine secretion. Our data suggest an intrinsic proinflammatory role of AQP4 involving AQP4-dependent astrocyte swelling and cytokine release. Reduction in AQP4 water transport may be protective in neuroinflammatory CNS diseases.

Aquaporin-4 water channel expression by thymoma of patients with and without myasthenia gravis

BACKGROUND

Neuromyelitis optica (NMO) is a serious idiopathic inflammatory demyelinating disorder characterized by acute transverse myelitis and optic neuritis. A significant proportion of NMO patients are seropositive for NMO-IgG, an autoantibody targeting aquaporin-4 (AQP4) water channel. Paraneoplastic NMO associated various tumors were recently reported.

AIM

We studied the expression of AQP4 by thymoma from patients with and without myasthenia gravis (MG).

METHODS

Thymoma obtained from thymomectomy in patients with and without MG were studied by immunohistochemistry and western blot.

RESULTS

Ten thymoma patients (9 with MG) and two control patients without thymoma or MG were studied. Immunohistochemistry revealed AQP4 immunoreactivity in cell membrane of thymoma cells from all ten thymoma specimens whereas thymic tissues from patients without thymoma or MG were negative for AQP4 immunoreactivity. Western blot revealed that lysates of nine of the ten thymoma specimens reacted with anti-human AQP4 antibody with a band of ~30 kDa compatible with the molecular weight of AQP4. Interestingly, immunofluorescence revealed that IgG isolated from 2 NMO patients seropositive for NMO-IgG bound to cell membrane of thymoma cells from all ten thymoma specimens while IgG from healthy control subject did not.

CONCLUSION

Thymoma cells of patients with and without MG express AQP4. AQP4 autoantibodies from serum of NMO patients bound to AQP4 expressed on thymoma cell membrane.

AQP1 is not only a water channel: It contributes to cell migration through Lin7/beta-catenin

AQPs are water channel proteins. In particular, AQP1 was demonstrated to be involved in cell migration. According to the model proposed by Verkman and collaborators, AQP drives water influx, facilitating lamellipodia extension and cell migration. Investigating the possible connection between AQP1 and cytoskeleton, our group showed that such a water channel through Lin7/beta-catenin affects the organization of the cytoskeleton and proposed a model. All together, these data appear particularly intriguing since the use of AQP1 as target might be useful to modulate angiogenesis/vasculogenic mimicry.

Expression profile of multiple aquaporins in human gastric carcinoma and its clinical significance

BACKGROUND

New evidence for involvement of aquaporins (AQPs) in cell migration and proliferation adds AQPs to an expanding list of effectors in tumor biology. But there is few report concerning the expression and role of AQPs in human gastric carcinogenesis so far. The aim of this current study was to investigate the expression profile of AQPs in human gastric carcinoma and its significance.

METHODS

We screened the expression profile of AQP0 approximately AQP12 in gastric adenocarcinoma tissues and corresponding normal mucosa from 89 patients with gastric cancer by reverse transcriptase polymerase chain reaction (RT-PCR), Western blot analysis and immunochemical assay. The relationship between AQPs expression and clinicopathologic characteristics of patients was evaluated.

RESULTS

Based on RT-PCR of 13 AQPs examined, AQP1, 3, 4, 5 and 11 were expressed in human gastric cancers or normal gastric tissues, and AQP3, 4 and 5 exhibited differential expression between human gastric carcinomas and corresponding normal tissues, which was confirmed by Western blot analyses. Immunohistochemical assay showed that AQP4 protein was expressed mainly in the membrane of parietal cell and chief cell in the normal gastric mucosa, and absent in carcinoma tissues. AQP3 and AQP5 were detected remarkably stronger in the carcinoma tissues than that in normal mucosa by immunofluorescence. AQP3 expression in cases with undifferentiated tumor was more than that in cases with well-differentiated tumor. Both AQP3 and AQP5 expression were associated with lymph node metastasis and lymphovascular invasion in patients.

CONCLUSIONS

These findings of differential expressions of AQPs and their correlation with clinicopathologic characteristics implicated AQPs might play a role in human gastric carcinogenesis.

Live cell analysis of aquaporin-4 m1/m23 interactions and regulated orthogonal array assembly in glial cells

Aquaporin-4 (AQP4) can assemble into supramolecular aggregates called orthogonal arrays of particles (OAPs). In cells expressing single AQP4 isoforms, we found previously that OAP formation by AQP4-M23 requires N terminus interactions just downstream of Met-23 and that the inability of AQP4-M1 to form OAPs involves blocking by residues upstream of Met-23. Here, we studied M1/M23 interactions and regulated OAP assembly by nanometer-resolution tracking of quantum dot-labeled AQP4 in live cells expressing differentially tagged AQP4 isoforms and in primary glial cell cultures in which native AQP4 was labeled with a monoclonal recombinant neuromyelitis optica autoantibody. OAP assembly was assessed independently by Blue Native gel electrophoresis. We found that OAPs in native glial cells could be reproduced in transfected cells expressing equal amounts of AQP4-M1 and -M23. Mutants of M23 that do not themselves form OAPs, including M23-F26Q and M23-G28P, were able to fully co-associate with native M23 to form large immobile OAPs. Analysis of a palmitoylation-null M1 mutant (C13A/C17A) indicated palmitoylation-dependent OAP assembly only in the presence of M23, with increased M1 palmitoylation causing progressive OAP disruption. Differential regulation of OAP assembly by palmitoylation, calcium elevation, and protein kinase C activation was found in primary glial cell cultures. We conclude that M1 and M23 co-assemble in AQP4 OAPs and that specific signaling events can regulate OAP assembly in glial cells.

Aquaporin-1 tunes pain perception by interaction with Na(v)1.8 Na+ channels in dorsal root ganglion neurons

Aquaporin-1 (AQP1) water channels are expressed in the plasma membrane of dorsal root ganglion (DRG) neurons. We found reduced osmotic water permeability in freshly isolated DRG neurons from AQP1(-/-) versus AQP1(+/+) mice. Behavioral studies showed greatly reduced thermal inflammatory pain perception in AQP1(-/-) mice evoked by bradykinin, prostaglandin E(2), and capsaicin as well as reduced cold pain perception. Patch clamp of freshly isolated DRG neurons showed reduced action potential firing in response to current injections. Single action potentials after pulse current injections showed reduced maximum inward current, suggesting impaired Na(v)1.8 Na(+) function. Whole-cell Na(v)1.8 Na(+) currents in Na(v)1.8-expressing ND7-23 cells showed slowed frequency-dependent inactivation after AQP1 transfection. Immunoprecipitation studies showed AQP1- Na(v)1.8 Na(+) interaction, which was verified in live cells by single-particle tracking of quantum dot-labeled AQP1. Our results implicate the involvement of AQP1 in DRG neurons for the perception of inflammatory thermal pain and cold pain, whose molecular basis is accounted for, in part, by reduced Na(v)1.8-dependent membrane Na(+) current. AQP1 is, thus, a novel target for pain management.

Evidences for a leaky scanning mechanism for the synthesis of the shorter M23 protein isoform of aquaporin-4: implication in orthogonal array formation and neuromyelitis optica antibody interaction

Aquaporin-4 (AQP4) exists as two major isoforms that differ in the length of the N terminus, the shorter AQP4-M23 and the longer AQP4-M1. Both isoforms form tetramers, which can further aggregate in the plasma membrane to form typical orthogonal arrays of particles (OAPs) whose dimension depends on the ratio of the M1 and M23. In this study, we tested the hypothesis that the M23 isoform can be produced directly by the M1 mRNA. In cells transiently transfected with AQP4-M1 coding sequence we observed besides AQP4-M1 the additional presence of the AQP4-M23 isoform associated with the formation of typical OAPs observable by two-dimensional blue native/SDS-PAGE and total internal reflection microscopy. The mutation of the second in-frame methionine M23 in AQP4-M1 (AQP4-M1(M23I)) prevented the expression of the M23 isoform and the formation of OAPs. We propose "leaky scanning" as a translational mechanism for the expression of AQP4-M23 protein isoform and that the formation of OAPs may occur even in the absence of AQP4-M23 mRNA. This mechanism can have important pathophysiological implications for the cell regulation of the M1/M23 ratio and thus OAP size. In this study we also provide evidence that AQP4-M1 is mobile in the plasma membrane, that it is inserted and not excluded into immobile OAPs, and that it is an important determinant of OAP structure and size.

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.