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

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.

Water permeability through aquaporin-4 is regulated by protein kinase C and becomes rate-limiting for glioma invasion

Glial-derived tumors, gliomas, are highly invasive cancers that invade normal brain through the extracellular space. To navigate the tortuous extracellular spaces, cells undergo dynamic changes in cell volume, which entails water flux across the membrane through aquaporins (AQPs). Two members of this family, AQP1 and AQP4 are highly expressed in primary brain tumor biopsies and both have a consensus phosphorylation site for protein kinase C (PKC), which is a known regulator of glioma cell invasion. AQP4 colocalizes with PKC to the leading edge of invading processes and clustered with chloride channel (ClC2) and K(+)-Cl(-) cotransporter 1 (KCC1), believed to provide the pathways for Cl(-) and K(+) secretion to accomplish volume changes. Using D54MG glioma cells stably transfected with either AQP1 or AQP4, we show that PKC activity regulates water permeability through phosphorylation of AQP4. Activation of PKC with either phorbol 12-myristate 13-acetate or thrombin enhanced AQP4 phosphorylation, reduced water permeability and significantly decreased cell invasion. Conversely, inhibition of PKC activity with chelerythrine reduced AQP4 phosphorylation, enhanced water permeability and significantly enhanced tumor invasion. PKC regulation of AQP4 was lost after mutational inactivation of the consensus PKC phosphorylation site S180A. Interestingly, AQP1 expressing glioma cells, by contrast, were completely unaffected by changes in PKC activity. To demonstrate a role for AQPs in glioma invasion in vivo, cells selectively expressing AQP1, AQP4 or the mutated S180A-AQP4 were implanted intracranially into SCID mice. AQP4 expressing glioma cells showed significantly reduced invasion compared to AQP1 and S180 expressing tumors as determined by quantitative stereology, consistent with a differential role for AQP1 and AQP4 in this process.

Aquaporins: translating bench research to human disease

There is considerable potential for translating knowledge of aquaporin structure, function and physiology to the clinic. One area is in aquaporin-based diagnostics. The discovery of AQP4 autoantibodies as a marker of the neuromyelitis optica form of multiple sclerosis has allowed precise diagnosis of this disease. Other aquaporin-based diagnostics are possible. Another area is in aquaporin-based genetics. Genetic diseases caused by loss-of-function mutations in aquaporins include nephrogenic diabetes insipidus and cataracts, and functionally significant aquaporin polymorphisms are beginning to be explored. Perhaps of greatest translational potential is aquaporin-based therapeutics. Information largely from aquaporin knockout mice has implicated key roles of aquaporin-facilitated water transport in transepithelial fluid transport (urinary concentrating, gland fluid secretion), water movement into and out of the brain, cell migration (angiogenesis, tumor metastasis, wound healing) and neural function (sensory signaling, seizures). A subset of aquaporins that transport both water and glycerol, the 'aquaglyceroporins', regulate glycerol content in epidermal, fat and other tissues, and are involved in skin hydration, cell proliferation, carcinogenesis and fat metabolism. Aquaporin-based modulator drugs are predicted to be of broad potential utility in the treatment of edematous states, cancer, obesity, wound healing, epilepsy and glaucoma. These exciting possibilities and their associated challenges are reviewed.

Live-cell imaging of aquaporin-4 diffusion and interactions in orthogonal arrays of particles

Orthogonal arrays of particles (OAPs) have been visualized for many years by freeze-fracture electron microscopy. Our laboratory discovered that aquaporin-4 (AQP4) is the protein responsible for OAP formation by demonstrating OAPs in AQP4-transfected cells and absence of OAPs in AQP4 knockout mice. We recently developed live-cell, single-molecule imaging methods to study AQP4 diffusion and interactions in OAPs. The methods include single particle tracking of quantum-dot labeled AQP4, and total internal reflection fluorescence microscopy of green fluorescent protein (GFP) and small fluorophore-labeled AQP4. The full-length (M1) form of AQP4 diffuses freely in membranes and does not form OAPs, whereas the shorter (M23) form of AQP4 forms OAPs and is nearly immobile. Analysis of a series of AQP4 truncations, point mutants and chimeras revealed that OAP formation by AQP4-M23 is stabilized by hydrophobic tetramer-tetramer interactions involving N-terminus residues, and that absence of OAPs in AQP4-M1 results from blocking of this interaction by residues just upstream from Met23. These biophysical methods are being extended to identify the cellular site of AQP4 assembly, AQP4 isoform interactions, OAP size and dynamics, and the determinants of regulated OAP assembly.

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.

Drosophila big brain does not act as a water channel, but mediates cell adhesion

The neurogenic gene Drosophila big brain (bib) has a high sequence homology to aquaporin-4. However, its cellular functions in Drosophila neurogenesis have remained elusive. Here we investigated cell adhesion, and the ion and water permeability of Bib. The adhesive function was examined by a cell aggregation assay using L cells. Bib-transfected L cells formed aggregated clusters, while control-L cells remained as a single cell suspension. Ion permeation was not confirmed in L cells stably expressing Bib. When expressed in COS7 cells, Bib exhibited limited water permeability. This newly found cell adhesive function of Bib may be important for Drosophila neurogenesis.

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.

Aquaporin-1-facilitated keratocyte migration in cell culture and in vivo corneal wound healing models

Aquaporin-1 (AQP1) water channels are expressed in corneal keratocytes, which become activated and migrate following corneal wounding. The purpose of this study was to investigate the role of AQP1 in keratocyte migration. Keratocyte primary cell cultures from wildtype and AQP1-null mice were compared, as well as keratocyte cultures from pig cornea in which AQP1 expression was modulated by RNAi knockdown and adenovirus-mediated overexpression. AQP1 expression was found in a plasma membrane pattern in corneal stromal and cultured keratocytes. Osmotic water permeability, as measured by calcein fluorescence quenching, was AQP1-dependent in cultured keratocytes, as was keratocyte migration following a scratch wound. Keratocyte migration in vivo was compared in wildtype and AQP1 knockout mice by histology and immunofluorescence of corneal sections at different times after partial-thickness corneal stromal debridement. AQP1 expression in keratocytes was increased by 24h after corneal debridement. Wound healing and keratocyte appearance near the wound margin were significantly reduced in AQP1 knockout mice, and the number of neutrophils was increased. These results implicate AQP1 water permeability as a new determinant of keratocyte migration in cornea.

Actin cytoskeleton remodeling governs aquaporin-4 localization in astrocytes

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

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.

AQP4 knockout impairs proliferation, migration and neuronal differentiation of adult neural stem cells

Aquaporin-4 (AQP4), a key molecule for maintaining water and ion homeostasis in the central nervous system, is expressed in adult neural stem cells (ANSCs) as well as astrocytes. However, little is known about the functions of AQP4 in the ANSCs in vitro. Here we show that AQP4 knockout inhibits the proliferation, survival, migration and neuronal differentiation of ANSCs derived from the subventricular zone of adult mice. Flow cytometric cell cycle analysis revealed that AQP4 knockout increased the basal apoptosis and induced a G2-M arrest in ANSCs. Using Fluo-3 Ca2+ imaging, we show that AQP4 knockout alters the spontaneous Ca2+ oscillations by frequency enhancement and amplitude suppression, and suppresses KCl-induced Ca2+ influx. AQP4 knockout downregulated the expression of connexin43 and the L-type voltage-gated Ca2+ channel CaV1.2 subtype in ANSCs. Together, these findings suggest that AQP4 plays a crucial role in regulating the proliferation, migration and differentiation of ANSCs, and this function of AQP4 is probably mediated by its action on intracellular Ca2+ dynamics.

Dissecting the roles of aquaporins in renal pathophysiology using transgenic mice

Transgenic mice lacking renal aquaporins (AQPs), or containing mutated AQPs, have been useful in confirming anticipated AQP functions in renal physiology and in discovering new functions. Mice lacking AQPs 1-4 manifest defects in urinary concentrating ability to different extents. Mechanistic studies have confirmed the involvement of AQP1 in near-isosmolar fluid absorption in the proximal tubule, and in countercurrent multiplication and exchange mechanisms that produce medullary hypertonicity in the antidiuretic kidney. Deletion of AQPs 2-4 impairs urinary concentrating ability by reduction of transcellular water permeability in the collecting duct. Recently created transgenic mouse models of nephrogenic diabetes insipidus produced by AQP2 gene mutation offer exciting possibilities to test new drug therapies. Several unanticipated AQP functions in kidney have been discovered recently that are unrelated to their role in transcellular water transport. There is evidence for involvement of AQP1 in kidney cell migration after renal injury, of AQP7 in renal glycerol clearance, of AQP11 in prevention of renal cystic disease, and possibly of AQP3 in regulation of collecting duct cell proliferation. Future work in renal AQPs will focus on mechanisms responsible for these non-fluid-transporting functions, and on the development of small-molecule AQP inhibitors for use as aquaretic-type diuretics.

Evidence against involvement of aquaporin-4 in cell-cell adhesion

Aquaporin-4 (AQP4) water channels are expressed strongly in glial cells, where they play a role in brain water balance, neuroexcitation, and glial cell migration. Here, we investigated a proposed new role of AQP4 in facilitating cell-cell adhesion. Measurements were made in differentiated primary glial cell cultures from wild-type versus AQP4 knockout mice as well as in null versus AQP4-transfected L-cells, a cell type lacking endogenous adhesion molecules, and in null versus AQP4-transfected Chinese hamster ovary (CHO)-K1 cells and Fisher rat thyroid cells. Using established assays of cell-cell adhesion, we found no significant effect of AQP4 expression on adhesion in each of the cell types. As a positive control, transfection with E-cadherin greatly increased cell-cell adhesion. High-level AQP4 expression also did not affect aggregation of plasma membrane vesicles in a sensitive quasi-elastic light-scattering assay. Further, we found no specific AQP4 binding of a fluorescently labeled oligopeptide containing the putative adhesion sequence in the second extracellular loop of AQP4. These data provide evidence against involvement of AQP4 in cell-cell adhesion.

Aquaporins and cell migration

Aquaporin (AQP) water channels are expressed primarily in cell plasma membranes. In this paper, we review recent evidence that AQPs facilitate cell migration. AQP-dependent cell migration has been found in a variety of cell types in vitro and in mice in vivo. AQP1 deletion reduces endothelial cell migration, limiting tumor angiogenesis and growth. AQP4 deletion slows the migration of reactive astrocytes, impairing glial scarring after brain stab injury. AQP1-expressing tumor cells have enhanced metastatic potential and local infiltration. Impaired cell migration has also been seen in AQP1-deficient proximal tubule epithelial cells, and AQP3-deficient corneal epithelial cells, enterocytes, and skin keratinocytes. The mechanisms by which AQPs enhance cell migration are under investigation. We propose that, as a consequence of actin polymerization/depolymerization and transmembrane ionic fluxes, the cytoplasm adjacent to the leading edge of migrating cells undergoes rapid changes in osmolality. AQPs could thus facilitate osmotic water flow across the plasma membrane in cell protrusions that form during migration. AQP-dependent cell migration has potentially broad implications in angiogenesis, tumor metastasis, wound healing, glial scarring, and other events requiring rapid, directed cell movement. AQP inhibitors may thus have therapeutic potential in modulating these events, such as slowing tumor growth and spread, and reducing glial scarring after injury to allow neuronal regeneration.

Roles of aquaporin-3 in the epidermis

Aquaporin-3 (AQP3) is a membrane transporter of water and glycerol expressed in plasma membranes in the basal layer keratinocytes of epidermis in normal skin. AQP3 expression in human skin is increased in response to skin stress in diseases such as atopic eczema, to various agents such as retinoic acid, and in skin carcinomas. AQP3-knockout mice have reduced stratum corneum water content and elasticity compared with wild-type mice, as well as impaired wound healing and epidermal biosynthesis. Reduced AQP3-dependent glycerol transport in AQP3-deficient epidermis appears to be responsible for these phenotype findings, as evidenced by reduced glycerol content in epidermis and stratum corneum in AQP3-knockout mice, and correction of the phenotype abnormalities by glycerol replacement. Recent data implicate AQP3 as an important determinant in epidermal proliferation and skin tumorigenesis, in which AQP3-knockout mice are resistant to tumor formation by a mechanism that may involve reduced cell glycerol content and ATP energy for biosynthesis. AQP3 is thus a key player in epidermal biology and a potential target for drug development.

Impaired olfaction in mice lacking aquaporin-4 water channels

Aquaporin-4 (AQP4) is a water-selective transport protein expressed in glial cells throughout the central nervous system. AQP4 deletion in mice produces alterations in several neuroexcitation phenomena, including hearing, vision, epilepsy, and cortical spreading depression. Here, we report defective olfaction and electroolfactogram responses in AQP4-null mice. Immunofluorescence indicated strong AQP4 expression in supportive cells of the nasal olfactory epithelium. The olfactory epithelium in AQP4-null mice had identical appearance, but did not express AQP4, and had approximately 12-fold reduced osmotic water permeability. Behavioral analysis showed greatly impaired olfaction in AQP4-null mice, with latency times of 17 +/- 0.7 vs. 55 +/- 5 s in wild-type vs. AQP4-null mice in a buried food pellet test, which was confirmed using an olfactory maze test. Electroolfactogram voltage responses to multiple odorants were reduced in AQP4-null mice, with maximal responses to triethylamine of 0.80 +/- 0.07 vs. 0.28 +/- 0.03 mV. Similar olfaction and electroolfactogram defects were found in outbred (CD1) and inbred (C57/bl6) mouse genetic backgrounds. Our results establish AQP4 as a novel determinant of olfaction, the deficiency of which probably impairs extracellular space K(+) buffering in the olfactory epithelium.

Functions of aquaporins in the eye

The aquaporins (AQPs) are integral membrane proteins whose main function is to transport water across cell membranes in response to osmotic gradients. At the ocular surface, AQP1 is expressed in corneal endothelium, AQP3 and AQP5 in corneal epithelium, and AQP3 in conjunctival epithelium. AQPs are also expressed in lens fiber cells (AQP0), lens epithelium (AQP1), ciliary epithelium (AQP1, AQP4) and retinal Müller cells (AQP4). Mutations in AQP0 produce congenital cataracts in humans. Analysis of knockout mice lacking individual AQPs suggests their involvement in maintenance of corneal and lens transparency, corneal epithelial repair, intraocular pressure (IOP) regulation, retinal signal transduction and retinal swelling following injury. The mouse phenotype findings implicate AQPs as potential drug targets for therapy of elevated IOP and ocular disorders involving the cornea, lens and retina. However, much research remains in defining cell-level mechanisms for the ocular AQP functions, in establishing the relevance to human eye disease of conclusions from knockout mice, and in developing AQP-modulating drugs.

Mammalian aquaporins: diverse physiological roles and potential clinical significance

Aquaporins have multiple distinct roles in mammalian physiology. Phenotype analysis of aquaporin-knockout mice has confirmed the predicted role of aquaporins in osmotically driven transepithelial fluid transport, as occurs in the urinary concentrating mechanism and glandular fluid secretion. Aquaporins also facilitate water movement into and out of the brain in various pathologies such as stroke, tumour, infection and hydrocephalus. A major, unexpected cellular role of aquaporins was revealed by analysis of knockout mice: aquaporins facilitate cell migration, as occurs in angiogenesis, tumour metastasis, wound healing, and glial scar formation. Another unexpected role of aquaporins is in neural function - in sensory signalling and seizure activity. The water-transporting function of aquaporins is likely responsible for these roles. A subset of aquaporins that transport both water and glycerol, the 'aquaglyceroporins', regulate glycerol content in epidermal, fat and other tissues. Mice lacking various aquaglyceroporins have several interesting phenotypes, including dry skin, resistance to skin carcinogenesis, impaired cell proliferation, and altered fat metabolism. The various roles of aquaporins might be exploited clinically by development of drugs to alter aquaporin expression or function, which could serve as diuretics, and in the treatment of brain swelling, glaucoma, epilepsy, obesity and cancer.

Mechanisms of disease: aquaporin-4 antibodies in neuromyelitis optica

Neuromyelitis optica (NMO) is a rare CNS inflammatory disorder that predominantly affects the optic nerves and spinal cord. Recent serological findings strongly suggest that NMO is a distinct disease rather than a subtype of multiple sclerosis. In NMO, serum antibodies, collectively known as NMO-IgG, characteristically bind to cerebral microvessels, pia mater and Virchow-Robin spaces. The main target antigen for this immunoreactivity has been identified as aquaporin-4 (AQP4). The antibodies are highly specific for NMO, and they are also found in patients with longitudinally extensive transverse myelitis without optic neuritis, which is thought to be a precursor to NMO in some cases. An antibody-mediated pathogenesis for NMO is supported by several observations, including the characteristics of the AQP4 antibodies, the distinct NMO pathology--which includes IgG and complement deposition and loss of AQP4 from spinal cord lesions--and emerging evidence of the beneficial effects of B-cell depletion and plasma exchange. Many aspects of the pathogenesis, however, remain unclear.

Aquaporins--new players in cancer biology

The aquaporins (AQPs) are small, integral-membrane proteins that selectively transport water across cell plasma membranes. A subset of AQPs, the aquaglyceroporins, also transport glycerol. AQPs are strongly expressed in tumor cells of different origins, particularly aggressive tumors. Recent discoveries of AQP involvement in cell migration and proliferation suggest that AQPs play key roles in tumor biology. AQP1 is ubiquitously expressed in tumor vascular endothelium, and AQP1-null mice show defective tumor angiogenesis resulting from impaired endothelial cell migration. AQP-expressing cancer cells show enhanced migration in vitro and greater local tumor invasion, tumor cell extravasation, and metastases in vivo. AQP-dependent cell migration may involve AQP-facilitated water influx into lamellipodia at the front edge of migrating cells. The aquaglyceroporin AQP3, which is found in normal epidermis and becomes upregulated in basal cell carcinoma, facilitates cell proliferation in different cell types. Remarkably, AQP3-null mice are resistant to skin tumorigenesis by a mechanism that may involve reduced tumor cell glycerol metabolism and ATP generation. Together, the data suggest that AQP expression in tumor cells and tumor vessels facilitates tumor growth and spread, suggesting AQP inhibition as a novel antitumor therapy.

[Neuromyelitis optica]

Neuromyelitis optica (NMO, Devic syndrome) is a rare demyelinating disease of the central nervous system which mostly follows a relapsing course. Key features of this disorder include unilateral or bilateral optic neuritis and longitudinally extensive myelitis (> or = three segments). Brain lesions are rarely present at onset. They may however evolve during the course of disease but usually remain asymptomatic. The histopathology of NMO is suggestive of an underlying humoral autoimmune pathomechanism and indicates that NMO is a distinct entity rather than a variant of multiple sclerosis. The recent detection of NMO-specific serum autoantibodies against the water channel aquaporin-4 (Aqp4) is of significant diagnostic relevance and classifies NMO as the first inflammatory demyelinating disorder of the CNS with a defined autoantigen. More recent therapeutic strategies such as plasma exchange or pharmacological B-cell depletion are expected to improve long-term prognosis of NMO.

Water movements in the brain: role of aquaporins

About 80% of the brain is water. This review discusses the importance of the three brain water-channel proteins (AQP1, AQP4, AQP9) in brain physiology. AQP1 is expressed in the choroid plexus and participates in forming cerebrospinal fluid. AQP4, found in astrocyte foot processes, glia limitans and ependyma, facilitates water movement into and out of the brain, accelerates astrocyte migration and alters neuronal activity. Recently, AQP4 autoantibodies were discovered in patients with neuromyelitis optica, a demyelinating disease, and are now being used to diagnose this condition. AQP9 is present in some glia and neurons, but its function is unclear. Finally, we discuss how the discovery of AQP activators and inhibitors will be the next major step in this field.

Chronic erythropoietin-mediated effects on the expression of astrocyte markers in a rat model of contusive spinal cord injury

Using a standardized rat model of contusive spinal cord injury (SCI; [Gorio A, Gokmen N, Erbayraktar S, Yilmaz O, Madaschi L, Cichetti C, Di Giulio AM, Vardar E, Cerami A, Brines M (2002) Recombinant human erythropoietin counteracts secondary injury and markedly enhances neurological recovery from experimental spinal cord trauma. Proc Natl Acad Sci U S A 99:9450-9455]), we previously showed that the administration of recombinant human erythropoietin (rhEPO) improves both tissue sparing and locomotory outcome. In the present study, to better understand rhEPO-mediated effects on chronic astrocyte response to SCI in rat, we have used immunocytochemical methods combined with confocal and electron microscopy to investigate, 1 month after injury, the effects of a single rhEPO administration on the expression of a) aquaporin 4 (AQP4), the main astrocytic water channel implicated in edema development and resolution, and two molecules (dystrophin and syntrophin) involved in its membrane anchoring; b) glial fibrillary acidic protein (GFAP) and vimentin as markers of astrogliosis; c) chondroitin sulfate proteoglycans of the extracellular matrix which are upregulated after SCI and can inhibit axonal regeneration and influence neuronal and glial properties. Our results show that rhEPO administration after SCI modifies astrocytic response to injury by increasing AQP4 immunoreactivity in the spinal cord, but not in the brain, without apparent modifications of dystrophin and syntrophin distribution. Attenuation of astrogliosis, demonstrated by the semiquantitative analysis of GFAP labeling, was associated with a reduction of phosphacan/RPTP zeta/beta, whereas the levels of lecticans remained unchanged. Finally, the relative volume of a microvessel fraction was significantly increased, indicating a pro-angiogenetic or a vasodilatory effect of rhEPO. These changes were consistently associated with remarkable reduction of lesion size and with improvement in tissue preservation and locomotor recovery, confirming previous observations and underscoring the potentiality of rhEPO for the therapeutic management of SCI.

Cytotoxic edema: mechanisms of pathological cell swelling

✓Cerebral edema is caused by a variety of pathological conditions that affect the brain. It is associated with two separate pathophysiological processes with distinct molecular and physiological antecedents: those related to cytotoxic (cellular) edema of neurons and astrocytes, and those related to transcapillary flux of Na+and other ions, water, and serum macromolecules. In this review, the authors focus exclusively on the first of these two processes. Cytotoxic edema results from unchecked or uncompensated influx of cations, mainly Na+, through cation channels. The authors review the different cation channels that have been implicated in the formation of cytotoxic edema of astrocytes and neurons in different pathological states. A better understanding of these molecular mechanisms holds the promise of improved treatments of cerebral edema and of the secondary injury produced by this pathological process.

Aquaporin-4 and brain edema

Aquaporin-4 (AQP4) is a water-channel protein expressed strongly in the brain, predominantly in astrocyte foot processes at the borders between the brain parenchyma and major fluid compartments, including cerebrospinal fluid (CSF) and blood. This distribution suggests that AQP4 controls water fluxes into and out of the brain parenchyma. Experiments using AQP4-null mice provide strong evidence for AQP4 involvement in cerebral water balance. AQP4-null mice are protected from cellular (cytotoxic) brain edema produced by water intoxication, brain ischemia, or meningitis. However, AQP4 deletion aggravates vasogenic (fluid leak) brain edema produced by tumor, cortical freeze, intraparenchymal fluid infusion, or brain abscess. In cytotoxic edema, AQP4 deletion slows the rate of water entry into brain, whereas in vasogenic edema, AQP4 deletion reduces the rate of water outflow from brain parenchyma. AQP4 deletion also worsens obstructive hydrocephalus. Recently, AQP4 was also found to play a major role in processes unrelated to brain edema, including astrocyte migration and neuronal excitability. These findings suggest that modulation of AQP4 expression or function may be beneficial in several cerebral disorders, including hyponatremic brain edema, hydrocephalus, stroke, tumor, infection, epilepsy, and traumatic brain injury.