[Regulative effect of electroacupuncture on aquaporin-4 in rats with focal cerebral ischemia/reperfusion]

OBJECTIVE

To study the effect of electroacupuncture (EA) on changes of aquaporin-4 (AQP4) expression in rats with cerebral ischemia/reperfusion(CI/R).

METHODS

One hundred and seventy male Sprauge-Dawley rats were randomized into control (n = 50), CI/R model (n = 60) and EA (n = 60) groups, and the later 2 groups were further divided into 6 h, 12 h, 24 h, 48 h and 72 h subgroups, with 12 cases in each. CI/R model was established by occlusion of the middle cerebral artery (MCAO). EA (3.85/6.25 Hz, 0.8 - 1.0 mA) was applied to "Shuigou" (GV 26) and "Baihui" (GV 20) for 30 min. Transcardial perfusion was conducted with normal saline + paraformaldehyde in rats of different groups for taking brain tissue samples. The degree of brain edema was detected by cresyl violet stain method, permeability of the blood-brain barrier (BBB) reflected with IgG value in brain tissue was displayed by immunohistochemistry, and AQP4 protein and mRNA expression of corpora striata (CS) were detected by using immunohistochemistry (ABC) and RT-PCR techniques respectively.

RESULTS

Twelve hours after CI/R, the hemisphere of brain began to get swelling significantly and the exosmose state of IgG became evidently; compared with the corresponding phases of model group, the swell rates in 24 h, 48 h and 72 h subgroups of EA group were significantly lower (P < 0.05, 0.01), and nearly no IgG immunoreaction positive products were found in different phases of EA group. The expression of AQP4 protein and mRNA increased significantly from 12 h and 24 h on separately in model group, while the expression in EA group was markedly weaker. The ratios of AQP4 mRNA and beta-actin mRNA of CS on the affected side of the brain from 24 h to 72 h of EA group were significantly lower than those of model group (P < 0.05, 0.01).

CONCLUSION

EA can reduce the degree of brain edema induced by CI/R and improve the degree of BBB injury, which may be related to its effect in down-regulating AQP4 expression.

Generation of muscle aquaporin 4 overexpressing transgenic mouse: Its characterization at RNA and protein levels including freeze-fracture study

We generated the muscle aquaporin 4 (AQP4) overexpressing transgenic mice in order to investigate the skeletal muscle pathology at RNA and protein levels. At RNA level, the AQP4 mRNA expression of soleus, EDL and cardiac muscles in Tg mice was statistically significantly higher than that in wild mice by the real-time reverse transcription polymerase chain reaction method. At protein level examinations, we used the immunoblot, immunohistochemistry and freeze-fracture electron microscopy. The immunoblot showed the single band of 31kDa with anti-AQP4 antibody in the extracts of soleus and EDL muscles of wild mice but not in extract of wild cardiac muscle; while the reaction band was noted in cardiac muscle of Tg mice and the reaction band was stronger in the extracts of soleus and EDL muscles of Tg mice. The immunohistochemistry showed that the expression of AQP4 at the myofiber surface of soleus and EDL muscles of Tg mice was more marked than that of wild mice and, interestingly, the AQP4 expression of these muscles of Tg mice appeared to be more remarkable in type 1 slow twitch myofibers as judged by the positive slow myosin immunostaining of adjacent serial sections. The immunofluorescence staining with anti-AQP4 antibody of cardiac muscles of wild mice revealed the scarcely immunopositive myofibers; whereas the immunostaining cardiac muscles of Tg mice contained the numerous AQP4 immunopositive myofibers. The freeze-fracture electron microscopy demonstrated that the orthogonal array densities in soleus and EDL muscle plasma membranes of Tg mice were significantly higher than those of wild mice and that the orthogonal array like particle density of cardiac muscle plasma membranes of Tg mice appeared to be more numerous than that of cardiac myofibers of wild mice. Finally the clinical phenotype of Tg mice appeared to be similar to that of wild mice. Further physiological examination with devices may suggest some about the physiological difference.

Dexamethasone treatment modulates aquaporin-4 expression after intracerebral hemorrhage in rats

This study investigated whether dexamethasone (DEX) treatment could regulate the expression of aquaporin-4 (AQP4) in rats with intracerebral hemorrhage (ICH). The results demonstrated that DEX significantly reduced AQP4 mRNA level in the perihematomal area compared with control group, but it increased the level in the brain area surrounding the third ventricle at day 1 post-ICH. There was no difference in AQP4 protein levels between DEX group and control group at the two above-mentioned brain regions at day 1 after ICH. The changes in AQP4 protein induced by DEX were marked at day 3 following surgery and still lasted at day 5 post-ICH, which were accompanied by a reduction of brain edema. Our results demonstrated that the expression of AQP4 protein after ICH was region-specific, time-dependent, and also indicated that DEX-induced cerebral edema clearance was correlated with the regulation of AQP4 expression in different brain regions.

Aquaporin 4 regulates the effects of ovarian hormones on monoamine neurotransmission

Aquaporin 4 (AQP4) is the predominant water channels in the brain of mammals. Our previous study has reported that AQP4 knockout induced sex-specific alterations in neurotransmission, indicating that AQP4 might regulate the interaction between sex hormones and neurotransmission. In the present study, we found that AQP4 knockout decreased the concentrations of estrogen and progestogen. Further study showed that exogenous estrogen decreased DA and 5-HT in cortex, reduced DA and 5-HT in striatum, but increased 5-HT in hippocampus in AQP4+/+ male mice. However, in AQP4-/- male mice, exogenous estrogen almost did not alter the levels of neurotransmitters except for decreasing DA in cortex. In female mice, ovariectomy decreased DA in the striatum of AQP4+/+ mice, but did not alter the levels of DA in AQP4-/- mice. These findings reveal for the first time that AQP4 regulates not only water and ion homeostasis but also the functions of ovarian hormone and neurotransmitter.

Expression of the water-channel protein aquaporin 4 in the H-Tx rat: possible compensatory role in spontaneously arrested hydrocephalus

OBJECT

Aquaporin (AQP) water channels play an important role in water movement in the central nervous system. The authors used an animal model to examine the relationship between AQP4 expression and spontaneously arrested hydrocephalus.

METHODS

. Congenitally hydrocephalic H-Tx (hH-Tx) rats and nonhydrocephalic H-Tx (nH-Tx) rats were used in the study. Brain tissue sections were obtained from animals in both groups at 1 day, 1 week, 4 weeks, and 8 weeks of age. Sections were immunostained using AQP4 antibodies, and AQP4 expression was assessed. In the nH-Tx group, no AQP4 expression was seen in 1-day-old rats, and AQP4 expression was found in astrocytes around capillaries of the cerebral cortex and in ependymal cells lining the ventricles in 1-week-old rats. In the 4- and 8-week-old nH-Tx animals, AQP4 expression was seen in subpial zones of the cortex, on foot processes of pericapillary astrocytes, and in periventricular regions. A marked increase in cerebral cortical expression of AQP4 was observed at 8 weeks in the hH-Tx rats but not in the nH-Tx rats.

CONCLUSIONS

The authors hypothesize that the differences in cerebral AQP4 expression in the 1-day-old and 1-week-old nH-Tx rats compared with the 4- and 8-week-old nH-Tx rats may be related to the fact that the cerebrospinal fluid (CSF) circulation of newborns and infants differs from that of adults. It is also possible that the increased expression of AQP4 seen in the 8-week-old hH-Tx animals was related to the development of alternative pathways of CSF circulation, which also may occur in instances of spontaneously arrested hydrocephalus.

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.

The roles of aquaporin-4 in brain edema following neonatal hypoxia ischemia and reoxygenation in a cultured rat astrocyte model

Aquaporin-4 (AQP4), a water channel protein, is abundantly expressed in astrocytes and plays a key role in the development of brain edema. However, it is not clear whether AQP4 contributes to astrocytic swelling in hypoxia-ischemia (HI). To investigate the roles of AQP4 in astrocytic swelling during HI and reoxygenation, we measured AQP4 expression and astrocytic cellular volume in cultured rat astrocytes following HI and reoxygenation. RNA interference was used to knockdown AQP4 expression (AQP4(-/-)). Real-time polymerase chain reaction and Western blot analysis were used to detect the inhibitory efficiency of AQP4. We found that the maximal inhibition of AQP4 mRNA and protein in astrocytes after AQP4 siRNA transfection (AQP4(-/-)) was approximately 77 and 85%, respectively, compared to wild-type AQP4 (AQP4(+/+)) expression. Cellular volume in both AQP4(-/-) and AQP4(+/+) astrocytes was significantly increased during HI compared to cells cultured in normoxia (P<0.05). However, cellular volume during HI in AQP4(-/-) astrocytes was significantly less than that in AQP4(+/+) astrocytes (P<0.05). After reoxygenation, the cellular volume gradually decreased to control levels at 7 days in AQP4(-/-) but at 5 days in AQP4(+/+) astrocytes. The different roles of AQP4 during HI and reoxygenation suggest that AQP4 knockdown may protect against water influx in the formation of astrocyte swelling during HI, and may also delay water clearance in the resolution of astrocyte swelling during reoxygenation. In conclusion, AQP4 mediates bidirectional transport of water across astrocytes during HI and reoxygenation. AQP4 manipulation may serve as a novel therapeutic strategy during different periods of hypoxic-ischemic brain edema in neonates.

Expression pattern of the water channel aquaporin-4 in human gliomas is associated with blood–brain barrier disturbance but not with patient survival

Aquaporin-4 (AQP4), the most prominent CNS water channel, is restricted to the glia limitans and astrocytic endfeet. We previously showed the loss of spatial AQP4 expression in glioblastomas and a redistribution across the cell surface. However, opposing AQP4 functions have been described: protective in vasogenic but detrimental in cytotoxic brain edema. Thus, specific AQP4 induction to prevent or reduce vasogenic edema is suggested. To elucidate the AQP4 role in brain tumors, we investigated 189 WHO grade I-IV gliomas by immunohistochemistry and the prognostic significance for patients' survival. In gliomas, a remarkable de novo AQP4 redistribution was observed in comparison with normal CNS tissue. Surprisingly, the highest membraneous staining levels were seen in pilocytic astrocytomas WHO grade I and grade IV glioblastomas, both significantly higher than in WHO grade II astrocytomas. AQP4 up-regulation was associated with brain edema formation; however, no association between survival and WHO grade-dependent AQP4 expression was seen. Hence, AQP4 redistribution may go along with other tumor properties, such as vascular proliferation and resulting blood-brain barrier disturbance, features usually prominent in pilocytic astrocytomas WHO I and glioblastomas WHO grade IV. In summary, our findings question the protective role of AQP4 in vasogenic brain edema. Although AQP4 was associated with brain edema formation, one has to question the suitability of AQP4 induction as a promising approach in vasogenic brain edema prevention and treatment. In addition, our results provide unexpectedly high AQP4 levels in pilocytic astrocytomas and present AQP4 as tumor progression marker in WHO grade II-IV astrocytomas.

Different pattern of aquaporin-4 expression in extensor digitorum longus and soleus during early development

Aquaporin-4 (AQP4) is the neuromuscular water channel expressed at the sarcolemma of mammalian fast-twitch fibers that mediates a high water transport rate, which is important during muscle activity. Clinical interest in the neuromuscular expression of AQP4 has increased as it is associated with the protein complex formed by dystrophin, the product of the gene affected in Duchenne muscular dystrophy. The expression of AQP4 during development has not been characterized. In this study, we analyzed the expression of AQP4 in extensor digitorum longus (EDL) and soleus, a fast- and slow-twitch muscle, respectively, during the first weeks after birth. The results show that AQP4 expression in both types of skeletal muscle occurs postnatally. The time course of expression of AQP4 in the two types of muscles was also different. Whereas the expression of AQP4 protein levels in the EDL showed a progressive increase during the first month after birth, reaching levels found in adults by day 24, the levels of the protein in the soleus showed a transient peak between day 12 and day 24 and declined thereafter, an effect that may be related to the transient high number of fast motor units innervating the soleus muscle during this time. The results suggest that AQP4 expression in skeletal muscle is under neuronal influence and contribute to the understanding of the molecular events of fiber differentiation during development.

Thrombin inhibits aquaporin 4 expression through protein kinase C-dependent pathway in cultured astrocytes

Aquaporin 4 (AQP4) is a key molecule for maintaining water balance in the central nervous system, and its dysfunction might cause brain edema. However, little is known about the regulation of AQP4 expression. Because thrombin has been implicated in brain edema formation, the purpose of this study is to determine whether thrombin affects expression of AQP4 in astrocytes. Here, the effect of thrombin on AQP4 expression in vitro was evaluated using Western blot analysis and RT-PCR. Meanwhile, we investigated whether the effect of thrombin on AQP4 expression was due to protease-activated receptor 1 (PAR-1). In addition, we examined the role of protein kinase C (PKC) in the effect of thrombin on AQP4 expression using Western blot analysis. We found that thrombin did not affect cell viability at concentrations of 0.05, 0.5, 5, or 50 nM but killed astrocytes at concentrations of 500 nM, with approx 72% of astrocytes surviving at 500 nM thrombin. Our data showed that AQP4 protein expression achieved only 28% of controls in 500 nM thrombin treatment, even if astrocytes survived approx 72% of controls at 500 nM thrombin. Thrombin significantly inhibited AQP4 in a time- and dose dependent manner in vitro (p<0.05). Cathepsin-G, a thrombin PAR-1 inhibitor, reversed significantly (p<0.05) the effect of thrombin on AQP4 mRNA and protein expression in astrocytes. We also observed that PKC inhibitor H-7 or prolonged pretreatment with TPA can rapidly increase AQP4 expression (p<0.05). Thrombin might inhibit AQP4 expression in rat astrocytes, and this effect is possibly mediated by the PKC pathway.

Blood–retinal barrier disruption and ultrastructural changes in the hypoxic retina in adult rats: the beneficial effect of melatonin administration

Reactive changes in astrocytes and Müller cells in the retina of adult rats subjected to hypoxia were investigated. Along with this, the integrity of the blood-retinal barrier (BRB) was assessed using fluorescent and electron-dense tracers. In hypoxic rats, mRNA and protein expression of glial fibrillary acidic protein (GFAP) and aquaporin-4 (AQ4) were significantly increased. AQ4 immunoreactive cells were identified as astrocytes and Müller cells by double immunofluorescence labelling. Another alteration in the hypoxic retina was marked reduction in melatonin content compared to controls. In this connection, administration of exogenous melatonin reduced the tissue concentration of vascular endothelial growth factor (VEGF) and nitric oxide (NO); both were elevated in hypoxic rats. A major structural change in the hypoxic retina was swelling of astrocyte and Müller cell processes but this was noticeably attenuated after melatonin administration. Following an intraperitoneal or intravenous injection of rhodamine isothiocyanate (RhIC) or horseradish peroxidase (HRP), leakage of both tracers was observed in the retina in hypoxic rats but not in the controls, indicating that the functional integrity of the BRB is compromised in hypoxia/reoxygenation. It is suggested that enhanced tissue concentration of VEGF and NO production in the hypoxic retina contribute to increased permeability of the retinal blood vessels. The concurrent up-regulation of AQ4, a water-transporting protein, in astrocytes and Müller cells in hypoxia suggests its involvement in oedema formation. Since melatonin effectively reduced the vascular permeability in the retina of hypoxic rats, as evidenced by reduced leakage of RhIC, we suggest that its administration may be of potential benefit in the management of retinal oedema associated with retinal hypoxia.

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

We reported previously that astroglia cultured from aquaporin-4-deficient (AQP4-/-) mice migrate more slowly in vitro than those from wild-type (AQP4+/+) mice (J. Cell Sci. 2005;118, 5691-5698). Here, we investigate the migration of fluorescently labeled AQP4+/+ and AQP4-/- astroglia after implantation into mouse brains in which directional movement was stimulated by a planar stab wound 3 mm away from the axis of the injection needle. Two days after cell injection we determined the location, elongation ratio, and orientation of labeled cells. Migration of AQP4+/+ but not AQP4-/- cells toward the stab was greater than away from the stab. AQP4+/+ astroglia moved on average 1.5 mm toward the stab compared with 0.6 mm for AQP4-/- cells. More than 25% of the migrating AQP4+/+ cells but <3% of AQP4-/- cells appeared elongated (axial ratio>2.5). In transwell assays, AQP4+/+ astroglia migrated faster than AQP4-/- cells in a manner dependent on pore size. At 8 h, approximately 50% of AQP4+/+ cells migrated through 8-microm diameter pores, whereas equivalent migration of AQP4-/- cells was found for 12-microm diameter pores. These results provide in vivo evidence for AQP4-dependent astroglial migration and suggest that modulation of AQP4 expression or function might alter glial scarring.

Specific aquaporins facilitate the diffusion of hydrogen peroxide across membranes

The metabolism of aerobic organisms continuously produces reactive oxygen species. Although potentially toxic, these compounds also function in signaling. One important feature of signaling compounds is their ability to move between different compartments, e.g. to cross membranes. Here we present evidence that aquaporins can channel hydrogen peroxide (H2O2). Twenty-four aquaporins from plants and mammals were screened in five yeast strains differing in sensitivity toward oxidative stress. Expression of human AQP8 and plant Arabidopsis TIP1;1 and TIP1;2 in yeast decreased growth and survival in the presence of H2O2. Further evidence for aquaporin-mediated H2O2 diffusion was obtained by a fluorescence assay with intact yeast cells using an intracellular reactive oxygen species-sensitive fluorescent dye. Application of silver ions (Ag+), which block aquaporin-mediated water diffusion in a fast kinetics swelling assay, also reversed both the aquaporin-dependent growth repression and the H2O2-induced fluorescence. Our results present the first molecular genetic evidence for the diffusion of H2O2 through specific members of the aquaporin family.

Interleukin-1beta induces the expression of aquaporin-4 through a nuclear factor-kappaB pathway in rat astrocytes

Interleukin (IL)-1beta is known to play a role in the formation of brain edema after various types of injury. Aquaporin (AQP)4 is also reported to be involved in the progression of brain edema. We tested the hypothesis that AQP4 is induced in response to IL-1beta. We found that expression of AQP4 mRNA and protein was significantly up-regulated by IL-1beta in cultured rat astrocytes, and that intracerebroventricular administration of IL-1beta increased the expression of AQP4 protein in rat brain. The effects of IL-1beta on induction of AQP4 were concentration and time dependent. The effects of IL-1beta on AQP4 were mediated through IL-1beta receptors because they were abolished by co-incubation with IL-1 receptor antagonist. It appeared that IL-1beta increased the level of AQP4 mRNA without involvement of de novo protein synthesis because cycloheximide, a protein synthesis inhibitor, did not inhibit the effects of IL-1beta. Inhibition of the nuclear factor-kappaB (NF-kappaB) pathway blocked the induction of AQP4 by IL-1beta in a concentration-dependent manner. These findings show that IL-1beta induces expression of AQP4 through a NF-kappaB pathway without involvement of de novo protein synthesis in rat astrocytes.

Increased expression of water channel aquaporin 1 and aquaporin 4 in Creutzfeldt-Jakob disease and in bovine spongiform encephalopathy-infected bovine-PrP transgenic mice

Spongiform change is a cardinal feature in transmissible spongiform encephalopathies, including Creutzfeldt-Jakob disease (CJD) and bovine spongiform encephalopathy (BSE). It is characterized by swelling of the neuronal processes and vacuolization of the neuropil, leading to increased intraneuronal water content. The present study examines, by gel electrophoresis and Western blotting, the expression levels of the water channels aquaporin 1 (AQP1) and aquaporin 4 (AQP4) in the frontal cortex (area 8) homogenates of sporadic CJD cases (six men, four women; seven cases with methionine/methionine at codon 129 and PrP type 1; two cases with valine/valine at codon 129 and PrP type 2, and one case methionine/valine at codon 129 and PrP type 1) compared with age-matched controls, and cases with Alzheimer's disease (AD, stage VI of Braak and Braak) and diffuse Lewy body disease (DLB). AQP1 and AQP4 protein levels were also studied in the cerebral cortex of BSE-infected bovine-PrP transgenic mice (BoPrP-Tg110 mice) examined at 60, 150, 210 and 270 days post-inoculation (dpi) compared with healthy brain-inoculated control mice. Quantitative densitometry of AQP bands normalized for beta-actin was analyzed using Statgraphics plus 5.0 software from ANOVA and LSD statistical tests. Significant increased expression levels of AQP1 (as revealed with two different antibodies) and AQP4 were seen in CJD, but not in advanced AD and DLB cases when compared with controls. Immunohistochemistry revealed that AQP1 and AQP4 were expressed in astrocytes in diseased cases. No modifications in the expression levels of AQP1 and AQP4 were observed in BSE-infected bovine-PrP transgenic mice at 60, 150 and 210 dpi. However, a significant increase in the expression levels of AQP1 and AQP4 was found in mice at 270 dpi, the time corresponding with the appearance of PrP(res) immunoreactivity in Western blots and typical spongiform lesions in the brain. Together, these findings show increased expression of water channels in the brain in human and animal prion diseases. These modifications may have implications in the regulation of water transport in astrocytes and may account for an imbalance in water and ion homeostasis in prion diseases.

Aquaporin-4 deficiency down-regulates glutamate uptake and GLT-1 expression in astrocytes

The role of aquaporin-4 in water transport has been extensively investigated, while little information exists regarding its contribution to astrocytic functions such as the action to glutamatergic transmission. Since aquaporin-4 has been detected widely co-localized with glutamate transporter 1 (GLT-1) and glutamate transporters also present water transport properties, we investigated the regulative role of aquporin-4 on glutamate transporter using primary cultured astrocytes from aquaporin-4 knockout (AQP4(-/-)) mice. It was demonstrated that lack of aquaporin-4 down-regulated astrocytic expression of GLT-1 but not of glutamate/aspartate transporter (GLAST). The result from [(3)H]D,L-glutamate uptake analysis showed a lower uptake capability in AQP4(-/-) astrocytes. Furthermore, MTT and LDH assays indicated less cellular toxicity induced by excessive glutamate in AQP4(-/-) genotype. These findings provide direct evidences for the first time that aquaporin-4 plays an important role in the function of glutamate transporters. And the present study will improve our understanding of aquaporin-4-glutamanergic biology.

Acute and chronic changes in aquaporin 4 expression after spinal cord injury

The effect of spinal cord injury (SCI) on the expression levels and distribution of water channel aquaporin 4 (AQP4) has not been studied. We have found AQP4 in gray and white matter astrocytes in both uninjured and injured rat spinal cords. AQP4 was detected in astrocytic processes that were tightly surrounding neurons and blood vessels, but more robustly in glia limitans externa and interna, which were forming an interface between spinal cord parenchyma and cerebrospinal fluid (CSF). Such spatial distribution of AQP4 suggests a critical role that astrocytes expressing AQP4 play in the transport of water from blood/CSF to spinal cord parenchyma and vice versa. SCI induced biphasic changes in astrocytic AQP4 levels, including its early down-regulation and subsequent persistent up-regulation. However, changes in AQP4 expression did not correlate well with the onset and magnitude of astrocytic activation, when measured as changes in GFAP expression levels. It appears that reactive astrocytes began expressing increased levels of AQP4 after migrating to the wound area (thoracic region) two weeks after SCI, and AQP4 remained significantly elevated for months after SCI. We also showed that increased levels of AQP4 spread away from the lesion site to cervical and lumbar segments, but only in chronically injured spinal cords. Although overall AQP4 expression levels increased in chronically-injured spinal cords, AQP4 immunolabeling in astrocytic processes forming glia limitans externa was decreased, which may indicate impaired water transport through glia limitans externa. Finally, we also showed that SCI-induced changes in AQP4 protein levels correlate, both temporally and spatially, with persistent increases in water content in acutely and chronically injured spinal cords. Although correlative, this finding suggests a possible link between AQP4 and impaired water transport/edema/syringomyelia in contused spinal cords.

Functional down-regulation of volume-regulated anion channels in AQP4 knockdown cultured rat cortical astrocytes

In the brain, the astroglial syncytium is crucially involved in the regulation of water homeostasis. Accumulating evidence indicates that a dysregulation of the astrocytic processes controlling water homeostasis has a pathogenetic role in several brain injuries. Here, we have analysed by RNA interference technology the functional interactions occurring between the most abundant water channel in the brain, aquaporin-4 (AQP4), and the swelling-activated Cl(-) current expressed by cultured rat cortical astrocytes. We show that in primary cultured rat cortical astrocytes transfected with control small interfering RNA (siRNA), hypotonic shock promotes an increase in cellular volume accompanied by augmented membrane conductance mediated by volume-regulated anion channels (VRAC). Conversely, astroglia in which AQP4 was knocked down (AQP4 KD) by transfection with AQP4 siRNA changed their morphology from polygonal to process-bearing, and displayed normal cell swelling but reduced VRAC activity. Pharmacological manipulations of actin cytoskeleton in rat astrocytes, and functional analysis in mouse astroglial cells, which retain their morphology upon knockdown of AQP4, suggest that stellation of AQP4 KD rat cortical astrocytes was not causally linked to reduction of VRAC current. Molecular analysis of possible candidates of swelling-activated Cl(-) current provided evidence that in AQP4 KD astrocytes, there was a down-regulation of chloride channel-2 (CIC-2), which, however, was not involved in VRAC conductance. Inclusion of ATP in the intracellular saline restored VRAC activity upon hypotonicity. Collectively, these results support the view that in cultured astroglial cells, plasma membrane proteins involved in cell volume homeostasis are assembled in a functional platform.

Aquaporin-4 knockout regulated cocaine-induced behavior and neurochemical changes in mice

Aquaporin-4 (AQP4) is the predominant water channel of brain, which mediates transmembrane water movement at the blood-brain barrier and at the brain-cerebrospinal fluid interface. It has been reported that AQP4 deletion results in an increase of amino acid and monoamine levels in some brain regions of mice, suggesting that AQP4 may participate in region-specific alterations in brain amino acid and monoamine metabolism. In the present study, we examined whether AQP4 affects neurotransmission in acute and chronic cocaine exposure mice. For this purpose, both wild-type and AQP4 knockout mice were used with locomotor activity evaluation and microdialysis methods. The results reveal that AQP4 deletion attenuated locomotor activity in acute and repeated cocaine exposure mice, and induced a decrease of extracellular dopamine and glutamate levels in the nucleus accumbens (NAc), a brain region known to be critically involved in the addictive properties of cocaine. Therefore, AQP4 may play a role in regulating extracellular cocaine-induced dopamine and glutamate release in the brain reward center, and in turn AQP4 deletion may attenuate cocaine reinforcement and dependence.

Increased Expression of Renal Aquaporin Water Channels in Spontaneously Hypertensive Rats

AIMS

The present study was aimed to determine whether there exists an altered regulation of aquaporin (AQP) water channels in hypertension.

METHODS

Male spontaneously hypertensive rats (SHR) aged 10-12 weeks were used. Age-matched Wistar-Kyoto (WKY) rats served as control. The abundance of AQP1-4 proteins in the kidney was determined by Western blot analysis. The protein expression and activity of adenylyl cyclase were also determined.

RESULTS

The medullary expression of AQP2 and AQP3 proteins was increased in SHR compared with that in WKY rats. The expression of AQP1 protein was also significantly increased in the inner medulla, while that of AQP4 was not. Immunohistochemistry of AQP2 revealed that principal cells of the collecting duct have strong immunoreactivity, the degree of which was augmented with prominent apical labeling in SHR. The plasma level of arginine vasopressin (AVP) was higher in SHR; the adenylyl cyclase activity stimulated by AVP was augmented, along with increased expression of type VI adenylyl cyclase. The urine was more concentrated with its volume decreased in SHR.

CONCLUSION

The expression of AQP1-3 channels is increased in the kidney, in association with enhanced activity of the AVP/cAMP pathway, in SHR.

Aquaporin 4 changes in rat brain with severe hydrocephalus

Hydrocephalus is characterized by impaired cerebrospinal fluid (CSF) flow with enlargement of the ventricular cavities of the brain and progressive damage to surrounding tissue. Bulk water movement is altered in these brains. We hypothesized that increased expression of aquaporins, which are water-permeable channel proteins, would occur in these brains to facilitate water shifts. We used quantitative (real-time) RT-PCR, Western blotting and immunohistochemistry to evaluate the brain expression of aquaporins (AQP) 1, 4, and 9 mRNA and protein in Sprague-Dawley rats rendered hydrocephalic by injection of kaolin into cistern magna. AQP4 mRNA was significantly up-regulated in parietal cerebrum and hippocampus 4 weeks and 9 months after induction of hydrocephalus (P < 0.05). Although Western blot analysis showed no significant change, there was more intense perivascular AQP4 immunoreactivity in cerebrum of hydrocephalic brains at 3-4 weeks after induction. We did not detect mRNA or protein changes in AQP1 (located in choroid plexus) or AQP9 (located in select neuron populations). Kir4.1, a potassium channel protein linked to water flux, exhibited enhanced immunoreactivity in the cerebral cortex of hydrocephalic rats; the perineuronal distribution was entirely different from that of AQP4. These results suggest that brain AQP4 up-regulation might be a compensatory response to maintain water homeostasis in hydrocephalus.

Effect of electroacupuncture at acupoints of the governor vessel on aquaporin-4 in rat with experimental spinal cord injury

This study is to investigate the effects of electroacupuncture at acupoints of the Governor Vessel (GV) on aquaporin-4 (AQP-4) expression and on functions of the hind limbs in the rat of spinal cord injury. The functions of the hind limbs were detected with BBB scale on the 1d, 3d, 7 d and 21 d after the spinal cord injury, respectively, and AQP-4 expression in the spinal cord was determined with immunohistochemical method and analyzed quantitatively with image analyzer. The results indicated that on the 1 d after the spinal cord injury, increased AQP-4 expression can be seen significantly in both the gray matter and the white matter of the injured spinal cord, and it reached the peaks on the 3d after the spinal cord injury in both the electroacupuncture group and the spinal cord injury group. However, AQP-4 express was significantly decreased in the electroacupuncture group as compared with that in the control group on 7 d, 14 d and 21 d (P < 0.05 or P < 0.01). The decrease of AQP-4 expression almost went with the improvement of the neurological function, which suggested that electroacupuncture at the acupoints of the Governor Vessel can inhibit edema of the spinal cord to alleviate the secondary spinal cord injury by means of decreasing the AQP-4 expression after the spinal cord injury, so as to protect the residual normal spinal cord tissues and promote the rebuilding of nervous tissues.

Increased seizure duration and slowed potassium kinetics in mice lacking aquaporin-4 water channels

The glial water channel aquaporin-4 (AQP4) has been hypothesized to modulate water and potassium fluxes associated with neuronal activity. In this study, we examined the seizure phenotype of AQP4 -/- mice using in vivo electrical stimulation and electroencephalographic (EEG) recording. AQP4 -/- mice were found to have dramatically prolonged stimulation-evoked seizures after hippocampal stimulation compared to wild-type controls (33 +/- 2 s vs. 13 +/- 2 s). In addition, AQP4 -/- mice were found to have a higher seizure threshold (167 +/- 17 microA vs. 114 +/- 10 microA). To assess a potential effect of AQP4 on potassium kinetics, we used in vivo recording with potassium-sensitive microelectrodes after direct cortical stimulation. Although there was no significant difference in baseline or peak [K(+)](o), the rise time to peak [K(+)](o) (t(1/2), 2.3 +/- 0.5 s) as well as the recovery to baseline [K(+)](o) (t(1/2), 15.6 +/- 1.5 s) were slowed in AQP4 -/- mice compared to WT mice (t(1/2), 0.5 +/- 0.1 and 6.6 +/- 0.7 s, respectively). These results implicate AQP4 in the expression and termination of seizure activity and support the hypothesis that AQP4 is coupled to potassium homeostasis in vivo.