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Abstract
A major role for Müller cells in the retina is to buffer changes in the extracellular K+ concentration ([K+]o) resulting from light-evoked neuronal activity. The primary K+ conductance in Müller cells is the inwardly rectifying K+ channel Kir4.1. Since this channel is constitutively active, K+ can enter or exit Müller cells depending on the state of the [K+]o. This process of [K+]o buffering by Müller cells ("K+ siphoning") is enhanced by the precise accumulation of these K+ channels at discrete subdomains of Müller cell membranes. Specifically, Kir4.1 is localized to the perivascular processes of Müller cells in animals with vascular retinas and to the endfeet of Müller cells in all species examined. The water channel aquaporin-4 (AQP4) also appears to be important for [K+]o buffering and is expressed in Müller cells in a very similar subcellular distribution pattern to that of Kir4.1. To gain a better understanding of how Müller cells selectively target K+ and water channels to discrete membrane subdomains, we addressed the question of whether Kir4.1 and AQP4 associate with the dystrophin-glycoprotein complex (DGC) in the mammalian retina. Immunoprecipitation (IP) experiments were utilized to show that Kir4.1 and AQP4 are associated with DGC proteins in rat retina. Furthermore, AQP4 was also shown to co-precipitate with Kir4.1, suggesting that both channels are tethered together by the DGC in Müller cells. This work further defines a subcellular localization mechanism in Müller cells that facilitates [K+]o buffering in the retina.
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Affiliation(s)
- Nathan C Connors
- Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania, USA
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252
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Warskulat U, Borsch E, Reinehr R, Heller-Stilb B, Mönnighoff I, Buchczyk D, Donner M, Flögel U, Kappert G, Soboll S, Beer S, Pfeffer K, Marschall HU, Gabrielsen M, Amiry-Moghaddam M, Ottersen OP, Dienes HP, Häussinger D. Chronic liver disease is triggered by taurine transporter knockout in the mouse. FASEB J 2006; 20:574-6. [PMID: 16421246 DOI: 10.1096/fj.05-5016fje] [Citation(s) in RCA: 95] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Taurine is an abundant organic osmolyte with antioxidant and immunomodulatory properties. Its role in the pathogenesis of chronic liver disease is unknown. The liver phenotype was studied in taurine transporter knockout (taut-/-) mice. Hepatic taurine levels were ~21, 15 and 6 mumol/g liver wet weight in adult wild-type, heterozygous (taut+/-) and homozygous (taut-/-) mice, respectively. Immunoelectronmicroscopy revealed an almost complete depletion of taurine in Kupffer and sinusoidal endothelial cells, but not in parenchymal cells of (taut-/-) mice. Compared with wild-type mice, (taut-/-) and (taut+/-) mice developed moderate unspecific hepatitis and liver fibrosis with increased frequency of neoplastic lesions beyond 1 year of age. Liver disease in (taut-/-) mice was characterized by hepatocyte apoptosis, activation of the CD95 system, elevated plasma TNF-alpha levels, hepatic stellate cell and oval cell proliferation, and severe mitochondrial abnormalities in liver parenchymal cells. Mitochondrial dysfunction was suggested by a significantly lower respiratory control ratio in isolated mitochondria from (taut-/-) mice. Taut knockout had no effect on taurine-conjugated bile acids in bile; however, the relative amount of cholate-conjugates acid was decreased at the expense of 7-keto-cholate-conjugates. In conclusion, taurine deficiency due to defective taurine transport triggers chronic liver disease, which may involve mitochondrial dysfunction.
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Affiliation(s)
- Ulrich Warskulat
- Clinic for Gastroenterology, Hepatology, and Infectiology, Heinrich Heine University, Düsseldorf, Germany
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253
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Bragg AD, Amiry-Moghaddam M, Ottersen OP, Adams ME, Froehner SC. Assembly of a perivascular astrocyte protein scaffold at the mammalian blood–brain barrier is dependent on α-syntrophin. Glia 2006; 53:879-90. [PMID: 16609960 DOI: 10.1002/glia.20347] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
alpha-Syntrophin, a member of the dystrophin-associated protein complex, is required for proper localization of the water channel aquaporin-4 at the blood-brain barrier. Mice lacking alpha-syntrophin have reduced levels of aquaporin-4 in perivascular astroglial endfeet. Consequently, they exhibit reduced edema and infarct volume in brain trauma models and reduced K+ clearance from the neuropil, leading to increased seizure susceptibility. We have used the alpha-syntrophin null mice to investigate whether alpha-syntrophin is required for proper localization of other components of the dystrophin complex at the blood-brain barrier. We find that alpha-syntrophin is required for the full recruitment of gamma2-syntrophin and alpha-dystrobrevin-2 to glial endfeet in adult cerebellum. In contrast, the localization of beta1- and beta2-syntrophin and alpha-dystrobrevin-1 at the blood-brain barrier is not dependent on the presence of alpha-syntrophin. The localization patterns of alpha-dystrobrevin-1 and -2 in wild type cerebellum are strikingly different; while alpha-dystrobrevin-1 is present in glial endfeet throughout the cerebellum, alpha-dystrobrevin-2 is restricted to glial endfeet in the granular layer alone. Finally, we show that the enrichment of dystrophin in glial endfeet depends on the presence of alpha-syntrophin. This finding is the first demonstration that dystrophin localization is dependent on syntrophin. Since the localization of gamma2-syntrophin, alpha-dystrobrevin-2, and dystrophin is contingent on alpha-syntrophin, we conclude that alpha-syntrophin is a central organizer of the astrocyte dystrophin complex, an important molecular scaffold for localization of aquaporin-4 at the blood-brain barrier.
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Affiliation(s)
- April D Bragg
- Department of Physiology and Biophysics, Health Sciences Building, Rm G424, 1959 NE Pacific St, University of Washington, Seattle, 98195, USA.
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254
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Binder DK, Yao X, Verkman AS, Manley GT. Increased seizure duration in mice lacking aquaporin-4 water channels. ACTA NEUROCHIRURGICA. SUPPLEMENT 2006; 96:389-92. [PMID: 16671491 DOI: 10.1007/3-211-30714-1_80] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Aquaporins are intrinsic membrane proteins involved in water transport in fluid-transporting tissues. In the brain, aquaporin-4 (AQP4) is expressed widely by glial cells, but its function is unclear. Extensive basic and clinical studies indicate that osmolarity affects seizure susceptibility, and in our previous studies we found that AQP4 -/- mice have an elevated seizure threshold in response to the chemoconvulsant pentylenetetrazol. In this study, we examined the seizure phenotype of AQP4 -/- mice in greater detail using in vivo electroencephalographic recording. AQP4 -/- mice were found to have dramatically longer stimulation-evoked seizures following hippocampal stimulation as well as a higher seizure threshold. These results implicate AQP4 in water and potassium regulation associated with neuronal activity and seizures.
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Affiliation(s)
- D K Binder
- Department of Neurological Surgery, University of California, San Francisco, CA 94110, USA
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255
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Kucheryavykh YV, Kucheryavykh LY, Nichols CG, Maldonado HM, Baksi K, Reichenbach A, Skatchkov SN, Eaton MJ. Downregulation of Kir4.1 inward rectifying potassium channel subunits by RNAi impairs potassium transfer and glutamate uptake by cultured cortical astrocytes. Glia 2006; 55:274-81. [PMID: 17091490 DOI: 10.1002/glia.20455] [Citation(s) in RCA: 182] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Glial cell-mediated potassium and glutamate homeostases play important roles in the regulation of neuronal excitability. Diminished potassium and glutamate buffering capabilities of astrocytes result in hyperexcitability of neurons and abnormal synaptic transmission. The role of the different K+ channels in maintaining the membrane potential and buffering capabilities of cortical astrocytes has not yet been definitively determined due to the lack of specific K+ channel blockers. The purpose of the present study was to assess the role of the inward-rectifying K+ channel subunit Kir4.1 on potassium fluxes, glutamate uptake and membrane potential in cultured rat cortical astrocytes using RNAi, whole-cell patch clamp and a colorimetric assay. The membrane potentials of control cortical astrocytes had a bimodal distribution with peaks at -68 and -41 mV. This distribution became unimodal after knockdown of Kir4.1, with the mean membrane potential being shifted in the depolarizing direction (peak at -45 mV). The ability of Kir4.1-suppressed cells to mediate transmembrane potassium flow, as measured by the current response to voltage ramps or sequential application of different extracellular [K+], was dramatically impaired. In addition, glutamate uptake was inhibited by knock-down of Kir4.1-containing channels by RNA interference as well as by blockade of Kir channels with barium (100 microM). Together, these data indicate that Kir4.1 channels are primarily responsible for significant hyperpolarization of cortical astrocytes and are likely to play a major role in potassium buffering. Significant inhibition of glutamate clearance in astrocytes with knock-down of Kir4.1 highlights the role of membrane hyperpolarization in this process.
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Affiliation(s)
- Y V Kucheryavykh
- Department of Biochemistry, Universidad Central del Caribe, Bayamón, Puerto Rico
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256
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Puwarawuttipanit W, Bragg AD, Frydenlund DS, Mylonakou MN, Nagelhus EA, Peters MF, Kotchabhakdi N, Adams ME, Froehner SC, Haug FM, Ottersen OP, Amiry-Moghaddam M. Differential effect of alpha-syntrophin knockout on aquaporin-4 and Kir4.1 expression in retinal macroglial cells in mice. Neuroscience 2005; 137:165-75. [PMID: 16257493 DOI: 10.1016/j.neuroscience.2005.08.051] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2005] [Revised: 07/04/2005] [Accepted: 08/07/2005] [Indexed: 11/20/2022]
Abstract
Aquaporin-4 water channels and the inwardly rectifying potassium channels Kir4.1 are coexpressed in a highly polarized manner at the perivascular and subvitreal endfeet of retinal Müller cells and astrocytes. The present study was aimed at resolving the anchoring mechanisms responsible for the coexpression of these molecules. Both aquaporin-4 and Kir4.1 contain PDZ-domain binding motifs at their C-termini and it was recently shown that mice with targeted disruption of the dystrophin gene display altered distribution of aquaporin-4 and Kir4.1 in the retina. To test our hypothesis that alpha-syntrophin (a PDZ-domain containing protein of the dystrophin associated protein complex) is involved in aquaporin-4 and Kir4.1 anchoring in retinal cells, we studied the expression pattern of these molecules in alpha-syntrophin null mice. Judged by quantitative immunogold cytochemistry, deletion of the alpha-syntrophin gene causes a partial loss (by 70%) of aquaporin-4 labeling at astrocyte and Müller cell endfeet but no decrease in Kir4.1 labeling at these sites. These findings suggest that alpha-syntrophin is not involved in the anchoring of Kir4.1 and only partly responsible for the anchoring of aquaporin-4 in retinal endfeet membranes. Furthermore we show that wild type and alpha-syntrophin null mice exhibit strong beta1 syntrophin labeling at perivascular and subvitreal Müller cell endfeet, raising the possibility that beta1 syntrophin might be involved in the anchoring of Kir4.1 and the alpha-syntrophin independent pool of aquaporin-4.
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Affiliation(s)
- W Puwarawuttipanit
- Centre for Molecular Biology and Neuroscience, and Nordic Centre for Water Imbalance Related Disorders, University of Oslo, P.O. Box 1105 Blindern, N-0317 Oslo, Norway
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257
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Padmawar P, Yao X, Bloch O, Manley GT, Verkman AS. K+ waves in brain cortex visualized using a long-wavelength K+-sensing fluorescent indicator. Nat Methods 2005; 2:825-7. [PMID: 16278651 DOI: 10.1038/nmeth801] [Citation(s) in RCA: 189] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2005] [Accepted: 09/08/2005] [Indexed: 11/08/2022]
Abstract
We synthesized a water-soluble, long-wavelength K(+) sensor, TAC-Red, consisting of triazacryptand coupled to 3,6-bis(dimethylamino)xanthylium, whose fluorescence increased 14-fold at 0-50 mM K(+) with K(+)-to-Na(+) selectivity >30. We visualized K(+) waves in TAC-Red-stained brain cortex in mice during spreading depression, with velocity 4.4 +/- 0.5 mm/min, and K(+) release and reuptake half-times (t(1/2)) of 12 +/- 2 and 32 +/- 4 s, respectively. Aquaporin-4 (AQP4) deletion slowed K(+) reuptake about twofold, suggesting AQP4-dependent K(+) uptake by astroglia.
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Affiliation(s)
- Prashant Padmawar
- Department of Medicine, Cardiovascular Research Institute, 1246 Health Sciences East Tower, University of California, San Francisco, California 94143, USA
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258
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Gunnarson E, Axehult G, Baturina G, Zelenin S, Zelenina M, Aperia A. Lead induces increased water permeability in astrocytes expressing aquaporin 4. Neuroscience 2005; 136:105-14. [PMID: 16203098 DOI: 10.1016/j.neuroscience.2005.07.027] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2005] [Revised: 07/01/2005] [Accepted: 07/06/2005] [Indexed: 12/25/2022]
Abstract
The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes. There is now compelling evidence that AQP4 may contribute to an unfavorable course in brain edema. Acute lead intoxication is a condition that causes brain damage preceded by brain edema. Here we report that lead increases AQP4 water permeability (P(f)) in astrocytes. A rat astrocyte cell line that does not express aquaporin 4 was transiently transfected with aquaporin 4 tagged with green fluorescent protein (GFP). Using confocal laser scanning microscopy we measured water permeability in these cells and in AQP4-negative cells located on the same plate. AQP4-expressing astrocytes had a three-fold higher water permeability than astrocytes not expressing AQP4. Lead exposure induced a significant, 40%, increase in water permeability in astrocytes expressing AQP4, but had no effect on P(f) in astrocytes not expressing AQP4. The increase in water permeability persisted after lead washout, while treatment with a lead chelator, meso-2,3-dimercaptosuccinic acid, abolished the lead-induced increase in P(f). The effect of lead was attenuated in the presence of a calcium (Ca(2+))/calmodulin-dependent protein kinase II (CaMKII) inhibitor, but not in the presence of a protein kinase C inhibitor. In cells expressing AQP4 where the consensus site for CaMKII phosphorylation was mutated, lead failed to increase water permeability. Lead exposure also increased P(f) in rat astroglial cells in primary culture, which express endogenous AQP4. Lead had no effect on P(f) in astrocytes transfected with aquaporin 3. In situ hybridization studies on rat brain after oral lead intake for three days showed no change in distribution of AQP4 mRNA. It is suggested that lead-triggered stimulation of water transport in AQP4-expressing astrocytes may contribute to the pathology of acute lead intoxication.
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Affiliation(s)
- E Gunnarson
- Nordic Centre for Water Imbalance Related Disorders, Department of Woman and Child Health, Karolinska Institutet, Pediatric Unit, Research Laboratory, Q2:09 Astrid Lindgren Children's Hospital, 171 76 Stockholm, Sweden.
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259
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Bringmann A, Uckermann O, Pannicke T, Iandiev I, Reichenbach A, Wiedemann P. Neuronal versus glial cell swelling in the ischaemic retina. ACTA ACUST UNITED AC 2005; 83:528-38. [PMID: 16187988 DOI: 10.1111/j.1600-0420.2005.00565.x] [Citation(s) in RCA: 85] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Under normal conditions, the pigment epithelium dehydrates the outer retina while Müller glial cells mediate the rapid water transport within the inner retina. Gliotic alterations of Müller cells may be implicated in the development of oedema in the post-ischaemic retina. Here, we suggest a mechanism of Müller cell-supported neuronal cell swelling and apoptosis in the ischaemic retina. During ischaemia, over-excitation of ionotropic glutamate receptors leads to neuronal cell depolarization that causes excess Ca(2+) influx into the cells, and to activation of the apoptosis machinery. The ion fluxes into the retinal neurons are associated with water movements that are mediated by aquaporin-4 water channels expressed by Müller cells and result in neuronal cell swelling. After reperfusion, the glial cells may swell due to the down-regulation of their K(+) conductance, which results in intracellular K(+) overload and water movements from the blood and vitreous into the cells. An inhibition of the glial cell-mediated water movements during ischaemic episodes should reduce the ion shifts at the neuronal synapses, resulting in decreased neuronal cell swelling and apoptosis. An inhibition of the water movements in the post-ischaemic phase may prevent cytotoxic Müller cell swelling but may impair the fluid clearance from retinal tissue in the presence of vasogenic oedema. Thus, pharmacological modification of the ion and fluid clearance functions of Müller cells may become a novel way to resolve both cytotoxic and vasogenic oedema in the retina.
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Affiliation(s)
- Andreas Bringmann
- Department of Ophthalmology and Eye Clinic, Medical Faculty, University of Leipzig, Leipzig, Germany.
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260
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261
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Myslobodsky M. Phobic memory and somatic vulnerabilities in anorexia nervosa: a necessary unity? Ann Gen Psychiatry 2005; 4:15. [PMID: 16144551 PMCID: PMC1260012 DOI: 10.1186/1744-859x-4-15] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/26/2005] [Accepted: 09/06/2005] [Indexed: 11/10/2022] Open
Abstract
Anorexia nervosa is a clinically significant illness that may be associated with permanent medical complications involving almost every organ system. The paper raises a question whether some of them are associated with premorbid vulnerability such as subcellular ion channel abnormalities ('channelopathy') that determines the clinical expression of the bodily response to self-imposed malnutrition. Aberrant channels emerge as a tempting, if rather speculative alternative to the notion of cognitively-driven neurotransmitter modulation deficit in anorexia nervosa. The concept of channelopathies is in keeping with some characteristics of anorexia nervosa, such as a genetically-based predisposition to hypophagia, early onset, cardiac abnormalities, an appetite-enhancing efficacy of some antiepileptic drugs, and others. The purpose of this article is to stimulate further basic research of ion channel biophysics in relation to restrictive anorexia.
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262
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Aoki-Yoshino K, Uchihara T, Duyckaerts C, Nakamura A, Hauw JJ, Wakayama Y. Enhanced expression of aquaporin 4 in human brain with inflammatory diseases. Acta Neuropathol 2005; 110:281-8. [PMID: 16133546 DOI: 10.1007/s00401-005-1052-2] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2005] [Revised: 05/16/2005] [Accepted: 05/20/2005] [Indexed: 12/01/2022]
Abstract
Aquaporin 4 (AQP4), one of the water channel proteins on the plasma membrane of astrocytes, is up-regulated in various conditions with brain edema. Possible participation of AQP4 in various inflammatory lesions, more or less associated with edema, was examined in human autopsied brains. Immunohistochemistry was used to investigate AQP4 expression in autopsied brains with multiple sclerosis (MS), human immunodeficiency virus encephalitis (HIVE) or progressive multifocal leukoencephalopathy (PML). The cellular localization of AQP4 and its relation to inflammatory lesions were then examined with double-labeling immunohistochemistry. AQP4 immunoreactivity (IR) was restricted to astrocytes and localized to their entire processes, including their endfeet facing the abluminal surface of capillaries. In MS brains, AQP4-positive astrocytes were more abundant at the periphery of plaques than in their center, as seen in ischemic foci. Quantification of fluorescent signal demonstrated that AQP4 IR was greatly increased around plaques relative to that in unaffected area. Although the white matter was severely involved in HIVE and PML, AQP4-positive astrocytes were rare in the white matter even around perivascular active inflammatory foci. They were abundant in the gray matter and most prominent in the boundary between the gray and white matter, without apparent relation to inflammatory foci. Some bizarre astrocytes in PML exhibited AQP4 IR. Up-regulation of AQP4 was consistently found in astrocytes in various inflammatory lesions. However, the distribution of AQP4-positve astrocytes differed markedly according to disease and was not necessarily related to brain edema, indicating that functions and regulation of AQP4 in human brains are multiple.
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Affiliation(s)
- Kazuko Aoki-Yoshino
- Department of Neuropathology, Tokyo Metropolitan Institute for Neuroscience, 2-6 Musashi-dai, Fuchu, 183-8526 Tokyo, Japan
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263
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Vitellaro-Zuccarello L, Mazzetti S, Bosisio P, Monti C, De Biasi S. Distribution of Aquaporin 4 in rodent spinal cord: relationship with astrocyte markers and chondroitin sulfate proteoglycans. Glia 2005; 51:148-59. [PMID: 15789430 DOI: 10.1002/glia.20196] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Water balance between cells and extracellular compartments is essential for proper functioning of the central nervous system, as demonstrated by its perturbations in pathological conditions. Aquaporin 4 (AQP4) is the predominant water channel in brain and spinal cord, where it is present mainly on astrocytic endfeet contacting vessels. A role in water homeostasis control has been proposed also for the extracellular matrix, that in brain consists mainly of chondroitin sulfate proteoglycans (CSPGs). Using cytochemical and immunocytochemical techniques, we investigated their distribution in rodent spinal cord, to better understand the role of these two classes of molecules. The results show that in spinal gray matter AQP4 labeling is intense in all perivascular profiles and (1) displays a marked dorsoventral gradient in the neuropil; and (2) coexists extensively with glial glutamate transporter-1 (GLT-1) but scarcely with glial fibrillary acidic protein (GFAP). In white matter the overlap between AQP4, GLT-1, and GFAP is almost complete. Ultrastructural examination shows that AQP4-labeled astrocytic processes surround blood vessels, neuronal perikarya and processes, and both asymmetric and symmetric synapses, indicating that the protein may be involved in the regulation of water fluxes around both inhibitory and excitatory synapses. CSPGs, visualized by labeling with Wisteria floribunda agglutinin, show a distribution complementary to that of AQP4, being absent or weekly expressed in AQP4-enriched areas. These findings suggest that different mechanisms may contribute to the regulation of water homeostasis in different spinal cord regions.
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264
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Nicchia GP, Srinivas M, Li W, Brosnan CF, Frigeri A, Spray DC. New possible roles for aquaporin-4 in astrocytes: cell cytoskeleton and functional relationship with connexin43. FASEB J 2005; 19:1674-6. [PMID: 16103109 DOI: 10.1096/fj.04-3281fje] [Citation(s) in RCA: 129] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Aquaporin-4 (AQP4), the main water channel in the brain, is expressed in the perivascular membranes of mouse, rat, and human astrocytes. In a previous study, we used small interfering RNA (siRNA) to specifically knock down AQP4 in rat astrocyte primary cultures and found that together with reduced osmotic permeability, AQP4 knockdown (KD) led to altered cell morphology. However, a recent report on primary cultured astrocytes from AQP4 null mice (KO) showed no morphological differences compared with wild types. In this study, we compared the effect of AQP4 KD in mouse, rat, and human astrocyte primary cultures and found that AQP4 KD in human astrocytes resulted in a morphological phenotype similar to that found in rat. In contrast, AQP4 KD in mouse astrocytes caused only very mild morphological changes. The actin cytoskeleton of untreated astrocytes exhibited strong species-specific differences, with F-actin being organized in cortical bands in mouse and in stress fibers in rat and human astrocytes. Surprisingly, as a consequence of AQP4 KD, F-actin cytoskeleton was depolymerized in rat and human whereas it was completely rearranged in mouse astrocytes. Although AQP4 KD induced alterations of the cell cytoskeleton, we found that the expression of dystrophin (DP71), beta-dystroglycan, and alpha-syntrophin was not altered. AQP4 KD in cultured mouse astrocytes produced strong down-regulation of connexin43 (Cx43) with a concomitant reduction in cell coupling while no major alterations in Cx43 expression were found in rat and human cells. Taken together, these results demonstrate that with regard to these properties, human astrocytes in culture are more similar to rat than to mouse astrocytes. Moreover, even though AQP4 KD in mouse astrocytes did not result in a dramatic morphological phenotype, it induced a remarkable rearrangement of F-actin, not related to disruption of the dystrophin complex, indicating a primary role of this water channel in the cytoskeleton changes observed. Finally, the strong down-regulation of Cx43 and cell coupling in AQP4 KD mouse astrocytes indicate that a functional relationship likely exists between water channels and gap junctions in brain astrocytes.
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Affiliation(s)
- Grazia P Nicchia
- Department of General and Environmental Physiology and Centre of Excellence in Comparative Genomics (CEGBA), University of Bari, Bari, Italy.
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265
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Verkman AS. Novel roles of aquaporins revealed by phenotype analysis of knockout mice. Rev Physiol Biochem Pharmacol 2005. [DOI: 10.1007/s10254-005-0040-1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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266
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Abstract
Aquaporins (AQPs) are membrane proteins that transport water and, in some cases, also small solutes such as glycerol. AQPs are expressed in many fluid-transporting tissues, such as kidney tubules and glandular epithelia, as well as in non-fluid-transporting tissues, such as epidermis, adipose tissue and astroglia. Their classical role in facilitating trans-epithelial fluid transport is well understood, as in the urinary concentrating mechanism and gland fluid secretion. AQPs are also involved in swelling of tissues under stress, as in the injured cornea and the brain in stroke, tumor and infection. Recent analysis of AQP-knockout mice has revealed unexpected cellular roles of AQPs. AQPs facilitate cell migration, as manifested by reduced tumor angiogenesis in AQP1-knockout mice, by a mechanism that might involve facilitated water transport in lamellipodia of migrating cells. AQPs that transport both glycerol and water regulate glycerol content in epidermis and fat, and consequently skin hydration/biosynthesis and fat metabolism. AQPs might also be involved in neural signal transduction, cell volume regulation and organellar physiology. The many roles of AQPs could be exploited for clinical benefit; for example, treatments that modulate AQP expression/function could be used as diuretics, and in the treatment of brain swelling, glaucoma, epilepsy, obesity and cancer.
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Affiliation(s)
- A S Verkman
- Department of Medicine, Cardiovascular Research Institute, Room 1246, Box 0521 University of California San Francisco, San Francisco, CA 94143-0521, USA.
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267
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Abstract
Birth is a transition from an underwater life in the uterus to a terrestrial life in a milieu where supply of water is limited. Rapid adaptation to the new environment is crucial for survival and health of infants. The discovery of a family of molecules-aquaporin (AQP) water channels-that are responsible for regulated water transport across cell membranes has made it possible to identify the molecular mechanisms behind the postnatal homeostatic adaptation and to better understand water imbalance-related disorders in infancy and childhood. Thirteen mammalian AQP isoforms have been identified, most of them having a unique tissue-specific pattern of expression. Most mammalian AQPs can be dynamically regulated, which makes them potential targets for the development of new drugs for diseases associated with disturbances in water homeostasis. This review deals with AQP in kidney, lung, and brain. Evidence is presented that AQPs are expressed in a specific age-dependent manner and that the timed expression of AQPs may have a crucial role during the early postnatal period.
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Affiliation(s)
- Marina Zelenina
- Nordic Center of Excellence for Research in Water Imbalance Related Disorders, Department of Woman and Child Health, Karolinska Institutet, Stockholm, Sweden
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268
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Warth A, Mittelbronn M, Wolburg H. Redistribution of the water channel protein aquaporin-4 and the K+ channel protein Kir4.1 differs in low- and high-grade human brain tumors. Acta Neuropathol 2005; 109:418-26. [PMID: 15723236 DOI: 10.1007/s00401-005-0984-x] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2004] [Revised: 12/28/2004] [Accepted: 12/29/2004] [Indexed: 10/25/2022]
Abstract
The blood-brain barrier (BBB) regulation is characterized by an interplay between endothelial cells, subendothelial basal laminae and astrocytic cells. Astroglial cells are highly polarized by the differentiation of perivascular membrane domains. These domains are characterized by the aggregation of, among other molecules, the water channel protein aquaporin-4 (AQP4), the dystrophin-dystroglycan complex, and the inwardly rectifying potassium channel protein Kir4.1. Normally, this ion channel plays an important role in spatial buffering of extracellular K(+) in the central nervous system, which only can be performed due to the non-uniform distribution of Kir4.1 across the surface of the glial cell. In this study, we observed a mislocalization of Kir4.1 in various human brain tumors (low- and high-grade astrocytomas and oligodendrogliomas), suggesting that buffering capacity of glial cells may be compromised, leading to water influx (cytotoxic edema). Interestingly, whereas dystrophin remained regularly restricted at the endfeet membranes in all cases investigated, AQP4 was found to be redistributed only in high-grade astrocytomas, not in low-grade astrocytomas. If the mechanisms of redistribution of AQP4 and Kir4.1 are different in low- and high-grade gliomas, this may suggest that the mechanisms of clustering of AQP4 and Kir4.1 at the glial endfeet membrane domains are also different. The redistribution of AQP4 in glioblastoma cells is discussed as a reaction to the vasogenic edema, as induced by the breakdown of the BBB, to facilitate reabsorption of excess fluid.
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Affiliation(s)
- Arne Warth
- Institute of Pathology, University of Tübingen, Liebermeisterstrasse 8, 72076 Tübingen, Germany
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269
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Nicchia GP, Nico B, Camassa LMA, Mola MG, Loh N, Dermietzel R, Spray DC, Svelto M, Frigeri A. The role of aquaporin-4 in the blood-brain barrier development and integrity: studies in animal and cell culture models. Neuroscience 2005; 129:935-45. [PMID: 15561409 DOI: 10.1016/j.neuroscience.2004.07.055] [Citation(s) in RCA: 158] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/23/2004] [Indexed: 02/09/2023]
Abstract
Aquaporin-4 (AQP4) is the major water channel expressed in brain perivascular astrocyte processes. Although the role of AQP4 in brain edema has been extensively investigated, little information exists regarding its functional role at the blood-brain barrier (BBB). The purpose of this work is to integrate previous and recent data regarding AQP4 expression during BBB formation and depending on BBB integrity, using several experimental models. Results from studies on the chick optic tectum, a well-established model of BBB development, and the effect of lipopolysaccharide on the BBB integrity and on perivascular AQP4 expression have been analyzed and discussed. Moreover, data on the BBB structure and AQP4 expression in murine models of Duchenne muscular dystrophy are reviewed. In particular, published results obtained from mdx(3cv) mice have been analyzed together with new data obtained from mdx mice in which all the dystrophin isoforms including DP71 are strongly reduced. Finally, the role of the endothelial component on AQP4 cellular expression and distribution has been investigated using rat primary astrocytes and brain capillary endothelial cell co-cultures as an in vitro model of BBB.
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Affiliation(s)
- G P Nicchia
- Department of General and Environmental Physiology and Centre of Excellence in Comparative Genomics (CEGBA), University of Bari, via Amendola 165/A, I-70126 Bari, Italy
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270
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Affiliation(s)
- P Agre
- Departments of Biological Chemistry and Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205-2185, USA
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271
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Nagelhus EA, Mathiisen TM, Ottersen OP. Aquaporin-4 in the central nervous system: cellular and subcellular distribution and coexpression with KIR4.1. Neuroscience 2005; 129:905-13. [PMID: 15561407 DOI: 10.1016/j.neuroscience.2004.08.053] [Citation(s) in RCA: 380] [Impact Index Per Article: 19.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/23/2004] [Indexed: 12/16/2022]
Abstract
Aquaporin-4 (AQP4) is the predominant water channel in the neuropil of the central nervous system. It is expressed primarily in astrocytes, but also occurs in ependymocytes and endothelial cells. A striking feature of AQP4 expression is its polarized distribution in brain astrocytes and retinal Muller cells. Thus, immunogold analyses have revealed an enrichment of AQP4 in endfeet membranes in contact with brain microvessels or subarachnoidal space and a low but significant concentration in non-endfeet membranes, including those astrocyte membranes that ensheath glutamate synapses. The subcellular compartmentation of AQP4 mimics that of the potassium channel Kir4.1, which is implicated in spatial buffering of K(+). We propose that AQP4 works in concert with Kir4.1 and the electrogenic bicarbonate transporter NBC and that water flux through AQP4 contributes to the activity dependent volume changes of the extracellular space. Such volume changes are important as they affect the extracellular solute concentrations and electrical fields, and hence neuronal excitability. We conclude that AQP4-mediated water flux represents an integral element of brain volume and ion homeostasis.
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Affiliation(s)
- E A Nagelhus
- Nordic Centre of Excellence for Research in Water Imbalance Related Disorders and Centre for Molecular Biology and Neuroscience, Institute of Basic Medical Sciences, University of Oslo, POB 1105 Blindern, N-0317 Oslo, Norway.
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272
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Manley GT, Binder DK, Papadopoulos MC, Verkman AS. New insights into water transport and edema in the central nervous system from phenotype analysis of aquaporin-4 null mice. Neuroscience 2005; 129:983-91. [PMID: 15561413 DOI: 10.1016/j.neuroscience.2004.06.088] [Citation(s) in RCA: 228] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/22/2004] [Indexed: 12/25/2022]
Abstract
Aquaporin-4 (AQP4) is the major water channel in the CNS. Its expression at fluid-tissue barriers (blood-brain and brain-cerebrospinal fluid barriers) throughout the brain and spinal cord suggests a role in water transport under normal and pathological conditions. Phenotype studies of transgenic mice lacking AQP4 have provided evidence for a role of AQP4 in cerebral water balance and neural signal transduction. Primary cultures of astrocytes from AQP4-null mice have greatly reduced osmotic water permeability compared with wild-type astrocytes, indicating that AQP4 is the principal water channel in these cells. AQP4-null mice have reduced brain swelling and improved neurological outcome following water intoxication and focal cerebral ischemia, establishing a role of AQP4 in the development of cytotoxic (cellular) cerebral edema. In contrast, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema caused by freeze-injury and brain tumor, probably due to impaired AQP4-dependent brain water clearance. AQP4-null mice also have markedly reduced acoustic brainstem response potentials and significantly increased seizure threshold in response to chemical convulsants, implicating AQP4 in modulation of neural signal transduction. Pharmacological modulation of AQP4 function may thus provide a novel therapeutic strategy for the treatment of stroke, tumor-associated edema, epilepsy, traumatic brain injury, and other disorders of the CNS associated with altered brain water balance.
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Affiliation(s)
- G T Manley
- Department of Neurological Surgery, Cardiovascular Research Institute, University of California-San Francisco, 1001 Potrero Avenue, Building 1, Room 101, San Francisco, CA 94143-0112, USA.
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273
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McCarty JH. Cell Biology of the Neurovascular Unit: Implications for Drug Delivery Across the Blood–Brain Barrier. Assay Drug Dev Technol 2005; 3:89-95. [PMID: 15798399 DOI: 10.1089/adt.2005.3.89] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
The central nervous system (CNS) neurovascular unit is a dynamic structure consisting of vascular endothelial cells, pericytes, and closely juxtaposed astrocytes and neurons. Contact and communication events between cells of the neurovascular unit regulate CNS development, modulate cerebral blood flow, and influence permeability properties of the blood-brain barrier. Dysregulation of proper neurovascular unit function is linked to many common human CNS pathologies, making it a target for a variety of neurotherapeutic interventions. Furthermore, manipulation of the neurovascular unit to enhance the delivery of drugs to the CNS is an active area of interest. In this review I summarize current data concerning the cell and molecular biology of the neurovascular unit. Additionally, I suggest how manipulation of novel protein components of the neurovascular unit may enhance delivery of neurotherapeutic drugs across the blood-brain barrier.
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Affiliation(s)
- Joseph H McCarty
- Center for Cancer Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
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274
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Eid T, Lee TSW, Thomas MJ, Amiry-Moghaddam M, Bjørnsen LP, Spencer DD, Agre P, Ottersen OP, de Lanerolle NC. Loss of perivascular aquaporin 4 may underlie deficient water and K+ homeostasis in the human epileptogenic hippocampus. Proc Natl Acad Sci U S A 2005; 102:1193-8. [PMID: 15657133 PMCID: PMC545857 DOI: 10.1073/pnas.0409308102] [Citation(s) in RCA: 206] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
An abnormal accumulation of extracellular K+ in the brain has been implicated in the generation of seizures in patients with mesial temporal lobe epilepsy (MTLE) and hippocampal sclerosis. Experimental studies have shown that clearance of extracellular K+ is compromised by removal of the perivascular pool of the water channel aquaporin 4 (AQP4), suggesting that an efficient clearance of K+ depends on a concomitant water flux through astrocyte membranes. Therefore, we hypothesized that loss of perivascular AQP4 might be involved in the pathogenesis of MTLE. Whereas Western blot analysis showed an overall increase in AQP4 levels in MTLE compared with non-MTLE hippocampi, quantitative ImmunoGold electron microscopy revealed that the density of AQP4 along the perivascular membrane domain of astrocytes was reduced by 44% in area CA1 of MTLE vs. non-MTLE hippocampi. There was no difference in the density of AQP4 on the astrocyte membrane facing the neuropil. Because anchoring of AQP4 to the perivascular astrocyte endfoot membrane depends on the dystrophin complex, the localization of the 71-kDa brain-specific isoform of dystrophin was assessed by immunohistochemistry. In non-MTLE hippocampus, dystrophin was preferentially localized near blood vessels. However, in the MTLE hippocampus, the perivascular dystrophin was absent in sclerotic areas, suggesting that the loss of perivascular AQP4 is secondary to a disruption of the dystrophin complex. We postulate that the loss of perivascular AQP4 in MTLE is likely to result in a perturbed flux of water through astrocytes leading to an impaired buffering of extracellular K+ and an increased propensity for seizures.
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Affiliation(s)
- Tore Eid
- Department of Neurosurgery, Laboratory Medicine, and Psychiatry, Yale University School of Medicine, New Haven, CT 06520, USA.
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275
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Abstract
Extracellular [K+] can range within 2.5-3.5 mM under normal conditions to 50-80 mM under ischemic and spreading depression events. Sustained exposure to elevated [K+]o has been shown to cause significant neuronal death even under conditions of abundant glucose supply. Astrocytes are well equipped to buffer this initial insult of elevated [K] through extensive gap junctional coupling, Na+/K+ pump activity (with associated glycogen and glycolytic potential), and endfoot siphoning capability. Their abundant energy availability and alkalinizing mechanisms help sustain Na+/K+ ATPase activity under ischemic conditions. Furthermore, passive K+ uptake mechanisms and water flux mediated through aquaporin-4 channels in endfoot processes are important energy-independent mechanisms. Unfortunately, as the length of ischemic episode is prolonged, these mechanisms increase to a point where they begin to have repercussions on other important cellular functions. Alkalinizing mechanisms induce an elevation of [Na+]i, increasing the energy demand of Na+/K+ ATPase and leading to eventual detrimental reversal of the Na+/glutamate- cotransporter and excitotoxic damage. Prolonged ischemia also results in cell swelling and activates volume regulatory processes that release excessive excitatory amino acids, further exacerbating excitotoxic injury. In the days following ischemic injury, reactive astrocytes demonstrate increased cell size and process thickness, leading to improved spatial buffering capacity in regions outside the lesion core where there is better neuronal survival. There is a substantial heterogeneity among reactive astrocytes, with some close to the lesion showing decreased buffering capacity. However, it appears that both Na+/K+ ATPase activity (along with energy production processes) as well as passive K+ uptake mechanisms are upregulated in gliotic tissue outside the lesion to enhance the above-mentioned homeostatic mechanisms.
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Affiliation(s)
- Jerome A Leis
- Department of Physiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
| | - Lane K Bekar
- Department of Physiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
| | - Wolfgang Walz
- Department of Physiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
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276
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Lee TS, Eid T, Mane S, Kim JH, Spencer DD, Ottersen OP, de Lanerolle NC. Aquaporin-4 is increased in the sclerotic hippocampus in human temporal lobe epilepsy. Acta Neuropathol 2004; 108:493-502. [PMID: 15517312 DOI: 10.1007/s00401-004-0910-7] [Citation(s) in RCA: 105] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2004] [Revised: 05/28/2004] [Accepted: 05/28/2004] [Indexed: 11/26/2022]
Abstract
The hippocampus of patients with mesial temporal lobe epilepsy is often hardened and shrunken, a condition known as sclerosis. Magnetic resonance imaging reveals an increase in the T2-weighted signal, while diffusion weighted imaging shows a higher apparent diffusion coefficient in sclerotic hippocampi, indicating increased water content. As water transport appears to be coupled to K+ clearance and neuronal excitability [4], the molecular basis of the perturbed water homeostasis in the sclerotic hippocampus was explored. The expression of aquaporin-4 (AQP-4), the predominant water channel in the brain, was studied with quantitative real time PCR analysis, light microscopic immunohistochemistry and high-resolution immunogold labeling. A significant increase in AQP-4 was observed in sclerotic, but not in non-sclerotic, hippocampi obtained from patients with medically intractable temporal lobe epilepsy. This increase was positively correlated with an increase in the astrocyte marker glial fibrillary acidic protein. AQP-4 was localized to the plasma membranes of astrocytes including the perivascular end-feet. Gene expression associated with increased AQP-4 was evaluated by high throughput gene expression analysis using Affymetrix GeneChip U133A and related gene networks were investigated with Ingenuity Pathways Analysis. AQP-4 expression was associated with a decrease in expression of the dystrophin gene, a protein implicated in the anchoring of AQP-4 in perivascular endfeet. The decreased expression of dystrophin may indicate a loss of polarity in the distribution of AQP-4 in astrocytes. We conclude that the perturbed expression of AQP-4 and dystrophin may be one factor underlying the loss of ion and water homeostasis in the sclerotic hippocampus and hypothesize that the reported changes may contribute to the epileptogenic properties of the sclerotic tissue.
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Affiliation(s)
- Tih Shih Lee
- Department of Psychiatry, Yale University School of Medicine, New Haven, Connecticut, USA
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277
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Hibino H, Fujita A, Iwai K, Yamada M, Kurachi Y. Differential Assembly of Inwardly Rectifying K+ Channel Subunits, Kir4.1 and Kir5.1, in Brain Astrocytes. J Biol Chem 2004; 279:44065-73. [PMID: 15310750 DOI: 10.1074/jbc.m405985200] [Citation(s) in RCA: 129] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The inwardly rectifying K+ channel subunit Kir5.1 is expressed abundantly in the brain, but its precise distribution and function are still largely unknown. Because Kir5.1 is co-expressed with Kir4.1 in retinal glial Muller cells, we have compared the biochemical and immunological properties of Kir5.1 and Kir4.1 in the mouse brain. Immunoprecipitation experiments suggested that brain expressed at least two subsets of Kir channels, heteromeric Kir4.1/5.1 and homomeric Kir4.1. Immunolabeling using specific antibodies showed that channels comprising Kir4.1 and Kir5.1 subunits were assembled in a region-specific fashion. Heteromeric Kir4.1/5.1 was identified in the neocortex and in the glomeruli of the olfactory bulb. Homomeric Kir4.1 was confined to the hippocampus and the thalamus. Homomeric Kir5.1 was not identified. Kir4.1/5.1 and Kir4.1 expression appeared to occur only in astrocytes, specifically in the membrane domains facing the pia mater and blood vessels or in the processes surrounding synapses. Both Kir4.1/5.1 and Kir4.1 could be associated with PDZ domain-containing syntrophins, which might be involved in the subcellular targeting of these astrocyte Kir channels. Because heteromeric Kir4.1/5.1 and homomeric Kir4.1 have distinct ion channel properties (Tanemoto, M., Kittaka, N., Inanobe, A., and Kurachi, Y. (2000) J. Physiol. (Lond.) 525, 587-592 and Tucker, S. J., Imbrici, P., Salvatore, L., D'Adamo, M. C., and Pessia, M. (2000) J. Biol. Chem. 275, 16404-16407), it is plausible that these channels play differential physiological roles in the K+ -buffering action of brain astrocytes in a region-specific manner.
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Affiliation(s)
- Hiroshi Hibino
- Department of Pharmacology II, Graduate School of Medicine, Osaka University, 2-2 Yamada-oka, Suita, 565-0871, Japan
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278
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Connors NC, Adams ME, Froehner SC, Kofuji P. The potassium channel Kir4.1 associates with the dystrophin-glycoprotein complex via alpha-syntrophin in glia. J Biol Chem 2004; 279:28387-92. [PMID: 15102837 DOI: 10.1074/jbc.m402604200] [Citation(s) in RCA: 151] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
One of the major physiological roles of potassium channels in glial cells is to promote "potassium spatial buffering" in the central nervous system, a process necessary to maintain an optimal potassium concentration in the extracellular environment. This process requires the precise distribution of potassium channels accumulated at high density in discrete subdomains of glial cell membranes. To obtain a better understanding of how glial cells selectively target potassium channels to discrete membrane subdomains, we addressed the question of whether the glial inwardly rectifying potassium channel Kir4.1 associates with the dystrophin-glycoprotein complex (DGC). Immunoprecipitation experiments revealed that Kir4.1 is associated with the DGC in mouse brain and cultured cortical astrocytes. In vitro immunoprecipitation and pull-down assays demonstrated that Kir4.1 can bind directly to alpha-syntrophin, requiring the presence of the last three amino acids of the channel (SNV), a consensus PDZ domain-binding motif. Furthermore, Kir4.1 failed to associate with the DGC in brains from alpha-syntrophin knockout mice. These results suggest that Kir4.1 is localized in glial cells by its association with the DGC through a PDZ domain-mediated interaction with alpha-syntrophin and suggest an important role for the DGC in central nervous system physiology.
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Affiliation(s)
- Nathan C Connors
- Department of Neuroscience, University of Minnesota, 321 Church Street SE, Minneapolis, MN 55455, USA
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279
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Abstract
Brain function is inextricably coupled to water homeostasis. The fact that most of the volume between neurons is occupied by glial cells, leaving only a narrow extracellular space, represents an important challenge, as even small extracellular volume changes will affect ion concentrations and therefore neuronal excitability. Further, the ionic transmembrane shifts that are required to maintain ion homeostasis during neuronal activity must be accompanied by water. It follows that the mechanisms for water transport across plasma membranes must have a central part in brain physiology. These mechanisms are also likely to be of pathophysiological importance in brain oedema, which represents a net accumulation of water in brain tissue. Recent studies have shed light on the molecular basis for brain water transport and have identified a class of specialized water channels in the brain that might be crucial to the physiological and pathophysiological handling of water.
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Affiliation(s)
- Mahmood Amiry-Moghaddam
- Centre for Molecular Biology and Neuroscience, Institute of Basic Medical Sciences, University of Oslo, POB 1105 Blindern, N-0317 Oslo, Norway
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280
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Kobayashi H, Yanagita T, Yokoo H, Wada A. Molecular Mechanisms and Drug Development in Aquaporin Water Channel Diseases: Aquaporins in the Brain. J Pharmacol Sci 2004; 96:264-70. [PMID: 15557735 DOI: 10.1254/jphs.fmj04004x5] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022] Open
Abstract
Water homeostasis of the brain is essential for its neuronal activity. Changes in water content in the intra- and extra-cellular space affect ionic concentrations and therefore modify neuronal activity. Aquaporin (AQP) water channels may have a central role in keeping water homeostasis in the brain. Among AQP subtypes cloned in mammalian, only AQP1, AQP4, and AQP9 were identified in the brain. Changes in AQP expression may be correlated with edema formation of the brain. In this review, we describe the physiological function of AQPs and the regulatory mechanism of their expression in the brain.
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Affiliation(s)
- Hideyuki Kobayashi
- Department of Pharmacology, Miyazaki Medical College, University of Miyazaki, Kiyotake, Japan.
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281
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Simard M, Nedergaard M. The neurobiology of glia in the context of water and ion homeostasis. Neuroscience 2004; 129:877-96. [PMID: 15561405 DOI: 10.1016/j.neuroscience.2004.09.053] [Citation(s) in RCA: 424] [Impact Index Per Article: 20.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/30/2004] [Indexed: 10/26/2022]
Abstract
Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.
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Affiliation(s)
- M Simard
- Utah Diabetes Center, 615 Arapeen Drive, Suite 100, Salt Lake City, UT 84108, USA.
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282
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Haenggi T, Soontornmalai A, Schaub MC, Fritschy JM. The role of utrophin and Dp71 for assembly of different dystrophin-associated protein complexes (DPCS) in the choroid plexus and microvasculature of the brain. Neuroscience 2004; 129:403-13. [PMID: 15501597 DOI: 10.1016/j.neuroscience.2004.06.079] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/26/2004] [Indexed: 10/26/2022]
Abstract
In the brain, utrophin is present in the choroid plexus epithelium and vascular endothelial cells, whereas the short C-terminal isoform of dystrophin (Dp71) is localized in the glial end-feet surrounding blood vessels. Both proteins serve as anchors for the so-called dystrophin-associated protein complex (DPC), composed of isoforms of syntrophin, dystroglycan and dystrobrevin. Numerous transporter proteins and channels have a polarized distribution in vascular endothelial cells and in glial end-feet, suggesting an association with the DPC. We investigated the composition and localization of the DPC in dependence on the anchoring proteins in mice lacking either utrophin (utrophin0/0) or dystrophin isoforms (mdx3Cv). Three distinct complexes were identified: (i) associated with utrophin in the basolateral membrane of the choroid plexus epithelium, (ii) associated with utrophin in vascular endothelial cells, and (iii) associated with Dp71 in the glial end-feet. Upon ablation of utrophin or Dp71, the corresponding DPCs were disrupted and no compensation of the missing protein by its homologue was observed. Association of the water channel aquaporin 4 with the glial DPC likewise was disrupted in mdx3Cv mice. These results demonstrate the essential role of utrophin and Dp71 for assembly of the DPC and suggest that these proteins contribute to the proper functioning of the cerebrospinal fluid and blood-brain barriers.
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Affiliation(s)
- T Haenggi
- Institute of Pharmacology and Toxicology, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
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283
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Schrier RW, Chen YC, Cadnapaphornchai MA. From finch to fish to man: Role of aquaporins in body fluid and brain water regulation. Neuroscience 2004; 129:897-904. [PMID: 15561406 DOI: 10.1016/j.neuroscience.2004.06.043] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/30/2004] [Indexed: 11/28/2022]
Abstract
Charles Darwin, in his Origin of the Species, noted that different species of finches on the Galapagos Islands had adapted their beak size based on where they sought their food. Homer Smith, in his book From Fish to Philosopher, discussed the evolution of the nephron from a single conduit in salt water vertebrates, to nephrons with large glomerular capillaries and proximal and distal tubules in fresh water vertebrates, to smaller glomerular capillaries in amphibians, to nephrons with loops of Henle to allow for urinary concentration and dilution in mammals. The kidney with its million nephrons has emerged as the vital organ for regulating body fluid composition and volume. With the recent discovery of aquaporin water channels, our understanding of volume regulation has been greatly enhanced. This article reviews current knowledge regarding: 1) the unifying hypothesis of body fluid volume regulation; 2) brain aquaporins and their role in pathophysiologic states; and 3) function and regulation of renal aquaporins in the syndrome of inappropriate antidiuretic hormone secretion (SIADH).
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Affiliation(s)
- R W Schrier
- Department of Medicine, Division of Renal Diseases and Hypertension, University of Colorado Health Sciences Center, 4200 East 9th Avenue, Box B173, Denver, CO 80262, USA.
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284
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Abstract
Emerging evidence suggests that brain aquaporins (AQP) play important roles for the dynamic regulation of brain water homeostasis and for the regulation of cerebrospinal fluid production. This review deals with the short- and long-term regulation of AQP4 and AQP9, both expressed in astrocytes, and of AQP1, expressed in the choroid plexus. AQP1 and 4 have in other cell types been shown to be regulated by phosphorylation. Phosphorylation affects the gating of AQP4 and the trafficking and insertion into membrane of AQP1. Mercury inhibits the water permeability of AQP1 and AQP9, but not AQP4. The permeability of AQP4 is increased by lead. AQP4 is also regulated by protein-protein interaction. The assembly between AQP4 and syntrophin is required for the proper localization of AQP4 in the astrocyte plasma membrane that faces capillaries. There is evidence from studies on peripheral tissues that steroid hormones regulate the expression of AQP1, AQP4 and AQP9. There is also evidence that the expression of AQP1 can be regulated by ubiquitination, and that osmolality can regulate the expression of AQP1, AQP4 and AQP9. Further insight into the mechanisms by which brain AQPs are regulated will be of utmost clinical importance, since perturbed water flow via brain AQPs has been implicated in many neurological diseases and since, in brain edema, water flow via AQP4 may have a harmful effect.
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Affiliation(s)
- E Gunnarson
- Department of Woman and Child Health, Karolinska Institutet, Stockholm, Sweden
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285
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Iwata Y, Sampaolesi M, Shigekawa M, Wakabayashi S. Syntrophin is an actin-binding protein the cellular localization of which is regulated through cytoskeletal reorganization in skeletal muscle cells. Eur J Cell Biol 2004; 83:555-65. [PMID: 15679101 DOI: 10.1078/0171-9335-00415] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
We have characterized the interaction of syntrophin with F-actin. Subcellular fractionation of cardiac and skeletal muscle tissues showed that alpha-, beta1- and beta2-syntrophins were present in the soluble and the membrane fraction. Syntrophins are known to bind to the dystrophin-glycoprotein complex (DGC), but since the DGC is not present in the soluble fraction, it was concluded that some syntrophin did not associate with the DGC. Native syntrophins purified from the soluble fraction and recombinant syntrophins were both able to bind to F-actin, and binding occurred through several sites on syntrophin, including the second pleckstrin homology domain and the unique carboxyl-terminal domain. Syntrophin was also able to inhibit actin-activated myosin ATPase activity and actomyosin super-precipitation. alpha-Syntrophin co-localized with cortical F-actin fibers when expressed in Chinese hamster ovary cells, and deletion of the actin-binding region abolished co-localization. Most of exogenous or endogenous syntrophin also co-localized with stress fibers in endothelial and smooth muscle (A7r5) cells. However, syntrophins were mostly localized in the cytosol of serum-starved C2C12 or primary cultured skeletal muscle myotubes, and translocated to the membrane upon treatment with lysophosphatidic acid or the actin-stabilizing agent jasplakinolide. The actin-depolymerizing agent latrunculin-B abolished this syntrophin translocation. These findings suggest that syntrophin is an actin-binding protein the subcellular localization of which is regulated through cytoskeletal reorganization.
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Affiliation(s)
- Yuko Iwata
- Department of Molecular Physiology, National Cardiovascular Center Research Institute, Fujishiro-dai 5-7, Suita, Osaka 5658565, Japan.
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286
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Amiry-Moghaddam M, Frydenlund DS, Ottersen OP. Anchoring of aquaporin-4 in brain: Molecular mechanisms and implications for the physiology and pathophysiology of water transport. Neuroscience 2004; 129:999-1010. [PMID: 15561415 DOI: 10.1016/j.neuroscience.2004.08.049] [Citation(s) in RCA: 211] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/23/2004] [Indexed: 01/01/2023]
Abstract
Astrocytes show an enrichment of aquaporin-4 (AQP4) in those parts of the plasma membrane that are apposed to pial or perivascular basal laminae. This observation begged the following questions: 1, What are the molecular mechanisms that are responsible for the site specific anchoring of AQP4? 2, What are the physiological and pathophysiological roles of the AQP4 pools at these specialized membrane domains? Recent studies suggest that the site specific anchoring depends on the dystrophin complex. Further, alpha-syntrophin (a member of the dystrophin complex) is required to maintain a polarized expression of AQP4 in the perivascular membranes. Hence transgenic mice deficient in alpha-syntrophin provided a model where the perivascular pool of AQP4 could be removed for assessment of its functional roles. Data suggest that the perivascular pool of AQP4 plays a role in edema formation and that this pool (through its serial coupling with the AQP4 pools in other astrocyte membranes) is involved in K(+) siphoning. In the cerebral cortex, the astrocyte membrane domain contacting the pial basal lamina differs from the perivascular membrane domain in regard to the mechanisms for AQP anchoring. Thus deletion of alpha-syntrophin causes only a 50% loss of AQP4 from the former membrane (compared with a 90% loss in the latter), pointing to the existence of additional anchoring proteins. We will also discuss the subcellular distribution and anchoring of AQP4 in the other cell types that express this protein: endothelial cells, ependymal cells, and the specialized astrocytes of the osmosensitive organs.
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Affiliation(s)
- M Amiry-Moghaddam
- Centre for Molecular Biology and Neuroscience, Institute of Basic Medical Sciences, University of Oslo, POB 1105 Blindern, N-0317 Oslo, Norway.
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287
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MacAulay N, Hamann S, Zeuthen T. Water transport in the brain: Role of cotransporters. Neuroscience 2004; 129:1031-44. [PMID: 15561418 DOI: 10.1016/j.neuroscience.2004.06.045] [Citation(s) in RCA: 89] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/30/2004] [Indexed: 11/15/2022]
Abstract
It is generally accepted that cotransporters transport water in addition to their normal substrates, although the precise mechanism is debated; both active and passive modes of transport have been suggested. The magnitude of the water flux mediated by cotransporters may well be significant: both the number of cotransporters per cell and the unit water permeability are high. For example, the Na(+)-glutamate cotransporter (EAAT1) has a unit water permeability one tenth of that of aquaporin (AQP) 1. Cotransporters are widely distributed in the brain and participate in several vital functions: inorganic ions are transported by K(+)-Cl(-) and Na(+)-K(+)-Cl(-) cotransporters, neurotransmitters are reabsorbed from the synaptic cleft by Na(+)-dependent cotransporters located on glial cells and neurons, and metabolites such as lactate are removed from the extracellular space by means of H(+)-lactate cotransporters. We have previously determined water transport capacities for these cotransporters in model systems (Xenopus oocytes, cell cultures, and in vitro preparations), and will discuss their role in water homeostasis of the astroglial cell under both normo- and pathophysiologal situations. Astroglia is a polarized cell with EAAT localized at the end facing the neuropil while the end abutting the circulation is rich in AQP4. The water transport properties of EAAT suggest a new model for volume homeostasis of the extracellular space during neural activity.
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Affiliation(s)
- N MacAulay
- The Panum Institute, Department of Medical Physiology, University of Copenhagen, Blegdamsvej 3C, DK 2200N Copenhagen, Denmark
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288
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Nico B, Paola Nicchia G, Frigeri A, Corsi P, Mangieri D, Ribatti D, Svelto M, Roncali L. Altered blood–brain barrier development in dystrophic MDX mice. Neuroscience 2004; 125:921-35. [PMID: 15120852 DOI: 10.1016/j.neuroscience.2004.02.008] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2003] [Revised: 01/29/2004] [Accepted: 02/04/2004] [Indexed: 11/23/2022]
Abstract
In order to ascertain whether the alterations of the blood-brain barrier (BBB) seen in adult dystrophic mdx-mice [Glia 42 (2003) 235], a human model of Duchenne muscular dystrophy (DMD), are developmentally established and correlated with other dystrophin isoforms which are localized at the glial-vascular interface, we used immunocytochemistry to investigate the expression of dystrophin isoforms (Dp71) during BBB development in mdx fetuses and in adult mice. Parallelly, we used Western blot, immunocytochemistry and immunogold electron microscopy to analyze the expression of the zonula occludens (ZO-1), aquaporin-4 (AQP4) and glial fibrillary acidic (GFAP) proteins as endothelial and glial markers, and we evaluated the integrity of the mdx BBB by means of intravascular injection of horseradish peroxidase (HRP). The results show reduced dystrophin isoforms (Dp71) in the mdx mouse compared with the control, starting from early embryonic life. Endothelial ZO-1 expression was reduced, and the tight junctions were altered and unlabeled. AQP4 and GFAP glial proteins in mdx mice also showed modifications in developmental expression, the glial vascular processes being only lightly AQP4- and GFAP-labeled compared with the controls. Confocal microscopy and HRP assays confirmed the alteration in vessel glial investment, GFAP perivascular endfoot reactivity being strongly reduced and BBB permeability increasing. These results demonstrate that a reduction in dystrophin isoforms (Dp71) at glial endfeet leads to an altered development of the BBB, whose no-closure might contribute to the neurological dysfunctions associated with DMD.
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Affiliation(s)
- B Nico
- Department of Human Anatomy and Histology, University of Bari Medical School, Piazza Giulio Cesare, 11, Policlinico, I-70124 Bari, Italy.
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289
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Guadagno E, Moukhles H. Laminin-induced aggregation of the inwardly rectifying potassium channel, Kir4.1, and the water-permeable channel, AQP4, via a dystroglycan-containing complex in astrocytes. Glia 2004; 47:138-49. [PMID: 15185393 DOI: 10.1002/glia.20039] [Citation(s) in RCA: 94] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Dystroglycan (DG) is part of a multiprotein complex that links the extracellular matrix to the actin cytoskeleton of muscle fibers and that is involved in aggregating acetylcholine receptors at the neuromuscular junction. This complex is also expressed in regions of the central nervous system where it is localized to both neuronal and glial cells. DG and the inwardly rectifying potassium channels, Kir4.1, are concentrated at the interface of astroglia and small blood vessels. These channels are involved in siphoning potassium released into the extracellular space after neuronal excitation. This raises the possibility that DG may be involved in targeting Kir4.1 channels to specific domains of astroglia. To address this question, we used mixed hippocampal cultures to investigate the distribution of DG, syntrophin, dystrobrevin, and Kir4.1 channels, as well as aquaporin-permeable water channels, AQP4. These proteins exhibit a similar distribution pattern and form aggregates in astrocytes cultured on laminin. Both DG and syntrophin colocalize with Kir4.1 channel aggregates in astrocytes. Similarly, DG colocalizes with AQP4 channel aggregates. Quantitative studies show a significant increase of Kir4.1 and AQP4 channel aggregates in astrocytes cultured in the presence of laminin when compared with those in the absence of laminin. These findings show that laminin has a role in Kir4.1 and AQP4 channel aggregation and suggest that this may be mediated via a dystroglycan-containing complex. This study reveals a novel functional role for DG in brain including K+ buffering and water homeostasis.
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Affiliation(s)
- Eric Guadagno
- Département de Sciences Biologiques, Université de Montréal, Montréal, Québec, Canada
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290
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Furman CS, Gorelick-Feldman DA, Davidson KGV, Yasumura T, Neely JD, Agre P, Rash JE. Aquaporin-4 square array assembly: opposing actions of M1 and M23 isoforms. Proc Natl Acad Sci U S A 2003; 100:13609-14. [PMID: 14597700 PMCID: PMC263861 DOI: 10.1073/pnas.2235843100] [Citation(s) in RCA: 240] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Osmotic homeostasis in the brain involves movement of water through aquaporin-4 (AQP4) membrane channels. Perivascular astrocyte end-feet contain distinctive orthogonal lattices (square arrays) assembled from 4- to 6-nm intramembrane particles (IMPs) corresponding to individual AQP4 tetramers. Two isoforms of AQP4 result from translation initiation at methionine residues M1 and M23, but no functional differences are known. In this study, Chinese hamster ovary cells were transfected with M1, M23, or M1+M23 isoforms, and AQP4 expression was confirmed by immunoblotting, immunocytochemistry, and immunogold labeling. Square array organization was examined by freeze-fracture electron microscopy. In astrocyte end-feet, >90% of 4- to 6-nm IMPs were found in square arrays, with 65% in arrays of 13-30 IMPs. In cells transfected with M23, 95% of 4- to 6-nm IMPs were in large assemblies (rafts), 85% of which contained >100 IMPs. However, in M1 cells, >95% of 4- to 6-nm IMPs were present as singlets, with <5% in incipient arrays of 2-12 IMPs. In M1+M23 cells, 4- to 6-nm IMPs were in arrays of intermediate sizes, resembling square arrays in astrocytes. Structural cross-bridges of 1 x 2 nm linked >90% of IMPs in M23 arrays ( approximately 1,000 cross-bridges per microm2) but were rarely seen in M1 cells. These studies show that M23 and M1 isoforms have opposing effects on intramembrane organization of AQP4: M23 forms large square arrays with abundant cross-bridges; M1 restricts square array assembly.
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Affiliation(s)
- C Sue Furman
- Department of Biomedical Sciences and Program in Molecular, Cellular, and Integrative Neurosciences, Colorado State University, Fort Collins, CO 80523, USA
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