Lang, Florian, Gillian L. Busch, Markus Ritter, Harald Volkl, Siegfried Waldegger, Erich Gulbins, and Dieter Haussinger. Functional Significance of Cell Volume Regulatory Mechanisms. Physiol. Rev. ...

Functional Significance of Cell Volume Regulatory Mechanisms

Cerebral edema contributes significantly to morbidity and death associated with many common neurological disorders. However, current treatment options are limited to hyperosmolar agents and surgical decompression, therapies introduced more than 70 years ago. Here we show that mice deficient in aquaporin-4 (AQP4), a glial membrane water channel, have much better survival than wild-type mice in a model of brain edema caused by acute water intoxication. Brain tissue water content and swelling of pericapillary astrocytic foot processes in AQP4-deficient mice were significantly reduced. In another model of brain edema, focal ischemic stroke produced by middle cerebral artery occlusion, AQP4-deficient mice had improved neurological outcome. Cerebral edema, as measured by percentage of hemispheric enlargement at 24 h, was decreased by 35% in AQP4-deficient mice. These results implicate a key role for AQP4 in modulating brain water transport, and suggest that AQP4 inhibition may provide a new therapeutic option for reducing brain edema in a wide variety of cerebral disorders.

Aquaporin-4 deletion in mice reduces brain edema after acute water intoxication and ischemic stroke.

Membrane water transport is critically involved in brain volume homeostasis and in the pathogenesis of brain edema. The cDNA encoding aquaporin-4 (AQP4) water channel protein was recently isolated from rat brain. We used immunocytochemistry and high-resolution immunogold electron microscopy to identify the cells and membrane domains that mediate water flux through AQP4. The AQP4 protein is abundant in glial cells bordering the subarachnoidal space, ventricles, and blood vessels. AQP4 is also abundant in osmosensory areas, including the supraoptic nucleus and subfornical organ. Immunogold analysis demonstrated that AQP4 is restricted to glial membranes and to subpopulations of ependymal cells. AQP4 is particularly strongly expressed in glial membranes that are in direct contact with capillaries and pia. The highly polarized AQP4 expression indicates that these cells are equipped with specific membrane domains that are specialized for water transport, thereby mediating the flow of water between glial cells and the cavities filled with CSF and the intravascular space.

https://www.jneurosci.org/content/jneuro/17/1/171.full.pdf

Specialized membrane domains for water transport in glial cells : high-resolution immunogold cytochemistry of aquaporin-4 in rat brain

The discovery of aquaporin-1 (AQP1) answered the long-standing biophysical question of how water specifically crosses biological membranes. In the kidney, at least seven aquaporins are expressed at distinct sites. AQP1 is extremely abundant in the proximal tubule and descending thin limb and is essential for urinary concentration. AQP2 is exclusively expressed in the principal cells of the connecting tubule and collecting duct and is the predominant vasopressin-regulated water channel. AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2. Studies in patients and transgenic mice have demonstrated that both AQP2 and AQP3 are essential for urinary concentration. Three additional aquaporins are present in the kidney. AQP6 is present in intracellular vesicles in collecting duct intercalated cells, and AQP8 is present intracellularly at low abundance in proximal tubules and collecting duct principal cells, but the physiological function of these two channels remains undefined. AQP7 is abundant in the brush border of proximal tubule cells and is likely to be involved in proximal tubule water reabsorption. Body water balance is tightly regulated by vasopressin, and multiple studies now have underscored the essential roles of AQP2 in this. Vasopressin regulates acutely the water permeability of the kidney collecting duct by trafficking of AQP2 from intracellular vesicles to the apical plasma membrane. The long-term adaptational changes in body water balance are controlled in part by regulated changes in AQP2 and AQP3 expression levels. Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting are seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure. In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy, and syndrome of inappropriate antidiuretic hormone secretion, both AQP2 expression levels and apical plasma membrane targetting are increased, suggesting a role for AQP2 in the development of water retention. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiology and pathophysiology of water balance and water balance disorders.

Aquaporins in the Kidney: From Molecules to Medicine

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of astroglia

It was shown recently that water channel homologs MIWC (mercurial insensitive water channel) and GLIP (glycerol intrinsic protein) colocalized in basolateral membranes of kidney collecting duct, tracheal and colonic epithelia, and in brain pia mater. We report here an extensive immunolocalization study of MIWC and GLIP in non-epithelial and glandular epithelial tissues in rat. Immunogold electron microscopy confirmed colocalization of MIWC and GLIP in basolateral membrane of principal cells in kidney collecting duct. However, in other epithelia, MIWC but not GLIP was expressed in basolateral membrane of parietal cells in stomach, and in excretory tubules of salivary and lacrimal glands; GLIP but not MIWC was expressed in transitional epithelium of urinary bladder and skin epidermis. In the central nervous system, MIWC was strongly expressed in the ependymal layer lining the aqueductal system, and in astrocytes throughout the spinal cord and in selected regions of brain. MIWC was also expressed in a plasma membrane pattern in skeletal, but not smooth or cardiac muscle. Neither protein was expressed in small intestine, testis, liver, spleen and nerve. The tissue-specific expression of MIWC suggests a role in fluid transport and/or cell volume regulation in stomach and glandular epithelia. The functional role of MIWC expression in the neuromuscular system and of GLIP expression in skin and urinary bladder is uncertain. The specific cellular sites of MIWC expression (astrocytes, trachea, sarcolemma, gastric parietal cells and kidney principal cells) correspond exactly to sites where orthogonal arrays of particles (OAPs) have been visualized by freeze-fracture electron microscopy, suggesting that MIWC may be the OAP protein.

Localization of MIWC and GLIP water channel homologs in neuromuscular, epithelial and glandular tissues

The RNase protection assay was applied to quantify mRNA expression of five principal mammalian water channels in 18 different rat tissues, and to determine the influence of dehydration on renal water channel expression. Probes consisted of labeled cRNAs transcribed from cDNA fragments of rat CHIP28 (AQP-1, bp 238-575 of coding sequence), AQP-CD (AQP2, bp 53-606), MIWC (AQP4, bp 235-572), GLIP (AQP3, bp 219-604), and AQP5 (bp 56-612). Results were normalized to expression of rat beta-actin by quantitative densitometry of autoradiograms. CHIP28 mRNA was expressed strongly in heart, kidney > placenta, skeletal muscle, and urinary bladder and detected weakly in eye, lung, trachea, spleen, liver, colon, prostate, and skin. AQP-CD was detected only in kidney. MIWC mRNA expression was highest in brain, followed by eye, trachea, lung, stomach, kidney, and skeletal muscle. GLIP was found in eye, trachea, kidney, urinary bladder, skin, prostate, placenta, and skeletal muscle. AQP5 was detected in salivary gland, eye, lung, and trachea. An alternatively spliced form of MIWC (sMIWC) was also identified in lung and kidney by RNase protection assay, corresponding to deletion of exon 2 of MIWC. In response to dehydration (3 days, -15 % body weight), renal expression of CHIP28 and MIWC were unchanged, whereas expression of AQP-CD and GLIP were increased significantly by 2.18 +/- 0.04 and 1.36 +/- 0.11 fold (SE, n = 5), respectively. These results establish quantitative values for aquaporin transcript expression in multiple mammalian tissues. The sensitive RNase protection assay revealed the expression of water channels in several tissues not studied previously or in which mRNA levels were too low to detect by Northern blot analysis. The observation of GLIP up-regulation in kidney by dehydration suggests a role in the urinary concentrating mechanism.

Quantitative analysis of aquaporin mRNA expression in rat tissues by RNase protection assay.

cAMP regulated membrane diffusion of a green fluorescent protein-aquaporin 2 chimera.

Three members of the water channel (aquaporin) family are expressed in adult rat lung: CHIP28 (AQP-1), MIWC (AQP-4), and AQP-5. Because water channels may be important in the clearance of fluid from the newborn lung, the expression of water channels just before and after birth was investigated using the ribonuclease (RNAse) protection assay. RNA was isolated from lungs, brain, and heart of prenatal rats (fetal days F19, F20, and F21) and postnatal rats (days +1, +2, +5, +7, +21, and adult). Transcript expression was measured relative to a beta-actin control by quantitative densitometry. Whereas beta-actin mRNA expression was nearly constant over time, distinct expression patterns were observed for the three water channels. CHIP28 mRNA expression rose slowly from days F19 to +1, then strongly at day +2, and remained elevated over the first week. MIWC mRNA was weakly expressed prenatally, but strongly increased just after birth. AQP-5 mRNA increased slowly and monotonically between days F20 and +7. These patterns contrasted sharply with the developmental expression of CHIP28 in heart, which decreased over time, and MIWC in brain. Immunocytochemistry showed CHIP28 protein expression in capillary endothelia and MIWC in airway epithelia by day +1; quantitative immunoblot analysis showed increased CHIP28 protein expression over time. These findings are consistent with a role of lung water channels in perinatal fluid clearance; however, proof of physiologic significance will require functional measurements of air space-capillary water permeability.

Sharp Increase in Rat Lung Water Channel Expression in the Perinatal Period

Lung fluid is reabsorbed rapidly at birth to permit alveolar respiration. We reported previously that expression of aquaporins (AQP) 1, 4, and 5 in rat lung increased just after birth. The hypothesis was tested that the increased AQP expression is associated with increased osmotic water permeability (Pf) between the airspace and capillary compartments. Pf was measured in isolated perfused fetal and newborn rabbit lungs using a pleural surface fluorescence method (Carter, E.P., M.A. Matthay, J. Farinas, and A.S. Verkman. 1996. J. Gen. Physiol. 108:133-142). In response to perfusate osmolality increase from 300 to 600 mosM, initial rates of osmotic equilibration were 1.13+/-0.13 mosM/s at 0-12 h after birth, increasing to 1.52+/-0.19 mosM/s at 12-24 h, and 1.83+/-0.10 mosM/s at 24-84 h. Corresponding Pf values (in cm/s x 10(-2)), computed from d[mosM]/dt and alveolar surface-to-volume ratios, were 1.03+/-0.11 (0-12 h), 1.51+/-0.16 (12-24 h), and 1.88+/-0.09 (24-84 h). Pf was relatively low in prenatal (1.22-1.27, fetal days 29 and 31) and adolescent (1.25+/-0.08, 21-d) rabbit lungs. To test for involvement of molecular water channels, measurements were made of Arrhenius activation energy (Ea), mercurial inhibition, diffusional water permeability (Pd), and AQP expression. Temperature-dependence measurements showed a 25% decrease in Ea for Pf in lungs < 1 d vs. 4 d. Pf was decreased 30% by 0.5 mM HgCl2 in < 1-d lungs and 44% in 4-d lungs. Pd was 1.0 x 10(-)5 cm/s and did not change when Pf was increased by 75%. RNase protection assay showed increased transcript expression in the first 24 h after birth for rabbit isoforms of AQP1 and AQP4. These results provide the first functional data on water permeability in perinatal lung. The increased water permeability after birth may facilitate the maintenance of dry alveoli.

/pdf/developmental-changes-in-water-permeability-across-the-w964js13sx.pdf

Fuminori Umenishi

Papers

Localization of MIWC and GLIP water channel homologs in neuromuscular, epithelial and glandular tissues

Quantitative analysis of aquaporin mRNA expression in rat tissues by RNase protection assay.

cAMP regulated membrane diffusion of a green fluorescent protein-aquaporin 2 chimera.

Sharp Increase in Rat Lung Water Channel Expression in the Perinatal Period

Developmental changes in water permeability across the alveolar barrier in perinatal rabbit lung.