Effect of expressing the water channel aquaporin-1 on the CO2 permeability of Xenopus oocytes

Aquaporin-1-mediated cerebral edema following traumatic brain injury: effects of acidosis and corticosteroid administration

OBJECT

Dysregulation of water homeostasis induces cerebral edema. Edema is a major cause of morbidity and mortality following traumatic brain injury (TBI). Aquaporin-1 (AQP-1), a water channel found in the brain, can function as a transporter for CO2 across the cellular membrane. Additionally, AQP-1's promoter contains a glucocorticoid response element. Thus, AQP-1 may be involved with edema-related brain injury and might be modulated by external conditions such as the pH and the presence of steroids. In this study, the authors investigated the hypotheses that: 1) AQP-1 participates in brain water homeostasis following TBI; 2) secondary injury (for example, acidosis) alters the expression of AQP-1 and exacerbates cerebral edema; and 3) corticosteroids augment brain AQP-1 expression and differentially affect cerebral edema under nonacidotic and acidotic conditions.

METHODS

Anesthetized Sprague-Dawley rats were subjected to moderate to severe TBI (2.5-3.5 atm) or surgery without injury, and they were randomized to receive a 3-mg/kg bolus of intravenous dexamethasone within 10 minutes after injury or surgery, a 3-mg/kg bolus of dexamethasone followed by 1-mg/kg maintenance doses every 8 hours for 24 hours, or saline boluses at similar time intervals. A second group of animals was subjected to respiratory acidosis with target arterial blood pH 6.8-7.2 for 1 hour following the surgery or injury. To evaluate selective blockage of AQP-1, some animals received a single intraperitoneal dose of HgCl2 (0.3-30.0 mmol/L) within 30 minutes of injury or surgery. At 4 or 24 hours postinjury, animals were killed and their brains were harvested for mRNA, protein, or water content analyses.

RESULTS

The authors demonstrated elevated cerebral edema levels at 4 and 24 hours following TBI. Dexamethasone administration within 1 hour of TBI attenuated the cerebral edema under nonacidotic conditions but worsened it under acidotic conditions. Selective blockage of AQP-1 channels with HgCl2 attenuated the edematous effects of corticosteroids and acidosis. Reverse transcriptase polymerase chain reaction and immunohistochemical analyses demonstrated a paucity of AQP-1 in the cerebral cortices of the uninjured animals. In contrast, AQP-1 mRNA and protein levels were higher in the cerebral cortices of animals that sustained a TBI.

CONCLUSIONS

These findings implicate an important, modifiable role for AQP-1 in water homeostasis within the CNS following TBI.

Nitric oxide conduction by the brain aquaporin AQP4

Involvement of aquaporins in gas conduction across the membrane and the physiological significance of this process have attracted marked attention from both experimental and theoretical studies. Previous work demonstrated that AQP1 is permeable to both CO(2) and O(2). Here we employ various simulation techniques to examine the permeability of the brain aquaporin AQP4 to NO and O(2) and to describe energetics and pathways associated with these phenomena. The energy barrier to NO and O(2) permeation through AQP4 central pore is found to be only approximately 3 kcal mol(-1). The results suggest that the central pore of AQP4, similar to that of AQP1, can indeed conduct gas molecules. Interestingly, despite a longer and narrower central pore, AQP4 appears to provide an energetically more favorable permeation pathway for gas molecules than AQP1, mainly due to the different orientation of its charged residues near the pore entrance. Although the low barrier against gas permeation through AQP4 indicates that it can participate in gas conduction across the cellular membrane, physiological relevance of the phenomenon remains to be established experimentally, particularly since pure lipid bilayers appear to present a more favorable pathway for gas conduction across the membrane. With an energy well of -1.8 kcal mol(-1), the central pore of AQP4 may also act as a reservoir for NO molecules to accumulate in the membrane.

AQP1 mediates water transport in the carotid body

In this study, we explored the presence of aquaporins (AQPs), a family of membrane water channel proteins, in carotid body (CB) type I chemoreceptor cells. The CB is a polymodal chemoreceptor whose major function is to detect changes in arterial O2 tension to elicit hyperventilation during hypoxia. The CB has also been proposed to function as a systemic osmoreceptor, thus we hypothesized that the presence of AQPs in type I cell membrane may confer higher sensitivity to osmolarity changes and hence accelerate the activation of chemoreceptor cells. We detected the expression of AQP1, AQP7, and AQP8 in the CB and confirmed the location of AQP1 in type I cells. We have also shown that inhibition of AQP1 expression clearly reduced type I cell swelling after a hyposmotic shock, demonstrating that AQP1 has a major contribution in transmembrane water movement in these chemoreceptor cells. Interestingly, CB AQP1 expression levels change during postnatal development, increasing during the first postnatal weeks as the organ matures. In conclusion, in this study, we report the novel observation that AQPs are expressed in the CB. We also show that AQP1 mediates water transport across the cell membrane of type I cells, supporting the contribution of this protein to the osmoreception function of the CB.

Physiological roles of aquaporins in the choroid plexus

The choroid plexus is a specialized tissue that lines subdomains within the four ventricles of the brain where most of the cerebrospinal fluid is produced. Maintenance of an equilibrium in volume and composition of the cerebrospinal fluid (CSF) is vital for a normal brain function, ensuring an optimal environment for the neurons. The necessarily high water permeability of the choroid plexus barrier is made possible by the abundant expression of a water channel, Aquaporin-1 (AQP1), on the apical side of the membrane from early stages of development through adulthood. Data from studies of AQP1 suggest that it also can contribute as a gated ion channel, and suggest that the AQP1-mediated ionic conductance has physiological significance for the regulation of cerebrospinal fluid secretion. The regulation of AQP1 ion channels could be one of several transport mechanisms that contribute to the decreased CSF secretion in response to endogenous signaling molecules such as atrial natriuretic peptide. Numerous classes of ion channels and transporters are targeted specifically to each side of the cellular membrane, and they all work in concert to secrete CSF. Several signaling cascades have a direct effect on transporters and ion channels present in the choroid plexus epithelium, altering their transport activity and therefore modulating the net transcellular movement of solutes and water. Several neurotransmitters, neuropeptides, and growth factors can influence CSF secretion by direct effect on transport mechanisms of the epithelium. The mammalian choroid plexus receives innervation from noradrenergic sympathetic fibers, cholinergic and peptidergic fibers that modulate CSF secretion. Water imbalance in the brain can have life-threatening consequences resulting from altered excitability and neurodegeneration, disruption of the supply of nutrients, loss of signaling molecules, and the accumulation of unwanted toxins and metabolites. Understanding the mechanisms involved in the modulation of CSF secretion is of fundamental importance. An appreciation of AQP1 as an ion channel in addition to its role as a water channel should offer new targets for therapeutic strategies in diseases involving water imbalance in the brain.

110 years of the Meyer-Overton rule: predicting membrane permeability of gases and other small compounds

The transport of gaseous compounds across biological membranes is essential in all forms of life. Although it was generally accepted that gases freely penetrate the lipid matrix of biological membranes, a number of studies challenged this doctrine as they found biological membranes to have extremely low gas-permeability values. These observations led to the identification of several membrane-embedded "gas" channels, which facilitate the transport of biological active gases, such as carbon dioxide, nitric oxide, and ammonia. However, some of these findings are in contrast to the well-established solubility-diffusion model (also known as the Meyer-Overton rule), which predicts membrane permeabilities from the molecule's oil-water partition coefficient. Herein, we discuss recently reported violations of the Meyer-Overton rule for small molecules, including carboxylic acids and gases, and show that Meyer and Overton continue to rule.

Molecular physiology and genetics of Na+-independent SLC4 anion exchangers

Plasmalemmal Cl(-)/HCO(3)(-) exchangers are encoded by the SLC4 and SLC26 gene superfamilies, and function to regulate intracellular pH, [Cl(-)] and cell volume. The Cl(-)/HCO(3)(-) exchangers of polarized epithelial cells also contribute to transepithelial secretion and reabsorption of acid-base equivalents and Cl(-). This review focuses on Na(+)-independent electroneutral Cl(-)/HCO(3)(-) exchangers of the SLC4 family. Human SLC4A1/AE1 mutations cause the familial erythroid disorders of spherocytic anemia, stomatocytic anemia and ovalocytosis. A largely discrete set of AE1 mutations causes familial distal renal tubular acidosis. The Slc4a2/Ae2(-/-) mouse dies before weaning with achlorhydria and osteopetrosis. A hypomorphic Ae2(-/-) mouse survives to exhibit male infertility with defective spermatogenesis and a syndrome resembling primary biliary cirrhosis. A human SLC4A3/AE3 polymorphism is associated with seizure disorder, and the Ae3(-/-) mouse has increased seizure susceptibility. The transport mechanism of mammalian SLC4/AE polypeptides is that of electroneutral Cl(-)/anion exchange, but trout erythroid Ae1 also mediates Cl(-) conductance. Erythroid Ae1 may mediate the DIDS-sensitive Cl(-) conductance of mammalian erythrocytes, and, with a single missense mutation, can mediate electrogenic SO(4)(2-)/Cl(-) exchange. AE1 trafficking in polarized cells is regulated by phosphorylation and by interaction with other proteins. AE2 exhibits isoform-specific patterns of acute inhibition by acidic intracellular pH and independently by acidic extracellular pH. In contrast, AE2 is activated by hypertonicity and, in a pH-independent manner, by ammonium and by hypertonicity. A growing body of structure-function and interaction data, together with emerging information about physiological function and structure, is advancing our understanding of SLC4 anion exchangers.

RhCG is the major putative ammonia transporter expressed in the human kidney, and RhBG is not expressed at detectable levels

Rhesus glycoprotein homologs RhAG, RhBG, and RhCG comprise a recently identified branch of the Mep/Amt ammonia transporter family. Animal studies have shown that RhBG and RhCG are present in the kidney distal tubules. Studies in mouse and rat tissue suggest a basolateral localization for RhBG in cells of the distal tubules including the alpha-intercalated cells (alpha-IC), but no localization of RhBG has been reported in human tissue. To date RhCG localization has been described as exclusively apical plasma membrane in mouse and rat kidney, or apical and basolateral in humans, and some mouse and rat tissue studies. We raised novel antibodies to RhBG and RhCG to examine their localization in the human kidney. Madin-Darby canine kidney (MDCKI) cell lines stably expressing human green fluorescent protein-tagged RhBG or RhCG and human tissue lysates were used to demonstrate the specificity of these antibodies for detecting RhBG and RhCG. Using immunoperoxidase staining and antigen liberation techniques, both apical and basolateral RhCG localization was observed in the majority of the cells of the distal convoluted tubule and IC of the connecting tubule and collecting duct. Confocal microscopic imaging of normal human kidney cryosections showed that RhCG staining was predominantly localized to the apical membrane in these cells with some basolateral and intracellular staining evident. A proportion of RhCG staining labeled kAE1-positive cells, confirming that RhCG is localized to the alpha-IC cells. Surprisingly, no RhBG protein was detectable in the human kidney by Western blot analysis of tissue lysates, or by immunohistochemistry or confocal microscopy of tissue sections. The same antibodies, however, could detect RhBG in rat tissue. We conclude that under normal conditions, RhCG is the major putative ammonia transporter expressed in the human kidney and RhBG is not expressed at detectable levels.

Relative CO2/NH3 selectivities of AQP1, AQP4, AQP5, AmtB, and RhAG

The water channel aquaporin 1 (AQP1) and certain Rh-family members are permeable to CO(2) and NH(3). Here, we use changes in surface pH (pH(S)) to assess relative CO(2) vs. NH(3) permeability of Xenopus oocytes expressing members of the AQP or Rh family. Exposed to CO(2) or NH(3), AQP1 oocytes exhibit a greater maximal magnitude of pH(S) change (DeltapH(S)) compared with day-matched controls injected with H(2)O or with RNA encoding SGLT1, NKCC2, or PepT1. With CO(2), AQP1 oocytes also have faster time constants for pH(S) relaxation (tau(pHs)). Thus, AQP1, but not the other proteins, conduct CO(2) and NH(3). Oocytes expressing rat AQP4, rat AQP5, human RhAG, or the bacterial Rh homolog AmtB also exhibit greater DeltapH(S)(CO(2)) and faster tau(pHs) compared with controls. Oocytes expressing AmtB and RhAG, but not AQP4 or AQP5, exhibit greater DeltapH(S)(NH(3)) values. Only AQPs exhibited significant osmotic water permeability (P(f)). We computed channel-dependent (*) DeltapH(S) or P(f) by subtracting values for H(2)O oocytes from those of channel-expressing oocytes. For the ratio DeltapH(S)(CO(2))*/P(f)*, the sequence was AQP5 > AQP1 congruent with AQP4. For DeltapH(S)(CO(2))*/DeltapH(S)(NH(3))*, the sequence was AQP4 congruent with AQP5 > AQP1 > AmtB > RhAG. Thus, each channel exhibits a characteristic ratio for indices of CO(2) vs. NH(3) permeability, demonstrating that, like ion channels, gas channels can exhibit selectivity.

Extra- and intracellular unstirred layer effects in measurements of CO2 diffusion across membranes--a novel approach applied to the mass spectrometric 18O technique for red blood cells

We have developed an experimental approach that allows us to quantify unstirred layers around cells suspended in stirred solutions. This technique is applicable to all types of transport measurements and was applied here to the (18)O technique used to measure CO(2) permeability of red cells (PCO2). We measure PCO2 in well-stirred red cell (RBC) suspensions of various viscosities adjusted by adding different amounts of 60 kDa dextran. Plotting 1/PCO2 vs. viscosity nu gives a linear relation, which can be extrapolated to nu=0. Theoretical hydrodynamics predicts that extracellular unstirred layers vanish at zero viscosity when stirring is maintained, and thus this extrapolation gives us an estimate of the PCO2 free from extracellular unstirred layer artifacts. The extrapolated value is found to be 0.16 cm s(-1) instead of the experimental value in saline of 0.12 cm s(-1) (+30%). This effect corresponds to an unstirred layer thickness of 0.5 microm. In addition, we present a theoretical approach modelling the actual geometrical and physico-chemical conditions of (18)O exchange in our experiments. It confirms the role of an extracellular unstirred layer in the determination of PCO2. Also, it allows us to quantify the contribution of the so-called intracellular unstirred layer, which results from the fact that in these transport measurements--as in all such measurements in general--the intracellular space is not stirred. The apparent thickness of this intracellular unstirred layer is about 1/4-1/3 of the maximal intracellular diffusion distance, and correction for it results in a true PCO2 of the RBC membrane of 0.20 cm s(-1). Thus, the order of magnitude of this is PCO2 unaltered compared to our previous reports. Discussion of the available evidence in the light of these results confirms that CO(2) channels exist in red cell and other membranes, and that PCO2 of red cell membranes in the absence of these channels is quite low.

Discovery of the aquaporins and development of the field

The study of water transport began long before the molecular identification of water channels with studies of water-permeable tissues. The discovery of the first aquaporin, AQP1, occurred during experiments focused on the identity of the Rh blood group antigens. Since then the field has expanded dramatically to study aquaporins in all types of organisms. In mammals, some of the aquaporins transport only water. However, there are some family members that collectively transport a diverse set of solutes. The aquaporins can be regulated by factors that affect channel permeability or subcellular localization. An extensive set of studies examines the physiological role of many of the mammalian aquaporins. However, much is still to be discovered about the physiological role of this membrane protein family.

Dynamics and energetics of permeation through aquaporins. What do we learn from molecular dynamics simulations?

Aquaporins (AQPs) are a family of integral membrane proteins, which facilitate the rapid and yet highly selective flux of water and other small solutes across biological membranes. Molecular dynamics (MD) simulations contributed substantially to the understanding of the molecular mechanisms that underlie this remarkable efficiency and selectivity of aquaporin channels. This chapter reviews the current state of MD simulations of aquaporins and related aquaglyceroporins as well as the insights these simulations have provided. The mechanism of water permeation through AQPs and methods to determine channel permeabilities from simulations are described. Protons are strictly excluded from AQPs by a large electrostatic barrier and not by an interruption of the Grotthuss mechanism inside the pore. Both the protein's electric field and desolvation effects contribute to this barrier. Permeation of apolar gas molecules such as CO(2) through AQPs is accompanied by a large energetic barrier and thus can only be expected in membranes with a low intrinsic gas permeability. Additionally, the insights from simulations into the mechanism of glycerol permeation through the glycerol facilitator GlpF from E. coli are summarized. Finally, MD simulations are discussed that revealed that the aro-matic/arginine constriction region is generally the filter for uncharged solutes, and that AQP selectivity is controlled by a hydrophobic effect and steric restraints.

The AQP structure and functional implications

Progress in the structure determination of AQPs has led to a deep understanding of water and solute permeation by these small integral membrane proteins. The atomic structures now available have allowed the water permeation and exclusion of protons to be monitored by molecular dynamics simulations, and have provided a framework for assessing the water and solute permeation in great detail by site-directed mutations. In spite of this, further structural and molecular dynamics analyses are required to elucidate the basis for regulation as well as for gas permeation, processes that are still to be deciphered.

The expression pattern of plasma membrane aquaporins in maize leaf highlights their role in hydraulic regulation

Leaves are key organs for evaporation and photosynthesis and play a crucial role in plant growth and development. In order to function properly, they need to maintain a balanced water content. Water movement through a leaf occurs by a combination of different pathways: water can follow an apoplastic route through the cell wall or a cell-to-cell route via the symplastic and transcellular paths. As aquaporins (AQPs) play an important role in regulating transcellular water flow and CO(2) conductance, studies on AQP mRNA and protein expression in leaves are essential to better understand their role in these physiological processes. Here, we quantified and localized the expression of Zea mays plasma membrane aquaporins (ZmPIPs, plasma membrane intrinsic proteins) in the leaf using quantitative RT-PCR and immunodetection. All ZmPIP genes except ZmPIP2;7 were expressed in leaves. Expression was found to be dependent on the developmental stage of the leaf tissue, with, in general, an increase in expression at the end of the elongation zone and a decrease in mature leaf tissue. These data correlated with the cell water permeability, as determined using a protoplast swelling assay. The diurnal expression of ZmPIPs was also investigated and expression was found to be higher during the first hours of the light period than at night. Immunocytochemical localization of four ZmPIP isoforms indicated that they are involved in leaf radial water movement, in particular in vascular bundles and the mesophyll.

Deactivation of aquaporins decreases internal conductance to CO2 diffusion in tobacco leaves grown under long-term drought

We compared the diffusion conductance to CO₂ from the intercellular air space to the chloroplasts (internal conductance (g _i)) between tobacco leaves acclimated to long-term drought (drought-acclimated (DA)) and those grown under sufficient irrigation (well-watered (WW)), and analysed the changes in g _i in relation to the leaf anatomical characteristics and a possible CO₂ transporter, aquaporin. The g _i, which was estimated by combined analyses of CO₂ gas exchange with chlorophyll fluorescence, in the DA plants was approximately half of that in the WW plants. The mesophyll and chloroplast surface areas exposing the intercellular air space, which potentially affect g _i, were not significantly different between the WW and DA plants. The amounts of plasma membrane aquaporins (PIP), immunochemically determined using radish PIP antibodies, were unrelated to g _i. After treatment with HgCl₂, an aquaporin inhibitor, the water permeability of the leaf tissues (measured as the weight loss of fully-turgid leaf disks without the abaxial epidermis in 1 m sorbitol) in WW plants decreased with an increase in HgCl₂ concentration. The g _i in the WW plants decreased to similar levels to the DA plants when the detached leaflets were fed with 0.5 mm HgCl₂. In contrast, both water permeability and g _i were insensitive to HgCl₂ treatments in DA plants. These results suggest that deactivation of aquaporins is responsible for the significant reduction in g _i observed in plants growing under long-term drought.

Expanding roles of plant aquaporins in plasma membranes and cell organelles

Aquaporins facilitate water transport across biomembranes in a manner dependent on osmotic pressure and water-potential gradient. The discovery of aquaporins has facilitated research on intracellular and whole-plant water transport at the molecular level. Aquaporins belong to a ubiquitous family of membrane intrinsic proteins (MIP). Plants have four subfamilies: plasma-membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), and small basic intrinsic protein (SIP). Recent research has revealed a diversity of plant aquaporins, especially their physiological functions and intracellular localisation. A few PIP members have been reported to be involved in carbon dioxide permeability of cells. Newly identified transport substrates for NIP members of rice and Arabidopsis thaliana have been demonstrated to transport silicon and boron, respectively. Ammonia, glycerol, and hydrogen peroxide have been identified as substrates for plant aquaporins. The intracellular localisation of plant aquaporins is diverse; for example, SIP members are localised on the ER membrane. There has been much progress in the research on the functional regulation of water channel activity of PIP members including phosphorylation, formation of hetero-oligomer, and protonation of histidine residues under acidic condition. This review provides a broad overview of the range of potential aquaporins, which are now believed to participate in the transport of several small molecules in various membrane systems in model plants, crops, flowers and fruits.

Mechanism of selectivity in aquaporins and aquaglyceroporins

Aquaporins and aquaglyceroporins form a family of pore proteins that facilitate the efficient and selective flux of small solutes across biological membranes. We studied the selectivity of aquaporin-1 (AQP1) and the bacterial glycerol facilitator, GlpF, for O(2), CO(2), NH(3), glycerol, urea, and water. Using molecular dynamics simulations, we calculated potentials of mean force for solute permeation along the aquaporin channels and compared them with the alternative pathway across the lipid bilayer. For small solutes permeating through AQP1, a remarkable anticorrelation between permeability and solute hydrophobicity was observed, whereas the opposite trend was observed for permeation through the membrane. This finding renders AQP1 a selective filter for small polar solutes, whereas GlpF was found to be highly permeable for small solutes and permeable for larger solutes. Surprisingly, not solute-channel but water-channel interactions were found to be the key determinant underlying the selectivity mechanism of aquaporins. Hence, a hydrophobic effect, together with steric restraints, determines the selectivity of aquaporins.

Role of aquaporins in lung liquid physiology

Aquaporins (AQPs) are small, integral membrane proteins that facilitate water transport across cell membranes in response to osmotic gradients. Water transport across epithelia and endothelia in the peripheral lung and airways occurs during airway hydration, alveolar fluid transport and submucosal gland secretion. Several AQPs are expressed in the lung and airways: AQP1 in microvascular endothelia, AQP3 and AQP4 in airway epithelia, and AQP5 in type I alveolar epithelial cells, submucosal gland acini, and a subset of airway epithelial cells. Phenotype analysis of transgenic knockout mice lacking AQPs has defined their roles in the lung and airways. AQP1 and AQP5 provide the principal route for osmotically driven water transport between airspace and capillary compartments; however, alveolar fluid clearance in the neonatal and adult lung is not affected by their deletion, nor is lung fluid accumulation in experimental models of lung injury. In the airways, though AQP3 and AQP4 facilitate osmotic water transport, their deletion does not impair airway hydration, regulation of airway surface liquid, or fluid absorption. In contrast to these negative findings, AQP5 deletion in submucosal glands reduced fluid secretion by >50%. The substantially slower fluid transport in the lung compared to renal and secretory epithelia probably accounts for the lack of functional significance of AQPs in the lung and airways. Recent data outside of the lung implicating the involvement of AQPs in cell migration and proliferation suggests possible new roles for lung AQPs to be explored.

RhAG protein of the Rhesus complex is a CO2channel in the human red cell membrane

We have determined CO2 permeabilities, P(CO2), of red cells of normal human blood and of blood deficient in various blood group proteins by a previously described mass spectrometric technique. While P(CO2) of normal red cells is approximately 0.15 cm/s, we find in red blood cells (RBCs) lacking the Rh protein complex (Rh(null)) a significantly reduced P(CO2) of 0.07 cm/s +/-0.02 cm/s (P<0.02). This value is similar to the value we have reported previously for RBCs lacking aquaporin-1 protein (AQP-1(null)), suggesting that each of the Rh and AQP-1 proteins is responsible for approximately 1/2 of the normal CO2 permeability of the RBC membrane. Four other blood group deficiencies tested lack diverse membrane proteins but exhibit normal CO2 permeability. The CO2 pathway constituted by Rh proteins was inhibitable at pH(e)= 7.4 by NH4Cl with an I50 of approximately 10 mM corresponding to an I50 for NH3 of approximately 0.3 mM. The pathway independent of Rh proteins, presumably that constituted by AQP-1, was not inhibitable by NH4Cl/NH3. However, both pathways were strongly inhibited by DIDS, which accounts for the marked inhibitory effect of DIDS on normal P(CO2), while in contrast another AE1 inhibitor, DiBAC, does not inhibit P(CO2), although it markedly reduces P(HCO3-). We conclude that Rh protein, presumably the Rh-associated glycoprotein RhAG, possesses a gas channel that allows passage of CO2 in addition to NH3.

Development of cytosolic hypoxia and hypoxia-inducible factor stabilization are facilitated by aquaporin-1 expression

O(2) is essential for aerobic life, and the classic view is that it diffuses freely across the plasma membrane. However, measurements of O(2) permeability of lipid bilayers have indicated that it is much lower than previously thought, and therefore, the existence of membrane O(2) channels has been suggested. We hypothesized that, besides its role as a water channel, aquaporin-1 (AQP-1) could also work as an O(2) transporter, because this transmembrane protein appears to be CO(2)-permeable and is highly expressed in cells with rapid O(2) turnover (erythrocytes and microvessel endothelium). Here we show that in mammalian cells overexpressing AQP-1 and exposed to hypoxia, the loss of cytosolic O(2), as well as stabilization of the O(2)-dependent hypoxia-inducible transcription factor and expression of its target genes, is accelerated. In normoxic endothelial cells, knocking down AQP-1 produces induction of hypoxia-inducible genes. Moreover, lung AQP-1 is markedly up-regulated in animals exposed to hypoxia. These data suggest that AQP-1 has O(2) permeability and thus could facilitate O(2) diffusion across the cell membrane.

Expression of water channel proteins (aquaporins) in the rat Eustachian tube and middle ear mucosa

CONCLUSION

Diverse expression of the different subtypes of aquaporins in different parts of the Eustachian tube and middle ear suggests region-specific functions of the aquaporins in the normal physiology of the tubotympanum and also suggests that they may play roles in the pathophysiology of otitis media.

OBJECTIVES

The epithelial cells of the middle ear and Eustachian tube must maintain adequate water balance for normal function of the mucociliary system. Since aquaporins (AQPs) are known to play critical roles in water homeostasis, we investigated their expression in the tubotympanum of the rat.

METHODS

The expression of AQP subtypes 1, 2, 4, 5, and 7 were examined in the rat Eustachian tube and middle ear using RT-PCR, Western blotting, and immunohistochemistry.

RESULTS

Transcripts for AQP 1, 4, and 5 were detected in the Eustachian tube and middle ear. Expression of these molecules at the protein level was confirmed by Western blot analysis. Immunohistochemical analysis demonstrated that AQP 4 was localized to the basolateral membranes of ciliated epithelial cells while AQP 5 was localized to the apical surface of serous gland cells, but not goblet cells, in the rat Eustachian tube. AQP 1 was found to be expressed by the subepithelial fibroblasts.

Molecular mechanisms of conduction and selectivity in aquaporin water channels

Aquaporins (AQP) are a family of membrane channels primarily responsible for conducting water across cellular membranes. The availability of a large body of high resolution structural data along with numerous atomic-scale simulation studies have resulted in an unprecedented level of understanding of the mechanism of function and selectivity in AQP. In this article, after summarizing major highlights of structure-functional studies of AQP, we will report on some of our recent large-scale molecular dynamics simulations investigating the mechanisms of permeation of various substances through pure lipid bilayers and through multiple pathways provided by tetrameric structures of different AQP. Comparison of the results obtained for structurally highly homologous, but functionally distinct, AQP allowed us to identify novel mechanisms of gating and selectivity of these channels and to design mutants with experimentally verified, altered properties. When applicable, special attention will be given to specific aromatic amino acids and their involvement in various functional aspects of AQP.

Carbonic anhydrase II increases the activity of the human electrogenic Na+/HCO3- cotransporter

Several acid/base-coupled membrane transporters, such as the electrogenic sodium-bicarbonate cotransporter (NBCe1), have been shown to bind to different carbonic anhydrase isoforms to create a "transport metabolon." We have expressed NBCe1 derived from human kidney in oocytes of Xenopus leavis and determined its transport activity by recording the membrane current in voltage clamp, and the cytosolic H(+) and Na(+) concentrations using ion-selective microelectrodes. When carbonic anhydrase isoform II (CAII) had been injected into oocytes, the membrane current and the rate of cytosolic Na(+) rise, indicative for NBCe1 activity, increased significantly with the amount of injected CAII (2-200 ng). The CAII inhibitor ethoxyzolamide reversed the effects of CAII on the NBCe1 activity. Co-expressing wild-type CAII or NH(2)-terminal mutant CAII together with NBCe1 provided similar results, whereas co-expressing the catalytically inactive CAII mutant V143Y had no effect on NBCe1 activity. Mass spectrometric analysis and the rate of cytosolic H(+) change following addition of CO(2)/HCO(3)(-) confirmed the catalytic activity of injected and expressed CAII in oocytes. Our results show that the transport capacity of NBCe1 is enhanced by the catalytic activity of CAII, in line with the notion that CAII forms a transport metabolon with NBCe1.

Exploring gas permeability of cellular membranes and membrane channels with molecular dynamics

Aquaporins are a family of membrane proteins specialized in rapid water conduction across biological membranes. Whether these channels also conduct gas molecules and the physiological significance of this potential function have not been well understood. Here we report 140 ns of molecular dynamics simulations of membrane-embedded AQP1 and of a pure POPE bilayer addressing these questions. The permeability of AQP1 to two types of gas molecules, O2 and CO2, was investigated using two complementary methods, namely, explicit gas diffusion simulation and implicit ligand sampling. The simulations show that the central (tetrameric) pore of AQP1 can be readily used by either gas molecule to permeate the channel. The two approaches produced similar free energy profiles associated with gas permeation through the central pore: a -0.4 to -1.7 kcal/mol energy well in the middle, and a 3.6-4.6 kcal/mol energy barrier in the periplasmic vestibule. The barrier appears to be mainly due to a dense cluster of water molecules anchored in the periplasmic mouth of the central pore by four aspartate residues. Water pores show a very low permeability to O2, but may contribute to the overall permeation of CO2 due to its more hydrophilic nature. Although the central pore of AQP1 is found to be gas permeable, the pure POPE bilayer provides a much larger cross-sectional area, thus exhibiting a much lower free energy barrier for CO2 and O2 permeation. As such, gas conduction through AQP1 may only be physiologically relevant either in membranes of low gas permeability, or in cells where a major fraction of the cellular membrane is occupied by AQPs.

CO2 sensing in fungi and beyond

Carbon dioxide is not only an important gaseous molecule for maintenance of the biosphere homeostasis, but is also a crucial signalling cue in living cells. Fungal pathogens, including Candida albicans and Cryptococcus neoformans, must adapt to dramatic changes in CO2 levels during colonization and subsequent infection of their human host. Recent reports provide insight into how pathogenic fungi sense environmental CO2 and the role of carbonic anhydrase and fungal adenylyl cyclase in CO2 sensing.

Comparative efficacy of HgCl2 with candidate aquaporin-1 inhibitors DMSO, gold, TEA+ and acetazolamide

Aquaporin-1 (AQP1) inhibitors are predicted to have multiple clinical applications. Hg(++) is a non-specific and toxic AQP1 blocker. We compared compounds with reported AQP1 inhibition activity, including DMSO, Au(+++), Ag(+), tetraethylammonium and acetazolamide. Water permeability was measured by stopped-flow light scattering in erythrocytes and volume marker dilution in epithelial cells. Au(+++) inhibited AQP1 with IC50 approximately 14 microM, similar to 10 microM for Hg(++). DMSO slowed osmotic equilibration; however, the apparent inhibition was due to 'osmotic clamp' rather than AQP1 inhibition. Neither tetraethylammonium nor acetazolamide (to 10 mM) inhibited AQP1. Our data indicate the need to identify new AQP1 inhibitors.

Evidence against a direct interaction between intracellular carbonic anhydrase II and pure C-terminal domains of SLC4 bicarbonate transporters

Based on solid-phase binding assays with enzyme-linked immunosorbent assay detection, previous investigators suggested that intracellular carbonic anhydrase II (CA II) interacts at high affinity with the C-terminal (Ct) domains of SLC4 bicarbonate-transport proteins, expressed as glutathione S-transferase (GST) fusion proteins, to form functional HCO3- metabolons. Here we re-evaluated this protein-protein interaction using two solid-phase binding assays. We first compared the ability of the Ct domain of three SLC4 transporters, SLC4-A1 (AE1), SLC4-A4 (NBCe1), and SLC4-A8 (NDCBE), to bind immobilized CA II, using enzyme-linked immunosorbent assay detection. We found that when expressed as GST fusion proteins, all three bind to CA II (Kd 300-600 nM) better than does pure GST. However, we detected no binding of pure SLC4-Ct peptides to immobilized CA II. Second, we reversed assay orientation by immobilizing the SLC4-Ct fusion proteins or peptides. We found that more CA II binds to GST than to any of the three GST-SLC4-Ct fusion proteins. Furthermore, we detected no binding of CA II to any of the immobilized pure SLC4-Ct peptides. Finally, we used surface plasmon resonance to detect possible rapid interactions between CA II and the pure peptides. Although we detected acetazolamide binding to immobilized CA II and specific antibodies binding to immobilized SLC4-Ct peptides, we detected no binding of CA II to immobilized SLC4-Ct or vice versa. Thus, although an HCO3 metabolon may exist, CA II cannot bind directly to pure SLC4-Ct peptides and can bind to GST-SLC4-Ct fusion proteins only when the CA II is immobilized and the fusion protein is soluble, and not vice versa.

Tobacco aquaporin NtAQP1 is involved in mesophyll conductance to CO2 in vivo

Leaf mesophyll conductance to CO(2) (g(m)) has been recognized to be finite and variable, rapidly adapting to environmental conditions. The physiological basis for fast changes in g(m) is poorly understood, but current reports suggest the involvement of protein-facilitated CO(2) diffusion across cell membranes. A good candidate for this could be the Nicotiana tabacum L. aquaporin NtAQP1, which was shown to increase membrane permeability to CO(2) in Xenopus oocytes. The objective of the present work was to evaluate its effect on the in vivo mesophyll conductance to CO(2), using plants either deficient in or overexpressing NtAQP1. Antisense plants deficient in NtAQP1 (AS) and NtAQP1 overexpressing tobacco plants (O) were compared with their respective wild-type (WT) genotypes (CAS and CO). Plants grown under optimum conditions showed different photosynthetic rates at saturating light, with a decrease of 13% in AS and an increase of 20% in O, compared with their respective controls. CO(2) response curves of photosynthesis also showed significant differences among genotypes. However, in vitro analysis demonstrated that these differences could not be attributed to alterations in Rubisco activity or ribulose-1,5-bisphosphate content. Analyses of chlorophyll fluorescence and on-line (13)C discrimination indicated that the observed differences in net photosynthesis (A(N)) among genotypes were due to different leaf mesophyll conductances to CO(2), which was estimated to be 30% lower in AS and 20% higher in O compared with their respective WT. These results provide evidence for the in vivo involvement of aquaporin NtAQP1 in mesophyll conductance to CO(2).

Evidence that aquaporin 1 is a major pathway for CO2 transport across the human erythrocyte membrane

We report here the application of a previously described method to directly determine the CO2 permeability (P(CO2)) of the cell membranes of normal human red blood cells (RBCs) vs. those deficient in aquaporin 1 (AQP1), as well as AQP1-expressing Xenopus laevis oocytes. This method measures the exchange of (18)O between CO2, HCO3(-), and H2O in cell suspensions. In addition, we measure the alkaline surface pH (pH(S)) transients caused by the dominant effect of entry of CO2 vs. HCO3(-) into oocytes exposed to step increases in [CO2]. We report that 1) AQP1 constitutes the major pathway for molecular CO2 in human RBCs; lack of AQP1 reduces P(CO2) from the normal value of 0.15 +/- 0.08 (SD; n=85) cm/s by 60% to 0.06 cm/s. Expression of AQP1 in oocytes increases P(CO2) 2-fold and doubles the alkaline pH(S) gradient. 2) pCMBS, an inhibitor of the AQP1 water channel, reduces P(CO2) of RBCs solely by action on AQP1 as it has no effect in AQP1-deficient RBCs. 3) P(CO2) determinations of RBCs and pH(S) measurements of oocytes indicate that DIDS inhibits the CO2 pathway of AQP1 by half. 4) RBCs have at least one other DIDS-sensitive pathway for CO2. We conclude that AQP1 is responsible for 60% of the high P(CO2) of red cells and that another, so far unidentified, CO2 pathway is present in this membrane that may account for at least 30% of total P(CO2).

Acid-base transport by the renal proximal tubule

One of the major tasks of the renal proximal tubule is to secrete acid into the tubule lumen, thereby reabsorbing approximately 80% of the filtered HCO3- as well as generating new HCO3- for regulating blood pH. This review summarizes the cellular and molecular events that underlie four major processes in HCO3- reabsorption. The first is CO2 entry across the apical membrane, which in large part occurs via a gas channel (aquaporin 1) and acidifies the cell. The second process is apical H+ secretion via Na-H exchange and H+ pumping, processes that can be studied using the NH4+ prepulse technique. The third process is the basolateral exit of HCO3- via the electrogenic Na/HCO3 co-transporter, which is the subject of at least 10 mutations that cause severe proximal renal tubule acidosis in humans. The final process is the regulation of overall HCO3- reabsorption by CO2 and HCO3- sensors at the basolateral membrane. Together, these processes ensure that the proximal tubule responds appropriately to acute acid-base disturbances and thereby contributes to the regulation of blood pH.

Myocardial water handling and the role of aquaporins

Cardiac surgery is performed in approximately 770,000 adults and 30,000 children in the United States of America annually. In this review we outline the mechanistic links between post-operative myocardial stunning and the development of myocardial edema. These interrelated processes cause a decline in myocardial performance that account for significant morbidity and mortality after cardiac surgery. Factors leading to myocardial edema include hemodilution, ischemia and reperfusion as well as osmotic gradients arising from pathological change. Several members of the aquaporin family of water transport proteins have been described in the myocardium although their role in the pathogenesis and resolution of cardiac edema is not established. This review examines evidence for the involvement of aquaporins in myocardial water handling during normal and pathological conditions.

Aquaporin-1 Transports NO Across Cell Membranes

NO plays a role in the regulation of blood pressure through its effects on renal, cardiovascular, and central nervous system function. It is generally thought to freely diffuse through cell membranes without need for a specific transporter. The water channel aquaporin-1 transports low molecular weight gases in addition to water and is expressed in cells that produce or are the targets of NO. Consequently, we tested the hypothesis that aquaporin-1 transports NO. In cells expressing aquaporin-1, NO permeability correlated with water permeability. NO transport was reduced by 71% by HgCl2, an inhibitor of aquaporin-1. Transport of NO by aquaporin-1 saturated at 3 micromol/L NO and displayed a K(1/2) (the concentration of NO that produces half of the maximum transport rate) of 0.54 micromol/L. Reconstitution of purified aquaporin-1 into lipid vesicles increased NO influx by 316%. In endothelial cells, lowering aquaporin-1 expression with a small interfering RNA (siRNA) blunted aquaporin-1 expression by 54% and NO release by 44%. We conclude that NO transport by aquaporin-1 may allow cells to control intracellular NO levels and effects. NO transport by aquaporin-1 may play a role in central nervous system, vascular and renal function, and consequently blood pressure. Disruption of NO transport by aquaporin-1 offers an alternate cause for diseases currently explained by inadequate NO bioavailability.

Does CO2 permeate through aquaporin-1?

Aquaporins facilitate water permeation across biological membranes. Additionally, glycerol and other small neutral solutes are permeated by related aquaglyceroporins. The role of aquaporins in gas permeation has been a long-standing and controversially discussed issue. We present an extensive set of atomistic molecular dynamics simulations that address the question of CO(2) permeation through human aquaporin-1. Free energy profiles derived from the simulations display a barrier of approximately 23 kJ/mol in the aromatic/arginine constriction region of the water pore, whereas a barrier of approximately 4 kJ/mol was observed for a palmitoyloleoylphosphatidylethanolamine lipid bilayer membrane. The results indicate that significant aquaporin-1-mediated CO(2) permeation is to be expected only in membranes with a low intrinsic CO(2) permeability.

Aquaporins in plants

Although very often exposed to a rapid changing environment, plants are in general unable to evade from unfavourable conditions. Therefore, a fine tuned adaptation of physiology including the water balance appears to be of crucial importance. As a consequence a relatively large number of aquaporin genes are present in plant genomes. So far aquaporins in plants were shown to be involved in root water uptake, reproduction or photosynthesis. Accordingly, plant aquaporin classification as simple water pores has changed corresponding to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the respective physiological process could be achieved by regulation mechanisms, which range from post-translational modification, molecular trafficking to heteromerization of aquaporin isoforms. Here the function of the four plant aquaporin family subclasses with regard to substrate specificity, regulation and physiological relevance is described.

Structure-Function Relationships in Aquaporins

The identification of members of the aquaporin family as the primary water channels of cell membranes has been followed up by an intense effort to determine how these channels work. Specifically, investigators have sought to learn why these channels are selective for water and how they exclude proton trafficking. Molecular-dynamics studies using elegant, extremely detailed computer models based on accurate crystallographic maps of the channels show the basis for the selectivity of the channel. Channel size, the location of hydrophobic amino-acid side chains, and specific interactions of water dipoles with a charged residue near the most constricted point of the channel indicate that water molecules travel in single file through the center of the channel, and that the orientation of water molecules is manipulated to prevent the formation of a water wire spanning the channel. Finally, the number of water molecules calculated to be aligned in single file in the channel constriction fits predictions based on classic studies of the osmotic permeability: diffusive permeability ratios in water-permeable membranes.

Functional aquaporin diversity in plants

Due to the fact that most plants are immobile, a rapid response of physiological processes to changing environmental conditions is essential for their survival. Thus, in comparison to many other organisms, plants might need a more sophisticated tuning of water balance. Among others, this is reflected by the comparable large amount of aquaporin genes in plant genomes. So far, aquaporins were shown to be involved in many physiological processes like root water uptake, reproduction or photosynthesis. Their classification as simple water pores has changed according to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the corresponding physiological process could be achieved by regulation mechanisms. Concerning aquaporins these range from posttranslational modification, molecular trafficking to heteromerization of aquaporin isoforms. The aim of this review is to underline the function of the four plant aquaporin family subclasses with regard to the substrate specificity, regulation and physiological relevance.

Modulating the expression of aquaporin genes in planta: A key to understand their physiological functions?

Aquaporins (AQPs) are believed to act as "cellular plumbers", allowing plants to rapidly alter their membrane water permeability in response to environmental cues. This study of AQP regulation at both the RNA and protein levels has revealed a large number of possible mechanisms. Currently, modulation of AQP expression in planta is considered the strategy of choice for elucidating the role of AQPs in plant physiology. This review highlights the fact that this strategy is complicated by many factors, such as the incomplete characterization of transport selectivity of the targeted AQP, the fact that AQPs might act as multifunctional channels with multiple physiological roles, and the number of post-translational regulation mechanisms. The classification of AQPs as constitutive or stress-responsive isoforms is also proposed.

Red cell membrane CO2 permeability in normal human blood and in blood deficient in various blood groups, and effect of DIDS

The red cell membrane has an exceptionally high permeability for CO2, PCO2 approximately 0.15 cm/s, which is two to three orders of magnitude greater than that of some epithelial membranes and similarly greater than the permeability of the red cell membrane for HCO3-. As shown previously, this high PCO2 can be drastically inhibited by 10 microM 4,4'-diisothiocyanato-2,2'-stilbenedisulfonate (DIDS), indicating that membrane proteins may be involved in this high gas permeability. Here, we have studied the possible contribution of several blood group proteins to CO2 permeation across the red cell membrane by comparing PCO2 of red cells deficient in specific blood group proteins with that of normal red cells. While PCO2 of normal red cells is approximately 0.15 cm/s and that of Fy(null) and Jk(null) red cells is similar, PCO2's of Colton null (deficient in aquaporin-1) and Rh(null) cells (deficient in Rh/RhAG) are both reduced to about 0.07 cm/s, i.e. to about one half. In addition, the inhibitory effect of DIDS is about half as great in Rh(null) and in Colton null red cells as it is in normal red cells. We conclude that aquaporin-1 and Rh/RhAG proteins contribute substantially to the high permeability of the human red cell membrane for CO2. Together these proteins are responsible for 50% or more of the CO2 permeability of red cell membranes. The CO2 pathways of both proteins can be partly inhibited by DIDS, which is why this compound very effectively reduces membrane CO2 permeability.

Electrogenic ammonium transport by renal Rhbg

The recently cloned, non-erythrocyte Rh glycoproteins (Rhbg and Rhcg) are expressed in the intercalated cells of the renal collecting duct. The apical Rhcg and the basolateral Rhbg are likely involved in NH3 and/or NH4+ transport, yet the characteristics of this transport are not yet certain. In this study we investigated the mechanism of NH4+ transport by Rhbg and Rhcg expressed in Xenopus oocytes. We used a two-electrode voltage-clamp and ion-selective microelectrodes to measure NH4+-induced currents (I(NH4)) and changes in pHi, respectively. In oocytes expressing Rhcg, exposure to bath [NH4+] of 2.5-20 mM induced inward currents that were slightly more than those in H2O-injected (control) oocytes. I-V plots in the presence of NH4+ showed a small increase in slope conductance only at positive potentials. On the other hand, in oocytes expressing Rhbg, 5 mM NH4+ induced an inward I(NH4) of -79 nA, decreased pHi (DeltapHi) by 0.13 at a rate (dpHi/dt) of -2 7 x 10(-4) pH/s and depolarized the cell by 45 mV. These changes were significantly more than those in control oocytes. I-V plots in the presence of NH4+ showed substantial increase in conductance. Amiloride (1 mM) inhibited I(NH4), DeltapHi and dpHi/dt in oocytes expressing Rhbg but not in control oocytes. Raising bath [NH4+] in increments from 1 to 20 mM elicited a faster dpHi/dt, a larger decrease in pHi and a larger depolarization. Net NH4+ flux by Rhbg (estimated from dpHi/dt) was proportional to [NH4+] gradient and followed saturation kinetics with an apparent Km of 2.3 mM. Methyl ammonium (5 mM) induced a current of -63 nA in Rhbg oocytes but did not cause any change in control oocytes. These data indicate that: 1) Rhbg transport of NH4+ is electrogenic. 2) Methyl ammonium is transported by Rhbg. 3) NH4+ transport by Rhbg is saturated at high concentrations with Michaelis-Menten kinetics.

Role of RhAG and AQP1 in NH3 and CO2 gas transport in red cell ghosts: a stopped-flow analysis

To clarify the potential role Rh/RhAG and AQP1 proteins in erythrocyte gas transport, NH3 and CO2 transport was measured in erythrocyte ghost membrane vesicles from rare human variants (Rh(null), CO(null),) and knockout mice (homozygous AQP1-/-, Rh-/- and Rhag-/-) exhibiting well-characterized protein defects. Transport was measured from intracellular pH (pHi) changes in a stopped-flow fluorimeter. NH3 transport was measured in chloride-free conditions with ghosts exposed to 20 mM inwardly directed gradients of gluconate salts of ammonium, hydrazine and methylammonium at 15 degrees C. Alkalinization rates of control samples were 6.5+/-0.3, 4.03+/-0.17, 0.95+/-0.08 s(-1) for each solute, respectively, but were significantly reduced for Rh(null) and CO(null) samples that are deficient in RhAG and AQP1 proteins, respectively. Alkalinization rates of Rh(null) ghosts were about 60%, 83% and 94% lower than that in control ghosts, respectively, for each solute. In CO(null) ghosts, the lack of AQP1 resulted in about 30% reduction of the alkalinization rates as compared to controls, but the transport selectivity of RhAG for the three solutes was preserved. Similar observations were made with ghosts from KO mice Rhag-/- and AQP1-/-. These results confirm the major contribution of RhAG/Rhag in the NH3 conductance of erythrocytes and suggest that the reduction of transport rates in the absence of AQP1 would be better explained by a direct or indirect effect on RhAG/Rhag-mediated transport. When ghosts were preloaded with carbonic anhydrase and exposed to a 25 mM CO2/HCO3- gradient at 6 degrees C, an extremely rapid kinetics of acidification corresponding to CO2 influx was observed. The rate constants were not significantly different between controls and human variants (125+/-6 s(-1)), or between wild-type and KO mice, suggesting no major role of RhAG or AQP1 in CO2 transport, at least in our experimental conditions.

Irradiance and phenotype: comparative eco-development of sun and shade leaves in relation to photosynthetic CO2 diffusion

The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO2 diffusion within a leaf is examined first. Because affinity of Rubisco for CO2 is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O2, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C3 plants to maintain the CO2 concentration in the chloroplast as high as possible. Since the internal conductance for CO2 diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C3 leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO2 dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or 'how-oriented'. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.

Quantitative modeling of chloride conductance in yeast TRK potassium transporters

So-called TRK proteins are responsible for active accumulation of potassium in plants, fungi, and bacteria. A pair of these proteins in the plasma membrane of Saccharomyces cerevisiae, ScTrk1p and ScTrk2p, also admit large, adventitious, chloride currents during patch-recording (Cl- efflux). Resulting steady-state current-voltage curves can be described by two simple kinetic models, most interestingly, voltage-driven channeling of ions through a pair of activation-energy barriers that lie within the membrane dielectric, near the inner (alpha) and outer (beta) surfaces. Two barrier heights (E(alpha) and E(beta)) and two relative distances (a1 and b2) from the surfaces specify the model. Measured current amplitude parallels intracellular chloride concentration and is strongly enhanced by acidic extracellular pH. The former implies an exponential variation of a1, between approximately 0.2 and approximately 0.4 of the membrane thickness, whereas the latter implies a linear variation of E(beta), by 0.69 Kcal mol(-1)/pH. The model requires membrane slope conductance to rise exponentially with increasingly large negative membrane voltage, as verified by data from a few yeast spheroplasts that tolerated voltage clamping at -200 to -300 mV. The behaviors of E(beta) and a1 accord qualitatively with a hypothetical structural model for fungal TRK proteins, suggesting that chloride ions flow through a central pore formed by symmetric aggregation of four TRK monomers.

Aquaporin Water Channels

Thank you very much. I am humbled, I am delighted; I am honored. This is every scientist's dream: to give the Nobel Lecture in Stockholm. But I would not be honest if I did not tell you that I am having a little anxiety being on this platform. I have lectured a number of times in Sweden, and I thought I would share with you some events preceding a special lecture that I gave here a few years ago. Arriving at Arlanda Airport, I waited in line at the Pass Control behind a group of businessmen in suits with briefcases. I heard the first in line asked by the control officer to state the purpose of his visit to Sweden. When the man replied “business,” the officer approved and stamped his passport. One at a time, each stepped forward and was asked the same thing; each answered “business” and was approved. Eventually it was my turn, and I was dressed in rumpled clothes after spending the night in the Economy Minus section of an SAS jetliner. The officer asked me the purpose of my visit, and I said “I am here to give the von Euler Lecture at Karolinska Institute.” The officer immediately looked up, stared at me, and asked, “Are you nervous?” At that point I became intensely nervous and said “Yes, I am a little nervous.” The officer looked up again and stated “Well, you should be!“ So if the lecturers look a little nervous, the problem is at Arlanda.

Low carbon dioxide permeability of the apical epithelial membrane of guinea-pig colon

We have investigated the apical membrane permeability for CO2 of intact epithelia of proximal and distal colon of the guinea pig. The method used was the mass spectrometric 18O-exchange technique previously described. In a first step, we determined the intraepithelial carbonic anhydrase (CA) activity by studying vital isolated colonocytes before and after lysis with Triton X-100. Intraepithelial CA activity was found to be 41,000 and 900 for proximal and distal colon, respectively. Then 18O-exchange measurements were done with stripped intact epithelial layers, which on their apical side were exposed to the reaction solution containing 18O-labelled CO2 and HCO3-. The mass spectrometric signals in these measurements are determined by the intracellular epithelial CA activity, and by the apical membrane permeabilities for CO2 and HCO3-, P(CO2) and P(HCO3). From the signals, we calculated the two permeabilities while inserting the CA activities obtained from isolated colonocytes. From layers of intact colon epithelium, the apical P(CO2) was determined to be 1.5 x 10(-3) cm s(-1) for proximal and 0.77 x 10(-3) cm s(-1) for distal colon. These values are > or =200 times lower than the P(CO2) of the human red cell membrane as studied with the same technique (0.3 cm s(-1)). We conclude that the apical membrane offers a significant resistance towards CO2 diffusion, which implies that a major drop in CO2 partial pressure (pCO2) will occur across the apical membrane when luminal pCO2 is higher than basolateral or capillary pCO2. In view of the very high pCO2 that can occur in the colonic lumen, this property of the apical membrane constitutes a significant protection of the cell against the high acid load associated with high pCO2.