Molecular mechanisms for passive and active transport of water

Poplar aquaporin PIP1;1 promotes Arabidopsis growth and development

BACKGROUND

Root hydraulic conductance is primarily determined by the conductance of living tissues to radial water flow. Plasma membrane intrinsic proteins (PIPs) in root cortical cells are important for plants to take up water and are believed to be directly involved in cell growth.

RESULTS

In this study, we found that constitutive overexpression of the poplar root-specific gene PtoPIP1;1 in Arabidopsis accelerated bolting and flowering. At the early stage of the developmental process, PtoPIP1;1 OE Arabidopsis exhibited faster cell growth in both leaves and roots. The turgor pressure of plants was correspondingly increased in PtoPIP1;1 OE Arabidopsis, and the water status was changed. At the same time, the expression levels of flowering-related genes (CRY1, CRY2 and FCA) and hub genes in the regulatory networks underlying floral timing (FT and SOC1) were significantly upregulated in OE plants, while the floral repressor FLC gene was significantly downregulated.

CONCLUSIONS

Taken together, the results of our study indicate that constitutive overexpression of PtoPIP1;1 in Arabidopsis accelerates bolting and flowering through faster cell growth in both the leaf and root at an early stage of the developmental process. The autonomous pathway of flowering regulation may be executed by monitoring developmental age. The increase in turgor and changes in water status with PtoPIP1;1 overexpression play a role in promoting cell growth.

The Balance of Fluid and Osmotic Pressures across Active Biological Membranes with Application to the Corneal Endothelium

The movement of fluid and solutes across biological membranes facilitates the transport of nutrients for living organisms and maintains the fluid and osmotic pressures in biological systems. Understanding the pressure balances across membranes is crucial for studying fluid and electrolyte homeostasis in living systems, and is an area of active research. In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments. The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges. The steady-state analysis on active membranes suggests a new pressure mechanism which balances the fluid pressure together with the osmotic pressure. The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport. We apply the proposed theory to study the transendothelial fluid pressure in the in vivo cornea, which is a crucial factor maintaining the hydration and transparency of the tissue. The results show the importance of the proposed pressure mechanism in mediating stromal fluid pressure and provide a new interpretation of the pressure modulation mechanism in the in vivo cornea.

Claudin-2, a component of the tight junction, forms a paracellular water channel

Whether or not significant amounts of water pass the tight junction (TJ) of leaky epithelia is still unresolved, because it is difficult to separate transcellular water flux from TJ-controlled paracellular water flux. Using an approach without differentiating technically between the transcellular and paracellular route, we measured transepithelial water flux with and without selective molecular perturbation of the TJ to unequivocally attribute changes to the paracellular pathway. To this end, MDCK C7 cells were stably transfected with either claudin-2 or claudin-10b, two paracellular cation-channel-forming TJ proteins that are not endogenously expressed in this cell line. Claudin-2 is typical of leaky, water-transporting epithelia, such as the kidney proximal tubule, whereas claudin-10b is present in numerous epithelia, including water-impermeable segments of the loop of Henle. Neither transfection altered the expression of endogenous claudins or aquaporins. Water flux was induced by an osmotic gradient, a Na+ gradient or both. Under all conditions, water flux in claudin-2-transfected cells was elevated compared with vector controls, indicating claudin-2-mediated paracellular water permeability. Na+-driven water transport in the absence of an osmotic gradient indicates a single-file mechanism. By contrast, claudin-10b transfection did not alter water flux. We conclude that claudin-2, but not claudin-10b, forms a paracellular water channel and thus mediates paracellular water transport in leaky epithelia.

A collaboration of aquaporins handles water transport in relation to the estrous cycle in the bitch uterus

Fluid movement through uterine cell membranes is crucial, as it can modulate the tissue imbibition pattern in the different phases of the estrous cycle. To gain insight into the mechanisms underlying steroid-controlled water handling, the presence and distribution of aquaporins (AQPs), integral membrane channel proteins permitting rapid passive water movement, was explored in bitch uterine tissues. Immunohistochemistry and Western immunoblot analysis were used to study the presence of AQP1, AQP2, and AQP5 in the layers of the bitch uterine wall during the different estrous phases. Presence of endothelial nitric oxide-generating enzyme NO synthase (NOS3) was also investigated, as it is known that the vasodilator NOS3 might be involved in the development of uterine edema. The results demonstrated the following: (1) AQP1, AQP2, and AQP5 were present in the uterus of cycling bitches. (2) AQP1 was localized within uterine mesometrial, myometrial, and endometrial blood vessels and in the circular and longitudinal layers of myometrium. AQP1 localization and expression were unaffected by the estrous cycle. (3) The estrogenic milieu was probably at the basis of AQP2 expression in the glandular and luminal epithelium of the endometrium. (4) AQP5 water channels were present in the apical plasma membrane of uterine epithelial cells in coincidence with plasma progesterone increase. (5) NOS3 was localized in the myometrial and epithelial tissues as well as in blood vessels indicating a contribution of this vasoactive peptide to the uterine imbibition processes. Thus, we can hypothesize that a functional and distinctive collaboration exists among diverse AQPs in water handling during the different functional uterine phases.

Relevance of diffusion through bacterial spore coats/membranes and the associated concentration boundary layers in the initial lag phase of inactivation: A case study for Bacillus subtilis with ozone and monochloramine

Disinfectants are generally used to inactivate microorganisms in solutions. The process of inactivation involves the disinfectant in the liquid diffusing towards the bacteria sites and thereafter reacting with bacteria at rates determined by the respective reaction rates. Such processes have demonstrated an initial lag phase followed by an active depletion phase of bacteria. This paper attempts to study the importance of the combined effects of diffusion of the disinfectant through the outer membrane of the bacteria and transport through the associated concentration boundary layers (CBLs) during the initial lag phase. Mathematical equations are developed correlating the initial concentration of the disinfectant with time required for reaching a critical concentration (C*) at the inner side of the membrane of the cell based on diffusion of disinfectant through the outer membranes of the bacteria and the formation of concentration boundary layers on both sides of the membranes. Experimental data of the lag phases of inactivation already available in the literature for inactivation of Bacillus subtilis spores with ozone and monochloramine are tested with the equations. The results seem to be in good agreement with the theoretical equations indicating the importance of diffusion process across the outer cell membranes and the resulting CBL's during the lag phase of disinfection.

Expression and immunolocalization of aquaporin-7 in rat gastrointestinal tract

BACKGROUND INFORMATION

In the gastrointestinal tract of mammals, water can either be secreted with digestive juices or absorbed by the small and large intestine. Transcellular water movement can be mediated by the transmembrane protein family of AQPs (aquaporins), as has also been recently identified in the gastrointestinal tract. However, the localization, expression and functioning of AQPs in the gastrointestinal tract have not been completely characterized. For the present study, we investigated: (1) the expression of AQP7 in some portions of rat gastrointestinal tract by semiquantitative reverse transcriptase-PCR and by immunoblotting and (2) the cellular and subcellular localization of AQP7 by immunohistochemistry.

RESULTS

AQP7 mRNA and proteins were highly expressed in the small intestine, weakly in the caecum, colon and rectum and were absent in the stomach. Immunoblotting analysis using rat gastrointestinal tract membrane fractions showed two major bands corresponding to a molecular mass of approx. 34 and 40 kDa for the AQP7 protein. No bands were observed when the anti-AQP7 antibody was preadsorbed with the immunizing peptide. Immunohistochemistry revealed strong AQP7 labelling in the surface epithelial cells of duodenum, jejunum, ileum, caecum, colon and rectum, whereas weak or no labelling was observed in the crypt cells. The labelling was manifest particularly in the apical membrane but intracellular staining was also observed.

CONCLUSIONS

The results indicate that AQP7 is present in the small and large intestine. The higher expression of AQP7 protein at the apical pole of the superficial epithelial cells suggests its involvement in rapid fluid movement through the villus epithelium.

Evolution of concentration field in a membrane system

This paper deals with the evolution of concentration field at a single membrane system. Concentration field evolution is described by concentration effect of stable boundary layers, which originate in this system. The concentration effect of boundary layers (CBLE) is studied experimentally on the basis concentration profiles obtained from computer analysis of interferometric pictures of near-membrane regions. Besides experimental results, we also report theoretical investigations and numerical calculations of this effect for two models of membranes (an infinite thin wall and the wall of thickness l). Evolution of concentration field at different distances from membrane surface describes accurately the spatio-temporal structure of the concentration boundary layers (CBLs). Results have shown that their spatial structure is fully established and these layers develop diffusively.

Water transport in the brain: Role of cotransporters

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.

Isolation and characterization of NaCl-sensitive mutants of Caulobacter crescentus

An attempt to characterize Caulobacter crescentus genes important for the response to high concentrations of NaCl was initiated by the isolation of mutants defective in survival in the presence of 85 mM NaCl. A transposon Tn5 library was screened, and five strains which contained different genes disrupted by the transposon were isolated. Three of the mutants had the Tn5 in genes involved in lipopolysaccharide biosynthesis, one had the Tn5 in the nhaA gene, which encodes a Na(+)/H(+) antiporter, and one had the Tn5 in the ppiD gene, which encodes a peptidyl-prolyl cis-trans isomerase. All the mutant strains showed severe growth arrest in the presence of 85 mM NaCl, but only the nhaA mutant showed decreased viability under these conditions. All the mutants except the nhaA mutant showed a slightly reduced viability in the presence of 40 mM KCl, but all the strains showed a more severe reduction in viability in the presence of 150 mM sucrose, suggesting that they are defective in responding to osmotic shock. The promoter regions of each disrupted gene were cloned in lacZ reporter vectors, and the pattern of expression in response to NaCl and sucrose was determined; this showed that both agents induced ppiD and nhaA gene expression but did not induce the other genes. Furthermore, the ppiD gene was not induced by heat shock, indicating that it does not belong to the sigma(32) regulon, as opposed to what was observed for its Escherichia coli homolog.

Peripheral opioidergic regulation of the tracheobronchial mucociliary transport system

We hypothesized that, in the airway mucosa, opioids are inhibitory neural modulators that cause an increase in net water absorption in the airway mucosa (as in the gut). Changes in bidirectional water fluxes across ovine tracheal mucosa in response to basolateral application of the opioid peptides beta-endorphin, dynorphin A-(1-8), and [d-Ala(2), d-Leu(5)]-enkephalin (DADLE) were measured. beta-Endorphin and dynorphin A-(1-8) decreased luminal-to-basolateral water fluxes, and dynorphin A-(1-8) and DADLE increased basolateral-to-luminal water flux. These responses were electroneutral. In seven beagle dogs, administration of aerosolized beta-endorphin (1 mg) to the tracheobronchial airways decreased the clearance of radiotagged particles from the bronchi in 1 h from 34.7 to 22.0% (P < 0.001). Naloxone abrogated the beta-endorphin-induced changes in vitro and in vivo. Contrary to our hypothesis, the opioid-induced changes in water fluxes would all lead to a predictable increase in airway surface fluid. The beta-endorphin-induced increases in airway fluid together with reduced bronchial mucociliary clearance may produce procongestive responses when opioids are administered as antitussives.

Aquaporin water channel genes are differentially expressed and regulated by ovarian steroids during the periimplantation period in the mouse

The periimplantation period is marked by edematous changes in the uterus. In the mouse, increased uterine vascular permeability occurs in response to estrogen and certain vasoactive mediators, but the mechanisms that regulate fluid transport during implantation are not fully understood. Aquaporins (AQPs) are a family of membrane channel proteins that facilitate bulk water transport. To assess their role in implantation, we examined the expression of AQPs 0-9 in the mouse uterus on d 1-8 of pregnancy. Our results show distinct uterine expression patterns for AQP1, AQP4, and AQP5. AQP1 is localized to the inner circular myometrium throughout the periimplantation period. AQP4 is highly expressed in the luminal epithelium on d 1 of pregnancy but barely detectable at the time of implantation. AQP5 is expressed at low levels in the glandular epithelium during early pregnancy but is markedly increased on d 5. By immunohistochemistry, AQP5 is localized in the basolateral region of the uterine glands. Treatment of adult ovariectomized mice with replacement steroids demonstrates an estrogen-induced shift in AQP1 signals from the myometrium to the uterine stromal vasculature, suggesting a role in uterine fluid imbibition. In contrast, AQP5 is induced only in estrogen-treated, progesterone-primed uteri. We also observed expression of AQP8 in the inner-cell mass and AQP9 in the mural trophectoderm of the implanting blastocyst. Collectively, these results suggest that members of the AQP family are involved in embryo and uterine fluid homeostasis during implantation.

General models for water transport across leaky epithelia

The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.

Passive water transport in biological pores

Three kinds of membrane proteins have been shown to have water channels properties: the aquaporins, the cotransporters, and the uniports. A molecular-kinetic description of water transport in pores is compared to analytical models based on macroscopic parameters such as pore diameter and length. The use and limitations of irreversible thermodynamics is discussed. Experimental data on water and solute permeability in aquaporins are reviewed. No unifying transport model based on macroscopic parameters can be set up; for example, there is no correlation between solute diameter and permeability. Instead, the influence of hydrogen bonds between solute and pore, and the pH dependence of permeability, point toward a model based upon chemical interactions. The atomic model for AQP1 based on electron crystallographic data defines the dimensions and chemical nature of the aqueous pore. These structural data combined with quantum mechanical modeling and computer simulation might result in a realistic description of water transport. Data on water and solute permeability in cotransporters and uniports are reviewed. The function of these proteins as substrate transporters involves a series of conformational changes. The role of conformational equilibria on the water permeability will be discussed.

Cotransporters as molecular water pumps

Molecular water pumps are membrane proteins of the cotransport type in which a flux of water is coupled to substrate fluxes by a mechanism within the protein. Free energy can be exchanged between the fluxes. Accordingly, the flux of water may be relatively independent of the external water chemical potential and can even proceed uphill. In short, water is being cotransported. The evidence for water cotransport is reviewed with particular emphasis on electrogenic cotransporters expressed in Xenopus oocytes under voltage clamped conditions. Phenomena such as uphill water transport, tight coupling between water transport and clamp current, cotransport of small hydrophilic molecules, and shifts in reversal potentials with osmolarity are discussed with examples from the Na+/glutamate and Na+/glucose cotransporters. Unstirred layers and electrode artifacts as alternative explanations for such cotransport can be ruled out for both experimental and theoretical reasons. Indeed, substrate fluxes mediated by channels or ionophores generate much smaller water fluxes than those observed with cotransporters. Theoretical models, using reasonable values for the intracellular diffusion coefficient, indicate the presence of only small unstirred layers in the membranes studied.

Mobility of ions, sugar, and water in the cytoplasm of Xenopus oocytes expressing Na(+)-coupled sugar transporters (SGLT1)

A model was set up to study water transport in membrane proteins expressed in Xenopus oocytes. The model was tested experimentally using human and rabbit Na+-glucose cotransporters (SGLT1), and was used to explain controversies regarding unstirred layer effects. Cotransport of Na+, sugar and water was monitored by two-electrode voltage clamp and online measurements of oocyte volume. The specific resistance of the oocyte cytoplasm was found by means of microelectrodes to be 263 +/- 91 Omega cm (S.D., n = 52), or 2.5 times that of Kulori medium, in agreement with reported values of intracellular ion concentrations and diffusion constants. Osmotically induced volume and resistance changes were compatible with a model of the oocyte in which 37 +/- 17 % (S.D., n = 66) of the intracellular volume acts as a free solution while the remainder is inert, being occupied by organelles, etc. The model explains the results of several types of experiments: rapid changes in rates of water cotransport induced by changes in clamp voltage followed by osmotic equilibration in sugar-free conditions; volume changes induced by Na+ transport via the ionophore gramicidin; and uphill water transport. Ethanol (0.5 %) induced a marked swelling of the oocytes of about 16 pl x s(-1). If the specific inhibitor of SGLT1 phlorizin is added from stock solutions in ethanol, the effect of ethanol obfuscates the effects of the inhibitor. We conclude that the transport parameters derived for water cotransport by the SGLT1 can be attributed to the protein residing in the plasma membrane with no significant influences from unstirred layer effects.

mRNAs encoding aquaporins are present during murine preimplantation development

The present study was conducted to investigate the mechanisms underlying fluid movement across the trophectoderm during blastocyst formation by determining whether aquaporins (AQPs) are expressed during early mammalian development. AQPs belong to a family of major intrinsic membrane proteins and function as molecular water channels that allow water to flow rapidly across plasma membranes in the direction of osmotic gradients. Ten different AQPs have been identified to date. Murine preimplantation stage embryos were flushed from the oviducts and uteri of superovulated CD1 mice. Reverse transcription-polymerase chain reaction (RT-PCR) methods employing primer sets designed to amplify conserved sequences of AQPs (1-9) were applied to murine embryo cDNA samples. PCR reactions were conducted for up to 40 cycles involving denaturation of DNA hybrids at 95 degrees C, primer annealing at 52-60 degrees C and extension at 72 degrees C. PCR products were separated on 2% agarose gels and were stained with ethidium bromide. AQP PCR product identity was confirmed by sequence analysis. mRNAs encoding AQPs 1, 3, 5, 6, 7, and 9 were detected in murine embryos from the one-cell stage up to the blastocyst stage. AQP 8 mRNAs were not detected in early cleavage stages but were present in morula and blastocyst stage embryos. The results were confirmed in experimental replicates applied to separate embryo pools of each embryo stage. These results demonstrate that transcripts encoding seven AQP gene products are detectable during murine preimplantation development. These findings predict that AQPs may function as conduits for trophectoderm fluid transport during blastocyst formation.

Molecular water pumps

There is good evidence that cotransporters of the symport type behave as molecular water pumps, in which a water flux is coupled to the substrate fluxes. The free energy stored in the substrate gradients is utilized, by a mechanism within the protein, for the transport of water. Accordingly, the water flux is secondary active and can proceed uphill against the water chemical potential difference. The effect has been recognized in all symports studied so far (Table 1). It has been studied in details for the K+/Cl- cotransporter in the choroid plexus epithelium, the H+/lactate cotransporter in the retinal pigment epithelium, the intestinal Na+/glucose cotransporter (SGLT1) and the renal Na+/dicarboxylate cotransporter both expressed in Xenopus oocytes. The generality of the phenomenon among symports with widely different primary structures suggests that the property of molecular water pumps derives from a pattern of conformational changes common for this type of membrane proteins. Most of the data on molecular water pumps are derived from fluxes initiated by rapid changes in the composition of the external solution. There was no experimental evidence for unstirred layers in such experiments, in accordance with theoretical evaluations. Even the experimental introduction of unstirred layers did not lead to any measurable water fluxes. The majority of the experimental data supports a molecular model where water is cotransported: A well defined number of water molecules act as a substrate on equal footing with the non-aqueous substrates. The ratio of any two of the fluxes is constant, given by the properties of the protein, and is independent of the driving forces or other external parameters. The detailed mechanism behind the molecular water pumps is as yet unknown. It is, however, possible to combine well established phenomena for enzymes into a working model. For example, uptake and release of water is associated with conformational changes during enzymatic action; a specific sequence of allosteric conformations in a membrane bound enzyme would give rise to vectorial transport of water across the membrane. In addition to their recognized functions, cotransporters have the additional property of water channels. Compared to aquaporins, the unitary water permeability is about two orders of magnitude lower. It is suggested that the water permeability is determined from chemical associations between the water molecule and sites within the pore, probably in the form of hydrogen-bonds. The existence of a passive water permeability suggests an alternative model for the molecular water pump: The water flux couples to the flux of non-aqueous substrates in a hyperosmolar compartment within the protein. Molecular water pumps allow cellular water homeostasis to be viewed as a balance between pumps and leaks. This enables cells to maintain their intracellular osmolarity despite external variations. Molecular water pumps could be relevant for a wide range of physiological functions, from volume regulation in contractile vacuoles in amoeba to phloem transport in plants (Zeuthen 1992, 1996). They could be important building blocks in a general model for vectorial water transport across epithelia. A simplified model of a leaky epithelium incorporating K+/Cl-/H2O and Na+/glucose/H2O cotransport in combination with channels and primary active transport gives good quantitative predictions of several properties. In particular of how epithelial cell layers can transport water uphill.

Structural correlates of the transepithelial water transport

Transepithelial permeability is one of the fundamental problems in cell biology. Epithelial cell layers protect the organism from its environment and form a selective barrier to the exchange of molecules between the lumen of an organ and an underlying tissue. This chapter discusses some problems and analyzes the participation of intercellular junctions in the paracellular transport of water, migration of intramembrane particles in the apical membrane during its permeability changes for isotonic fluid in cells of leaky epithelia, insertion of water channels into the apical membrane and their cytoplasmic sources in cells of tight epithelia under ADH (antidiuretic hormone)-induced water flows, the osmoregulating function of giant vacuoles in the transcellular fluxes of hypotonic fluid across tight epithelia, and the role of actin filaments and microtubules in the transcellular transport of water across epithelia.

Osmosensing by bacteria: signals and membrane-based sensors

Bacteria can survive dramatic osmotic shifts. Osmoregulatory responses mitigate the passive adjustments in cell structure and the growth inhibition that may ensue. The levels of certain cytoplasmic solutes rise and fall in response to increases and decreases, respectively, in extracellular osmolality. Certain organic compounds are favored over ions as osmoregulatory solutes, although K+ fluxes are intrinsic to the osmoregulatory response for at least some organisms. Osmosensors must undergo transitions between "off" and "on" conformations in response to changes in extracellular water activity (direct osmosensing) or resulting changes in cell structure (indirect osmosensing). Those located in the cytoplasmic membranes and nucleoids of bacteria are positioned for indirect osmosensing. Cytoplasmic membrane-based osmosensors may detect changes in the periplasmic and/or cytoplasmic solvent by experiencing changes in preferential interactions with particular solvent constituents, cosolvent-induced hydration changes, and/or macromolecular crowding. Alternatively, the membrane may act as an antenna and osmosensors may detect changes in membrane structure. Cosolvents may modulate intrinsic biomembrane strain and/or topologically closed membrane systems may experience changes in mechanical strain in response to imposed osmotic shifts. The osmosensory mechanisms controlling membrane-based K+ transporters, transcriptional regulators, osmoprotectant transporters, and mechanosensitive channels intrinsic to the cytoplasmic membrane of Escherichia coli are under intensive investigation. The osmoprotectant transporter ProP and channel MscL act as osmosensors after purification and reconstitution in proteoliposomes. Evidence that sensor kinase KdpD receives multiple sensory inputs is consistent with the effects of K+ fluxes on nucleoid structure, cellular energetics, cytoplasmic ionic strength, and ion composition as well as on cytoplasmic osmolality. Thus, osmoregulatory responses accommodate and exploit the effects of individual cosolvents on cell structure and function as well as the collective contribution of cosolvents to intracellular osmolality.

AVP V1 receptor-mediated decrease in Cl- efflux and increase in dark cell number in choroid plexus epithelium

The cerebrospinal fluid (CSF)-generating choroid plexus (CP) has many V1 binding sites for arginine vasopressin (AVP). AVP decreases CSF formation rate and choroidal blood flow, but little is known about how AVP alters ion transport across the blood-CSF barrier. Adult rat lateral ventricle CP was loaded with 36Cl-, exposed to AVP for 20 min, and then placed in isotope-free artificial CSF to measure release of 36Cl-. Effect of AVP at 10(-12) to 10(-7) M on the Cl- efflux rate coefficient (in s-1) was quantified. Maximal inhibition (by 20%) of Cl- extrusion at 10(-9) M AVP was prevented by the V1 receptor antagonist [beta-mercapto-beta, beta-cyclopentamethyleneproprionyl1,O-Me-Tyr2,Arg8]vasopressin. AVP also increased by more than twofold the number of dark and possibly dehydrated but otherwise morphologically normal choroid epithelial cells in adult CP. The V1 receptor antagonist prevented this AVP-induced increment in dark cell frequency. In infant rats (1 wk) with incomplete CSF secretory ability, 10(-9) M AVP altered neither Cl- efflux nor dark cell frequency. The ability of AVP to elicit functional and structural changes in adult, but not infant, CP epithelium is discussed in regard to ion transport, CSF secretion, intracranial pressure, and hydrocephalus.

Water does not flow across the tight junctions of MDCK cell epithelium

Although it has been known for decades that the tight junctions of fluid-transporting epithelia are leaky to ions, it has not been possible to determine directly whether significant transjunctional water movement also occurs. An optical microscopic technique was developed for the direct visualization of the flow velocity profiles within the lateral intercellular spaces of a fluid-absorptive, cultured renal epithelium (MDCK) and used to determine the velocity of the fluid flow across the tight junction. The flow velocity within the lateral intercellular spaces fell to near zero adjacent to the tight junction, showing that significant transjunctional flow did not occur, even when transepithelial fluid movement was augmented by imposition of osmotic gradients.

Cell membranes; barriers, regulators and transducers?

The concept of the cell membrane as a physical barrier has been progressively revised over the past 50 years to produce a model of a homeostatic regulatory system. Homeostatic feedback systems involve control and feedback transducers, with error detecting systems. It is, therefore, interesting to see how far the control of membrane fluidity, cell volume and ion composition can be interpreted in this way especially in epithelial systems. An attempt is made to consider the systems involved in extracellular, intracellular and stored calcium. When membranes are analysed in this way it is possible to identify questions about the extent to which cell regulation and cell transport might be in conflict, and how specific some membrane functions might be. These can then be related to the study of various adaptations to environmental stresses.

Routes and mechanism of fluid transport by epithelia

The mechanism of fluid transport by leaky epithelia and the route taken by the transported fluid are in dispute. A consideration of current mathematical models for coupling of solutes and water, as well as the methodologies for the study of fluid transport, shows that local osmosis best accounts for water movement. Although it seems virtually certain that the tight junctions are water permeable, the fraction of absorbed fluid that crosses the tight junction cannot yet be determined with confidence.

The human Na+-glucose cotransporter is a molecular water pump

1. The human Na+-glucose cotransporter (hSGLT1) was expressed in Xenopus laevis oocytes. The transport activity, given by the Na+ current, was monitored as a clamp current and the concomitant flux of water followed optically as the change in oocyte volume. 2. When glucose was added to the bathing solution there was an abrupt increase in clamp current and an immediate swelling of the oocyte. The transmembrane transport of two Na+ ions and one sugar molecule was coupled, within the protein itself, to the influx of 210 water molecules. 3. This stoichiometry was constant and independent of the external parameters: Na+ concentrations, sugar concentrations, transmembrane voltages, temperature and osmotic gradients. 4. The cotransport of water occurred in the presence of adverse osmotic gradients. In accordance with the Gibbs equation, energy was transferred within the protein from the downhill fluxes of Na+ and sugar to the uphill transport of water, indicative of secondary active transport of water. 5. Unstirred layer effects were ruled out on the basis of experiments on oocytes treated with gramicidin or other ionophores. Na+ currents maintained by ionophores did not lead to any initial water movements. 6. The finding of a molecular water pump allows for new models of cellular water transport which include coupling between ion and water fluxes at the protein level; the hSGLT1 could account for almost half the daily reuptake of water from the small intestine.

Membrane potential-generating malate (MleP) and citrate (CitP) transporters of lactic acid bacteria are homologous proteins. Substrate specificity of the 2-hydroxycarboxylate transporter family

Membrane potential generation via malate/lactate exchange catalyzed by the malate carrier (MleP) of Lactococcus lactis, together with the generation of a pH gradient via decarboxylation of malate to lactate in the cytoplasm, is a typical example of a secondary proton motive force-generating system. The mleP gene was cloned, sequenced, and expressed in a malolactic fermentation-deficient L. lactis strain. Functional analysis revealed the same properties as observed in membrane vesicles of a malolactic fermentation-positive strain. MleP belongs to a family of secondary transporters in which the citrate carriers from Leuconostoc mesenteroides (CitP) and Klebsiella pneumoniae (CitS) are found also. CitP, but not CitS, is also involved in membrane potential generation via electrogenic citrate/lactate exchange. MleP, CitP, and CitS were analyzed for their substrate specificity. The 2-hydroxycarboxylate motif R1R2COHCOOH, common to the physiological substrates, was found to be essential for transport although some 2-oxocarboxylates could be transported to a lesser extent. Clear differences in substrate specificity among the transporters were observed because of different tolerances toward the R substituents at the C2 atom. Both MleP and CitP transport a broad range of 2-hydroxycarboxylates with R substituents ranging in size from two hydrogen atoms (glycolate) to acetyl and methyl groups (citromalate) for MleP and two acetyl groups (citrate) for CitP. CitS was much less tolerant and transported only citrate and at a low rate citromalate. The substrate specificities are discussed in the context of the physiological function of the transporters.

AQUAPORINS AND WATER PERMEABILITY OF PLANT MEMBRANES

The mechanisms of plant membrane water permeability have remained elusive until the recent discovery in both vacuolar and plasma membranes of a class of water channel proteins named aquaporins. Similar to their animal counterparts, plant aquaporins have six membrane-spanning domains and belong to the MIP superfamily of transmembrane channel proteins. Their very high efficiency and selectivity in transporting water molecules have been mostly characterized using heterologous expression in Xenopus oocytes. However, techniques set up to measure the osmotic water permeability of plant membranes such as transcellular osmosis, pressure probe measurements, or stopped-flow spectrophotometry are now being used to analyze the function of plant aquaporins in their native membranes. Multiple mechanisms, at the transcriptional and posttranslational levels, control the expression and activity of the numerous aquaporin isoforms found in plants. These studies suggest a general role for aquaporins in regulating transmembrane water transport during the growth, development, and stress responses of plants. Future research will investigate the integrated function of aquaporins in long-distance water transport and cellular osmoregulation.

The effect of a transmembrane osmotic flux on the ion concentration distribution in the immediate membrane vicinity measured by microelectrodes

The osmotically induced transmembrane water flow is accompanied by solute concentration changes within the unstirred layer adjacent to membranes. Experimental concentration profiles, measured by means of microelectrodes in the immediate vicinity of a planar lipid bilayer, are compared with theoretical ones predicted from the standard physiological model in which the osmotic advection is countered by back-diffusion of the solute only. An increase of the apparent osmotic flow rate is induced by an increase of the osmotic gradient and by rigorous stirring. The polarization effect decreases in the latter case due to an increase of the transfer rate of solutes between the bulk solutions and the membrane surfaces, whereas it increases in the former case. The observations show that the concentration profile is not well described by the standard approximation. The discrepancy becomes increasingly large with increased volume flow. Based on a modified theoretical description of the interaction between water flux and diffusion, the hydraulic conductivity of the bilayer is calculated from the measured uniexponential concentration profiles. The common approximation that there is a discrete boundary between the stirred and unstirred regions adjacent to the membrane is substituted by the model of a stagnant point flow that takes into account a gradual change of the stirring velocity in the immediate membrane vicinity. Supported by experimental observations, this approach predicts a shortening of the unstirred layer if the transmembrane osmotic gradient is increased under gentle stirring conditions.

Cotransport of H+, lactate and H2O by membrane proteins in retinal pigment epithelium of bullfrog

1. The interaction between H+, lactate and H2O fluxes in the retinal membrane of the pigment epithelium from bullfrog Rana catesbiana was studied by means of ion-selective micro-electrodes. 2. Changes in intracellular pH and cell volume were recorded in response to abrupt changes in retinal solution concentration and/or osmolarity. 3. Two parallel pathways for water transport were identified across the retinal membrane, an osmotic one with a hydraulic water permeability of 3.2 x 10(-4) cm s-1 (osmol l-1)-1 and one which depended on the presence of lactate. 4. Addition of sodium lactate to the retinal solution caused cell shrinkages that were small compared with those produced by mannitol. The reflection coefficient for sodium lactate was 0.25. 5. Isosmotic replacement of Cl- with lactate caused an influx of water. Simultaneous acidification of the retinal solution from pH 7.4 to 6.4 enhanced the effect. The influx of water could proceed against osmotic gradients elicited by mannitol. 6. The interdependence of the fluxes of H+, lactate and H2O can be described as cotransport: the fluxes had a fixed ratio of about 109 mmol of lactic acid per litre of water, the flux of one species was able to energize the flux of the other two, and the fluxes exhibited saturation for increasing driving forces. 7. The Gibbs equation gives an accurate quantitative description of these coupled fluxes.

Cotransport of water by the Na+/glucose cotransporter

Water is transported across epithelial membranes in the absence of any hydrostatic or osmotic gradients. A prime example is the small intestine, where 10 liters of water are absorbed each day. Although water absorption is secondary to active solute transport, the coupling mechanism between solute and water flow is not understood. We have tested the hypothesis that water transport is directly linked to solute transport by cotransport proteins such as the brush border Na+/glucose cotransporter. The Na+/glucose cotransporter was expressed in Xenopus oocytes, and the changes in cell volume were measured under sugar-transporting and nontransporting conditions. We demonstrate that 260 water molecules are directly coupled to each sugar molecule transported and estimate that in the human intestine this accounts for 5 liters of water absorption per day. Other animal and plant cotransporters such as the Na+/CI-/gamma-aminobutyric acid, Na+/iodide and H+/amino acid transporters are also able to transport water and this suggests that cotransporters play an important role in water homeostasis.