From contractile vacuole to leaky epithelia. Coupling between salt and water fluxes in biological membranes

Cerebrospinal fluid production by the choroid plexus: a century of barrier research revisited

Cerebrospinal fluid (CSF) envelops the brain and fills the central ventricles. This fluid is continuously replenished by net fluid extraction from the vasculature by the secretory action of the choroid plexus epithelium residing in each of the four ventricles. We have known about these processes for more than a century, and yet the molecular mechanisms supporting this fluid secretion remain unresolved. The choroid plexus epithelium secretes its fluid in the absence of a trans-epithelial osmotic gradient, and, in addition, has an inherent ability to secrete CSF against an osmotic gradient. This paradoxical feature is shared with other 'leaky' epithelia. The assumptions underlying the classical standing gradient hypothesis await experimental support and appear to not suffice as an explanation of CSF secretion. Here, we suggest that the elusive local hyperosmotic compartment resides within the membrane transport proteins themselves. In this manner, the battery of plasma membrane transporters expressed in choroid plexus are proposed to sustain the choroidal CSF secretion independently of the prevailing bulk osmotic gradient.

Basic Biology of Trypanosoma cruzi

The present review addresses basic aspects of the biology of the pathogenic protozoa Trypanosoma cruzi and some comparative information of Trypanosoma brucei. Like eukaryotic cells, their cellular organization is similar to that of mammalian hosts. However, these parasites present structural particularities. That is why the following topics are emphasized in this paper: developmental stages of the life cycle in the vertebrate and invertebrate hosts; the cytoskeleton of the protozoa, especially the sub-pellicular microtubules; the flagellum and its attachment to the protozoan body through specialized junctions; the kinetoplast-mitochondrion complex, including its structural organization and DNA replication; glycosome and its role in the metabolism of the cell; acidocalcisome, describing its morphology, biochemistry, and functional role; cytostome and the endocytic pathway; the organization of the endoplasmic reticulum and Golgi complex; the nucleus, describing its structural organization during interphase and division; and the process of interaction of the parasite with host cells. The unique characteristics of these structures also make them interesting chemotherapeutic targets. Therefore, further understanding of cell biology aspects contributes to the development of drugs for chemotherapy.

A Functional Aqp1 Gene Product Localizes on The Contractile Vacuole Complex in Paramecium multimicronucleatum

In a ciliate Paramecium, the presence of water channels on the membrane of contractile vacuole has long been predicted by both morphological and physiological data, however, to date either the biochemical or the molecular biological data have not been provided. In the present study, to examine the presence of aquaporin in Paramecium, we carried out RT-PCR with degenerated primers designed based on the ParameciumDB, and an aquaporin cDNA (aquaporin 1, aqp1) with a full-length ORF encoding 251 amino acids was obtained from Paramecium multimicronucleatum by using RACE. The deduced amino acid sequence of AQP1 had NPA-NPG motifs, and the prediction of protein secondary structure by CNR5000 and hydropathy plot showed the presence of six putative transmembrane domains and five connecting loops. Phylogenetic analysis results showed that the amino acid sequence of AQP1 was close to that of the Super-aquaporin group. The AQP1-GFP fusion protein clearly demonstrated the subcellular localization of AQP1 on the contractile vacuole complex, except for the decorated spongiome membrane. The functional analyses of aqp1 were done by RNA interference-based gene silencing, using an established feeding method. The aqp1 was found to be crucial for the total fluid output of the cell, the function of contractile vacuole membranes.

An organelle K+ channel is required for osmoregulation in Chlamydomonas reinhardtii

Fresh water protozoa and algae face hypotonic challenges in their living environment. Many of them employ a contractile vacuole system to uptake excessive water from the cytoplasm and expel it to the environment to achieve cellular homeostasis. K(+), a major osmolyte in contractile vacuole, is predicted to create higher osmolarity for water influx. Molecular mechanisms for K(+) permeation through the plasma membrane have been well studied. However, how K(+) permeates organelles such as the contractile vacuole is not clear. Here, we show that the six-transmembrane K(+) channel KCN11 in Chlamydomonas is exclusively localized to contractile vacuole. Ectopic expression of KCN11 in HEK293T cells results in voltage-gated K(+) channel activity. Disruption of the gene or mutation of key residues for K(+) permeability of the channel leads to dysfunction of cell osmoregulation in very hypotonic conditions. The contractile cycle is inhibited in the mutant cells with a slower rate of contractile vacuole swelling, leading to cell death. These data demonstrate a new role for six-transmembrane K(+) channels in contractile vacuole functioning and provide further insights into osmoregulation mediated by the contractile vacuole.

The SEC6 protein is required for contractile vacuole function in Chlamydomonas reinhardtii

Contractile vacuoles (CVs) are essential for osmoregulation in many protists. To investigate the mechanism of CV function in Chlamydomonas, we isolated novel osmoregulatory mutants. Four of the isolated mutant cell lines carried the same 33,641 base deletion, rendering the cell lines unable to grow under strong hypotonic conditions. One mutant cell line (Osmo75) was analyzed in detail. The CV morphology was variable in mutant cells, and most cells had multiple small CVs. In addition, one or two enlarged CVs or no visible CVs at all, were observed by light microscopy. These findings suggest that the mutant is impaired in homotypic vacuolar and exocytotic membrane fusion. Furthermore the mutants had long flagella. One of the affected genes is the only SEC6 homologue in Chlamydomonas (CreSEC6). The SEC6 protein is a component of the exocyst complex that is required for efficient exocytosis. Transformation of the Osmo75 mutant with a CreSEC6-GFP construct rescued the mutant completely (osmoregulation and flagellar length). Rescued strains overexpressed CreSEC6 (as a GFP-tagged protein) and displayed a modified CV activity. CVs were larger, whereas the CV contraction interval remained unchanged, leading to increased water efflux rates. Electron microscopy analysis of Osmo75 cells showed that the mutant is able to form the close contact zones between the plasma membrane and the CV membrane observed during late diastole and systole. These results indicate that CreSEC6 is essential for CV function and required for homotypic vesicle fusion during diastole and water expulsion during systole. In addition, CreSEC6 is not only necessary for CV function, but possibly influences the CV cycle in an indirect manner and flagellar length in Chlamydomonas.

An osmoregulatory basis for shape oscillations in regenerating hydra

The freshwater polyp Hydra has considerable regeneration capabilities. A small fragment of tissue excised from an adult animal is sufficient to regenerate an entire Hydra in the course of a few days. During the initial stages of the regeneration process, the tissue forms a hollow sphere. Then the sphere exhibits shape oscillations in the form of repeated cycles of swelling and collapse. We propose a biophysical model for the swelling mechanism. Our model takes the osmotic pressure difference between Hydra's inner and outer media and the elastic forces of the Hydra shell into account. We validate the model by a comprehensive experimental study including variations in initial medium concentrations, Hydra sphere sizes and temperatures. Numerical simulations of the model provide values for the swelling rates that are in agreement with the ones measured experimentally. Based on our results we argue that the shape oscillations are a consequence of Hydra's osmoregulation.

Inhibition of Contractile Vacuole Function by Brefeldin A

Brefeldin A (BFA) causes a block in the secretory system of eukaryotic cells. In the scaly green flagellate Scherffelia dubia, BFA also interfered with the function of the contractile vacuoles (CVs). The CV is an osmoregulatory organelle which periodically expels fluid from the cell in many freshwater protists. Fusion of the CV membrane with the plasma membrane is apparently blocked by BFA in S. dubia. The two CVs of S. dubia swell and finally form large central vacuoles (LCVs). BFA-induced formation of LCVs depends on V-ATPase activity, and can be reversed by hypertonic media, suggesting that water accumulation in the LCVs is driven by osmosis. We suggest that the BFA-induced formation of LCVs represents a prolonged diastole phase. A normal diastole phase takes about 20 s and is difficult to investigate. Therefore, BFA-induced formation of LCVs in S. dubia represents a unique model system to investigate the diastole phase of the CV cycle.

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.

Osmoregulation and contractile vacuoles of protozoa

Protozoa living in fresh water are subjected to a hypotonic environment. Water flows across their plasma membrane since their cytosol is always hypertonic to the environment. Many wall-less protozoa have an organelle, the contractile vacuole complex (CVC), that collects and expels excess water. Recent progress shows that most, if not all, CVCs are composed of a two-compartment system encircled by two differentiated membranes. One membrane, which is often divided into numerous vesicles and tubules, contains many proton-translocating V-ATPase enzymes that provide an electrochemical gradient of protons and which fuses only with the membrane of the second compartment. The membrane of the second compartment lacks V-ATPase holoenzymes, expands into a reservoir for fluid storage, and is capable of fusing with the plasma membrane. It is this second compartment that periodically undergoes rounding ("contraction"), setting the stage for fluid expulsion. Rounding is accompanied by increased membrane tension. We review the current state of knowledge on osmolarity, ion concentrations, membrane permeability, and electrophysiological parameters of cells and their contractile vacuoles, where these criteria are helpful to our understanding of the function of the CVC. Effects of environmental stresses on the CVC function are also summarized. Finally, other functions suggested for CVCs based on molecular and physiological studies are reviewed.

Cell volume homeostasis: ionic and nonionic mechanisms. The sodium pump in the emergence of animal cells

Plant cells and bacterial cells are surrounded by a massive polysaccharide wall, which constrains their high internal osmotic pressure (tens of atmospheres). Animal cells, in contrast, are in osmotic equilibrium with their environment, have no restraining surround, and can take on a variety of shapes and can change these from moment to moment. This osmotic balance is achieved, in the first place, by the action of the energy-consuming sodium pump, one of the P-type ATPase transport protein family, members of which are found also in bacteria. The pump's action brings about a transmembranal electrochemical gradient of sodium ions, harnessed in a range of transport systems which couple the dissipation of this gradient to establishing a gradient of the coupled substrate. These transport systems include many which are responsible for short-term regulation of the cell's volume in response to acute changes of their osmotic balance. Thus, the primary role of the sodium pump as a regulator of cell volume has been built upon to provide the basis for an enormous variety of physiological functions.

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.

Role of carbon dioxide and ion transport in the formation of sub-embryonic fluid by the blastoderm of the Japanese quail

1. The explanted blastoderm of the Japanese quail was used to explore the role of ions and carbon dioxide in determining the rate of sub-embryonic fluid (SEF) production between 54 and 72 h of incubation. 2. Amiloride, an inhibitor of Na+/H+ exchange, at concentrations of 10(-3) to 10(-6) M substantially decreased the rate of SEF production when added to the albumen culture medium. N-ethylmaleimide, an inhibitor of V type H+ ATPase, also decreased this rate but only to a small extent at the highest dose applied, 10(-3) M. Both inhibitors had no effect on SEF production when added to the SEF. 3. The inhibitors of cellular bicarbonate and chloride exchange, 4-acetamido-4'-isothiocyano-2,2'-disulphonic acid (SITS) and 4,4' diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), had no effect upon SEF production. 4. Ouabain, an inhibitor of Na+/K+ ATPase, decreased SEF production substantially at all concentrations added to the SEF (10(-3) to 10(-6) M). Three sulphonamide inhibitors of carbonic anhydrase, acetazolamide, ethoxzolamide and benzolamide, decreased SEF production when added to the SEF at concentrations of 10(-3) to 10(-6) M. Benzolamide was by far the most potent. Neither ouabain nor the sulphonamides altered SEF production when added to the albumen culture medium. 5. Using a cobalt precipitation method, carbonic anhydrase activity was localised to the endodermal cells of the area vasculosa. The carbonic anhydrase activity was primarily associated with the lateral plasma membranes, which together with the potent inhibitory effect of benzolamide, suggests the carbonic anhydrase of these cells is the membrane-associated form, CA IV. 6. The changes in SEF composition produced by inhibitors were consistent with the production of SEF by local osmotic gradients. 7. It is concluded that a Na+/K+ ATPase is located on the basolateral membranes of the endodermal cells of the area vasculosa, and that a sodium ion/hydrogen ion exchanger is located on their apical surfaces. Protons for this exchanger would be provided by the hydration of CO2 catalysed by the membrane-associated carbonic anhydrase. Furthermore, it is proposed that the prime function of the endodermal cells of the area vasculosa is the production of SEF.

How external osmolarity affects the activity of the contractile vacuole complex, the cytosolic osmolarity and the water permeability of the plasma membrane in Paramecium multimicronucleatum

The rate of fluid expulsion, R(CVC), from the contractile vacuole complex (CVC) of Paramecium multimicronucleatum was estimated from the volume of the contractile vacuoles (CVs) immediately before the start of fluid discharge and from the time elapsing between discharges. The R(CVC) increased when the cell was exposed to a strongly hypotonic solution and decreased in a weakly hypotonic solution. When the cell was exposed to an isotonic or a hypertonic solution, R(CVC) fell to zero. The time constant, tau, used to describe the change in R(CVC) in response to a change in external osmolarity shortened after a short-term exposure to a strongly hypotonic solution and lengthened after a short-term exposure to a less hypotonic solution. A remarkable lengthening of tau occurred after a short-term exposure to isotonic or hypertonic solution. Under natural conditions, mechanisms for controlling R(CVC) are effective in maintaining the cytosolic osmolarity hypertonic within a narrow concentration range despite changes in the external osmolarity, which is normally hypotonic to the cytosol. Cells exposed to an isotonic or hypertonic solution resumed CV activity when left in the solution for 12 h. The cytosolic osmolarity was found to increase and to remain hypertonic to the external solution. This will permit cells to continue to acquire water. The increase in the cytosolic osmolarity occurred in a stepwise fashion, rather than linearly, as the external osmolarity increased. That is, the cytosolic osmolarity first remained more-or-less constant at an increased level until the external osmolarity exceeded this level. Thereupon, the cytosolic osmolarity increased to a new higher level in 12 h, so that the cytosol again became hypertonic to the external solution and the cells resumed CV activity. These results imply that the cell needs to maintain water segregation activity even after it has been exposed to an isotonic or hypertonic environment. This supports the idea that the CVC might be involved not only in the elimination of excess cytosolic water but also in the excretion of some metabolic waste substances.

The contractile vacuole and its membrane dynamics

The contractile vacuole (CV) is an osmoregulatory organelle whose mechanisms of function are poorly understood. Immunological studies in the last decade have demonstrated abundant proton-translocating V-type ATPases (V-ATPases) in its membrane that could provide the energy, from proton electrochemical gradients, for moving ions into the CV to be followed by water. This review emphasizes recent work on the contractile vacuole complex (CVC) of Paramecium including (1) CV expulsion, (2) a role for V-ATPases in sequestering fluid, (3) identifying ions in the cytosol and in the CV, (4) in situ electrophysiological parameters of the CVC membrane, and (5) a better understanding of the membrane dynamics of this organelle.

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.

The effect of colchicine on proximal tubular reabsorption

Aquaporins, expressed in the brush border membrane (BB) could play a pivotal role in glomerulo-tubular balance (GTB) by effecting adaptive changes of water permeability to the variations in the load of filtered solutes. Since aquaporin expression is modulated by microtubule-dependent trafficking between endoplasmic reticulum and cell membranes, we used the microtubule poison colchicine to assess the importance of aquaporins in mediating GTB. The effects of colchicine 1.6x10(-4)m on proximal tubule volume reabsorption was tested on 48 nephrons of ten rats by micropuncture techniques. Thirty proximal tubules were sampled from the last proximal convolution before, and recollected during and again after the microinjection (MIJ), into the early proximal convolution or Bowman's space, of colchicine added to a Ringer solution. We studied 18 proximal tubules in the same way before, during and after the microperfusion (MP) of colchicine added to an ultrafiltrate of plasma into the peritubular capillaries. During MIJ, SNGFR did not change significantly from baseline (17.7+/-1.3 vs 20.9+/-1.8 nl min(-1), P>0.12). Post-control values were superimposable upon their paired pre-MIJ controls, when available (15.8+/-1.3 vs 13.5+/-1.5 nl min(-1), P>0.25). The measurements of percentage reabsorption (49+/-5 during baseline, 45+/-7 during MIJ, and 55+/-5 in post-control, P>0.6) and absolute reabsorption (8.1+/-0.7, 11.1+/-2. 2, and 7.9+/-1.3 nl min(-1), respectively, P>0.18) were also unchanged. The three average measurements obtained in control conditions, during MP and again in post-MP control were not significantly different for SNGFR (19.8+/-3.0, 20.0+/-4.7, and 20. 2+/-3.5 nl min(-1), P>0.48), percentage (55+/-3, 59+/-5, and 47+/-3%, P>0.35) and absolute reabsorptions (12.5+/-2.2, 12.4+/-4.6, and 9. 4+/-1.0 nl min(-1), respectively, P>0.42). MIJ and MP of vehicles were devoid of any measurable effect. Colchicine does not acutely affect volume reabsorption in the proximal tubule. Aquaporin trafficking, if any, is not involved in mediating glomerulotubular balance in the proximal tubule, although aquaporin expression and function could still be important, although regulated by mechanisms different from microtubule-dependent shuttling between endoplasmic reticulum and BB.

A key function of non-planar membranes and their associated microtubular ribbons in contractile vacuole membrane dynamics is revealed by electrophysiologically controlled fixation of Paramecium

The contractile vacuole complex of the fresh water protozoan Paramecium multimicronucleatum exhibits periodic exocytotic activity. This keeps cytosolic osmolarity at a constant value. The contractile vacuole, the central exocytotic vesicle of the complex, becomes disconnected from its surrounding radial arms and rounds before its fluid content is expelled. We previously proposed a hypothesis that the rounding of the contractile vacuole corresponds to an increase in its membrane tension and that a periodic increase in membrane tension governs the exocytotic cycle. We also proposed a hypothesis that transformation of excess planar membrane of the contractile vacuole into 40 nm diameter tubules, that remain continuous with the contractile vacuole membrane, is a primary cause for the tension development in the planar membrane. In order to investigate tension development further, we have examined electron microscopically the contractile vacuole membrane at the rounding phase. To do this, we developed a computer-aided system to fix the cell precisely at the time that the contractile vacuole exhibited rounding. In this system a decrease in the electrical potential across the contractile vacuole membrane that accompanied the vacuole's rounding was monitored through a fine-tipped microelectrode inserted directly into the in vivo contractile vacuole. A decrease in membrane potential was used to generate an electric signal that activated an injector for injecting a fixative through a microcapillary against the cell at the precise time of rounding. Subsequent electron micrographs of the contractile vacuole membrane clearly demonstrated that numerous approximately 40 nm membrane-bound tubules formed in the vicinity of the vacuole's microtubule ribbons when the vacuole showed rounding. This finding suggested that membrane tubulation was the cause for topographical isolation of excess membrane from the planar membrane during the periodic rounding of the contractile vacuole. This together with stereo-pair images of the contractile vacuole complex membranes suggested that the microtubule ribbons were intimately involved in enhancing this membrane tubulation activity. Electron micrographs of the contractile vacuole complexes also showed that decorated tubules came to lie abnormally close to the contractile vacuole in these impaled cells. This suggested that the contractile vacuole was capable of utilizing the smooth spongiome membrane that lies around the ampullae and the collecting canals to increase its size.

Osmotic homeostasis in Dictyostelium discoideum: excretion of amino acids and ingested solutes

The response to osmotic stress in axenically cultured Dictyostelium discoideum was examined. Hypoosmotic buffers elicited two changes in the large (approximately 50 mM) cytosolic pool of amino acids: a) the total size of the pool diminished, while b) about half of the initial pool was excreted. Hyperosmotic stress had the opposite effect. Among the predominant amino acids in the pool were glycine, alanine and proline. Putrescine, the major diamine, was neither excreted nor modulated. Recently ingested radioactive amino acids were excreted in preference to those in the cytoplasm, suggesting that the endocytic pathway might be involved in water excretion. Furthermore, hypoosmotic stress stimulated the selective excretion of small, membrane-impermeable fluorescent dyes which had been ingested into endocytic vacuoles. Caffeine inhibited the excretion of the fluorophores but not the amino acids. We conclude that the response of Dictyostelium to osmotic stress is complex and includes both modulation of the cytoplasmic amino acid pool and the excretion of amino acids and other small solutes from the endocytic pathway.

The cell adhesion molecule DdCAD-1 in Dictyostelium is targeted to the cell surface by a nonclassical transport pathway involving contractile vacuoles

DdCAD-1 is a 24-kD Ca2+-dependent cell- cell adhesion molecule that is expressed soon after the initiation of development in Dictyostelium cells. DdCAD-1 is present on the cell surface as well as in the cytosol. However, the deduced amino acid sequence of DdCAD-1 lacks a hydrophobic signal peptide or any predicted transmembrane domain, suggesting that it may be presented on the cell surface via a nonclassical transport mechanism. Here we report that DdCAD-1 is transported to the cell surface via contractile vacuoles, which are normally involved in osmoregulation. Immunofluorescence microscopy and subcellular fractionation revealed a preferential association of DdCAD-1 with contractile vacuoles. Proteolytic treatment of isolated contractile vacuoles degraded vacuole-associated calmodulin but not DdCAD-1, demonstrating that DdCAD-1 was present in the lumen. The use of hyperosmotic conditions that suppress contractile vacuole activity led to a dramatic decrease in DdCAD-1 accumulation on the cell surface and the absence of cell cohesiveness. Shifting cells back to a hypotonic condition after hypertonic treatments induced a rapid increase in DdCAD-1-positive contractile vacuoles, followed by the accumulation of DdCAD-1 on the cell membrane. 7-chloro-4-nitrobenzo-2-oxa-1, 3-diazole, a specific inhibitor of vacuolar-type H+-ATPase and thus of the activity of contractile vacuoles, also inhibited the accumulation of DdCAD-1 on the cell surface. Furthermore, an in vitro reconstitution system was established, and isolated contractile vacuoles were shown to import soluble DdCAD-1 into their lumen in an ATP-stimulated manner. Taken together, these data provide the first evidence for a nonclassical protein transport mechanism that uses contractile vacuoles to target a soluble cytosolic protein to the cell surface.

A voltage- and K+-dependent K+ channel from a membrane fraction enriched in contractile vacuole of Dictyostelium discoideum

We obtained a membrane fraction enriched in the contractile vacuole by aqueous-polymer two-phase partitioning and its channel activities were analysed by incorporating it into artificial planar lipid bilayers. In asymmetrical KCl solutions (cis, 300 mM/100 mM, trans), we observed single-channel currents of a highly K(+)-selective channel with slope conductance of 102 pS and reversal potential of -20.4 mV, which corresponded to PK+/PCl- = 7. They showed bursts separated by infrequent quiescent periods. At 0 mV the mean open time was 2.0 ms. Among monovalent cations, Na+ and Li+ were impermeable, whereas Rb+ showed permeability equivalent to that of K+, although the unitary conductance was apparently reduced when the current flowed from the Rb+ containing side, suggesting that Rb+ is a permeant blocking ion. The open probability within bursts remained constant at approx.0.6 as long as the holding potential was positive on the cis side with respect to the trans side, but it decreased to 0 at negative potential. This channel was blocked by submillimolar concentrations of quinine and 30 mM TEA+. The open probability-voltage relationship showed a striking dependency on the KCl concentration on either side. This channel may play a role in water transport in this organelle.

Experimental studies of intestinal ion and water transport

A major advance in transport physiology was H. H. Ussing's development of the voltage-clamp method, and later the Koefoed-Johnsen-Ussing model for Na+ transport. In the same decade, J. C. Skou identified the Na(+)-K(+)-ATPase, which maintains the Na+ and K+ gradients that drive most epithelial transport processes. With this foundation, Danish scientists have pursued the mechanism of ion transport and the resulting solute-linked water flow. Recent contributions have been on isosmotic transport, suggesting solute recycling, and KCl-water cotransport in the basolateral epithelial cell membrane. Efficient small intestinal nutrient absorption is dependent on coupling to the Na+ gradient. Cotransport of Na+ and glucose is quantitatively the most important absorptive mechanism in the small intestine, as illustrated by the success of oral rehydration solutions in diarrhoea. The majority of amino acids are likewise transported by Na+ dependent carriers, but recent experiments have identified a concomitant Cl- dependency for some. Regulation of intestinal secretion, both under normal digestive processes, and in response to enterotoxins, has turned out to be very complex. It involves local and central neuronal regulation through an array of neurotransmitters and local actions of gastrointestinal hormones. Major effectors are the submucosal neurons and the main transmitters serotonin, vasoactive intestinal peptide, acetylcholine, substance P, and neurotensin. Development of antisecretagogues is impeded by the existence of several receptor subtypes and significant species differences. The Na+ and water-conserving properties of the large intestine have been shown to be regulated by adrenocortical hormones, with aldosterone as a potent stimulator of colonic Na+ absorption. A major colonic function is the symbiosis with the anaerobic bacterial population. The fermentation of carbohydrate to short-chain fatty acids, which can be absorbed, supplements small intestinal digestive function.

Hyperosmotic stress leads to reversible dissociation of the proton pump-bearing tubules from the contractile vacuole complex in Paramecium

To study the effect of hyperosmotic stress on the structure and function of the contractile vacuole complex of Paramecium multimicronucleatum, we employed two different monoclonal antibody markers: one to a decorated spongiome antigen (A4) and a second to an antigen found on all other membranes of the contractile vacuole complex (G4). A hyperosmotic condition was produced by adding sorbitol to the axenic culture medium which induced both dose- and time-dependent decreases in the vacuole's expulsion rate. The addition of 150 mM sorbitol to the medium (making a final osmolarity of 230 mOsmol) was sufficient to completely stop the expulsion of the contractile vacuole. Immunofluorescence demonstrated that the blocking of fluid output was accompanied by the disappearance of most fluorescence labeling from the decorated spongiome (the A4 antigen). Electron microscopy revealed that the disappearance of the labeling was accompanied by the disappearance of the decorated tubules from around the collecting canals. These tubules vesiculate. The other membranes of the contractile vacuole complex remained unaffected which was demonstrated by both electron microscopy and indirect immunolabeling using the mAb against the G4 antigen. These results show that the decorated spongiome is formed from a distinct membrane pool separate from that of the smooth spongiome, collecting canals and the contractile vacuole. Recovery of the decorated spongiome rapidly followed the return of the cell to an isotonic environment and was completed within 3 hours. Decorated tubule recovery paralleled the recovery of the function of the contractile vacuole. Recovery was also observed during continuous hyperosmotic treatment with the reappearance of the contractile vacuole activity starting at 3 hours and stabilizing at around 10 hours of incubation. Functional recovery under these conditions was accompanied by a reappearance of the decorated tubules but the total fluid output was always lower than for cells in an isotonic environment. Thus, cells were shown to be capable of adapting to high hyperosmotic conditions. We conclude that the dissociation and reassociation of the decorated spongiome is an important regulatory feature controlling the activity of the contractile vacuole complex and of intracellular osmoregulation in Paramecium.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.

Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus

1. The interaction between K+, Cl- and H2O fluxes was studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. 2. Three experimental strategies were adopted: the osmolarity of the ventricular solution was increased abruptly by addition of (i) mannitol or (ii) KCl; (iii) Na+ in the ventricular solution was replaced isosmotically by K+. 3. The mannitol experiments showed that H2O had two pathways across the ventricular membrane. One was purely passive, with a water permeability, L'p, of 0.64 x 10(-4) cm s-1 (osmol l-1)-1. This operated in parallel with an ion-dependent pathway of similar magnitude which was abolished in Cl(-)-free solutions. 4. When KCl was added there was a flow of H2O into the cell. Surprisingly, this took place despite the osmotic gradient which favoured an efflux of H2O. The effect was blocked by frusemide (furosemide), in which case KCl had the same effects as applications of NaCl or mannitol. 5. Replacement of Na+ with K+ caused an influx of H2O. This flux could proceed against osmotic gradients implemented by mannitol. 6. The present data and those of earlier publications show that the interdependence of the fluxes of K+, Cl- and H2O in the exit membrane can be described as cotransport. The fluxes have a fixed stoichiometry of 1:1:500, the flux of one species is able to energize the flux of the two others, and the transport exhibits saturation and is specific for K+ and Cl-. 7. A molecular model based upon a mobile barrier in a membrane spanning protein gives an accurate quantitative description of the data.

Vacuolar H(+)-ATPase of Dictyostelium discoideum. A monoclonal antibody study

A Dictyostelium membrane fraction rich in vacuolar proton pumps, previously described by Nolta et al. (J. Biol. Chem. 266, 18,318-18,323, 1991), was used as the immunogen for production of monoclonal antibodies. We obtained antibodies that recognized polypeptides of 100 kDa and 68 kDa, corresponding to the two largest subunits of the vacuolar proton pump. In indirect immunofluorescence experiments, these two subunits were located on an interconnected collection of tubules and vacuoles. On frozen thin sections they were found principally on membranes of vacuoles and collections of small vesicles typically located just internal to the plasma membrane. These vesicles and vacuoles had electron-translucent lumens. No other structures in axenically grown Dictyostelium cells were labeled to a significant extent by these two antibodies. Using an affinity-purified antibody to calmodulin and a monospecific antibody to the B subunit of the chromaffin granule vacuolar ATPase, markers known to label the membranes of the contractile vacuole complex in Dictyostelium (Zhu and Clarke, J. Cell Biol. 118, 347–358, 1992; Heuser et al., J. Cell Biol. 121, 1311–1327, 1993), we showed that the 100 kDa and 68 kDa subunits had the same distribution as these two markers. Co-localization was seen in both interphase and mitotic cells. Thus, our results support the conclusion that vacuolar proton pumps are located principally on the membranes of the contractile vacuole complex in Dictyostelium. In addition, in indirect immunofluorescence experiments, these monoclonal antibodies provided improved images of the organization of the contractile vacuole system.

Osmoregulation in Paramecium: the locus of fluid segregation in the contractile vacuole complex

In a previous study, monoclonal antibody DS-1 was found to specifically label the decorated spongiome along the radial arms of the contractile vacuole complexes in Paramecium multimicronucleatum. Fluorescein isothiocyanate-conjugated DS-1, when injected into cells, labels the radial arms initially, but with increasing postinjection time both the intensity of fluorescence and the number of fluorescently labeled radial arms were reduced. When these cells were fixed after 45 minutes and probed indirectly using a second fluorochrome, little new label was seen on the already fluorescein-labeled radial arms. Thin sections showed that the amount of decorated tubules along some collecting canals decreased from the proximal to the distal end and vesicles, which were never seen in control cells, appeared next to the decorated spongiome. These results suggested that the decorated spongiome was undergoing disassembly and sequestration into one region of the cell. The injected DS-1 also reduced the expulsion frequency of the contractile vacuoles in a dose-, time- and site-dependent manner. The contractile vacuole complexes near the injection site were affected more than those farther from the site, but the sizes of both contractile vacuoles were only transiently affected so that fluid output per cell was reduced by approximately 60%. Beyond 45 minutes postinjection, both the expulsion frequency and total fluid output began to recover as the DS-1 was sequestered into one part of the cell. This region persisted in cells up to 18 hours but disappeared by 24 hours, which coincided with the full recovery of the expulsion frequency and of decorated spongiome along the radial arms. The contractile vacuole, the collecting canals and the smooth spongiome were morphologically unaffected. These results indicate that when the decorated spongiome is dissociated from the contractile vacuole complex, the complex's function is strongly inhibited, showing the decorated spongiome to be the site of fluid segregation.

Volume regulation in cochlear outer hair cells

Many cells placed in a hypotonic medium initially swell and then rapidly undergo a regulatory volume decrease (RVD) to return towards original volume. Re-exposure to the isotonic solution results in the cells shrinking followed by a regulatory volume increase (RVI). Previous studies have shown that isolated outer hair cells (OHCs) placed in a hypotonic medium swell and maintain this shape until returned to the original medium. We re-examined this apparent lack of cell volume regulation in OHCs. OHCs were isolated from guinea pig cochleae, mechanically dissociated and dispersed, and placed in a Hank's balanced salt solution (HBS). In the cells studied, switching the perfusate to a hypotonic HBS (290-280 mmol/kg) for 15 min resulted in an immediate shortening of the OHCs (i.e., volume increase). In 26% of the cells, this increase was followed by a return to original length during the time the cell was perfused with the hypotonic medium, a RVD. Twelve percent of the cells demonstrating a RVD also displayed a RVI. Omitting collagenase and increasing Ca2+ concentration did not increase the percentage of cells displaying a RVD, while gadolinium (Gd3+, 10 microM) decreased the percentage to zero. This is the first report of isolated OHCs undergoing cell volume regulation.

Proton pumps populate the contractile vacuoles of Dictyostelium amoebae

Amoebae of the eukaryotic microorganism Dictyostelium discoideum were found to contain an interconnected array of tubules and cisternae whose membranes were studded with 15-nm-diameter "pegs." Comparison of the ultrastructure and freeze-fracture behavior of these pegs with similar structures found in other cells and tissues indicated that they were the head domains of vacuolar-type proton pumps. Supporting this identification, the pegs were observed to decorate and clump when broken amoebae were exposed to an antiserum against the B subunit of mammalian vacuolar H(+)-ATPase. The appearance of the peg-rich cisternae in quick-frozen amoebae depended on their osmotic environment: under hyperosmotic conditions, the cisternae were flat with many narrow tubular extensions, while under hypo-osmotic conditions the cisternae ranged from bulbous to spherical. In all cases, however, their contents deep etched like pure water. These properties indicated that the interconnected tubules and cisternae comprise the contractile vacuole system of Dictyostelium. Earlier studies had demonstrated that contractile vacuole membranes in Dictyostelium are extremely rich in calmodulin (Zhu, Q., and M. Clarke, 1992, J. Cell Biol. 118: 347-358). Light microscopic immunofluorescence confirmed that antibodies against the vacuolar proton pump colocalized with anti-calmodulin antibodies on these organelles. Time-lapse video recording of living amoebae imaged by interference-reflection microscopy, or by fluorescence microscopy after staining contractile vacuole membranes with potential-sensitive styryl dyes, revealed the extent and dynamic interrelationship of the cisternal and tubular elements in Dictyostelium's contractile vacuole system. The high density of proton pumps throughout its membranes suggests that the generation of a proton gradient is likely to be an important factor in the mechanism of fluid accumulation by contractile vacuoles.