Role of Cytoplasmic Vesicles in Volume Maintenance

Regulatory volume decrease in Leishmania mexicana: effect of anti-microtubule drugs

The trypanosomatid cytoskeleton is responsible for the parasite's shape and it is modulated throughout the different stages of the parasite's life cycle. When parasites are exposed to media with reduced osmolarity, they initially swell, but subsequently undergo compensatory shrinking referred to as regulatory volume decrease (RVD). We studied the effects of anti-microtubule (Mt) drugs on the proliferation of Leishmania mexicana promastigotes and their capacity to undergo RVD. All of the drugs tested exerted antiproliferative effects of varying magnitudes [ansamitocin P3 (AP3)> trifluoperazine > taxol > rhizoxin > chlorpromazine]. No direct relationship was found between antiproliferative drug treatment and RVD. Similarly, Mt stability was not affected by drug treatment. Ansamitocin P3, which is effective at nanomolar concentrations, blocked amastigote-promastigote differentiation and was the only drug that impeded RVD, as measured by light dispersion. AP3 induced 2 kinetoplasts (Kt) 1 nucleus cells that had numerous flagella-associated Kts throughout the cell. These results suggest that the dramatic morphological changes induced by AP3 alter the spatial organisation and directionality of the Mts that are necessary for the parasite's hypotonic stress-induced shape change, as well as its recovery.

Effects of medium calcium, and agents affecting cytoskeletal function, on cellular volume and morphology in liver tissue in vitro

The possible role of an exocytotic, vesicular mechanism in cellular volume regulation under iso-osmotic conditions has been studied in slices of rat liver. The effects of incubation conditions and agents affecting the actin cytoskeleton were examined for changes of water, ionic composition, and ultrastructure. Slices were pre-incubated at 1°C in an iso-osmotic buffered medium to induce swelling. Upon restoration to 37°C in the same medium, tissue lost water. The Na+-K+ adenosine triphosphatase (ATPase) inhibitor ouabain inhibited water extrusion of about 50%, an effect that was accompanied by the formation of characteristic vesicles in the cytoplasmic region between the Golgi apparatus and the bile canaliculi. Water extrusion in the presence of ouabain was partially inhibited by trifluoroperazine and completely inhibited when the medium was free of Ca2+. In the presence of ouabain, brefeldin A caused a small reduction of water extrusion, whereas phalloidin and cytochalasins A, D, or E caused a marked inhibition. In these conditions there was a marked increase in size and number of cytoplasmic vesicles and a more widespread distribution of them within the cells, lacking the more specific orientation to the Golgi and canalicular regions that was seen in the presence of ouabain alone. Water extrusion was inhibited by phalloidin and cytochalasins in the absence of ouabain. In conclusion, our results are consistent with the hypothesis that iso-osmotic expulsion of water from hepatocytes can proceed partly through an accumulation of water in cytoplasmic vesicles, followed by exocytosis. This mechanism does not depend on Na+-K+ ATPase activity.

A contractile vacuole complex is involved in osmoregulation in Trypanosoma cruzi

Acidocalcisomes are dense, acidic organelles with a high concentration of phosphorus present as pyrophosphate and polyphosphate complexed with calcium and other cations. Acidocalcisomes have been linked to the contractile vacuole complex in Chlamydomonas reinhardtii, Dictyostelium discoideum, and Trypanosoma cruzi. A microtubule- and cyclic AMP-mediated fusion of acidocalcisomes to the contractile vacuole complex in T. cruzi results in translocation of aquaporin and the resulting water movement which, in addition to swelling of acidocalcisomes, is responsible for the volume reversal not accounted for by efflux of osmolytes. Polyphosphate hydrolysis occurs during hyposmotic stress, probably increasing the osmotic pressure of the contractile vacuole and facilitating water movement.

Acidocalcisomes and the contractile vacuole complex are involved in osmoregulation in Trypanosoma cruzi

Trypanosoma cruzi, the etiologic agent of Chagas disease, resists extreme fluctuations in osmolarity during its life cycle. T. cruzi possesses a robust regulatory volume decrease mechanism that completely reverses cell swelling when submitted to hypo-osmotic stress. The efflux of amino acids and K+ release could account for only part for this volume reversal. In this work we demonstrate that swelling of acidocalcisomes mediated by an aquaporin and microtubule- and cyclic AMP-mediated fusion of acidocalcisomes to the contractile vacuole complex with translocation of this aquaporin and the resulting water movement are responsible for the volume reversal not accounted for by efflux of osmolytes. Contractile vacuole bladders were isolated by subcellular fractionation in iodixanol gradients, showed a high concentration of basic amino acids and inorganic phosphate, and were able to transport protons in the presence of ATP or pyrophosphate. Taken together, these results strongly support a role for acidocalcisomes and the contractile vacuole complex in osmoregulation and identify a functional role for aquaporin in protozoal osmoregulation.

A functional aquaporin co-localizes with the vacuolar proton pyrophosphatase to acidocalcisomes and the contractile vacuole complex of Trypanosoma cruzi

We cloned an aquaporin gene from Trypanosoma cruzi (TcAQP) that encodes a protein of 231 amino acids, which is highly hydrophobic. The protein has six putative transmembrane domains and the two signature motifs asparagine-proline-alanine (NPA) which have been shown, in other aquaporins, to be involved in the formation of an aqueous channel spanning the bilayer. TcAQP was sensitive to endo H treatment, suggesting that the protein is N-glycosylated. Oocytes of Xenopus laevis expressing TcAQP swelled under hyposmotic conditions indicating water permeability, which was abolished after preincubating oocytes with very low concentrations of the AQP inhibitors HgCl(2) and AgNO(3). glycerol transport was detected. No Immunofluorescence microscopy of T. cruzi expressing GFP-TcAQP showed co-localization of TcAQP with the vacuolar proton pyrophosphatase (V-H(+)-PPase), a marker of acidocalcisomes. This localization was confirmed by Western blotting and immunofluorescence staining using polyclonal antibodies against a C-terminal peptide of TcAQP. In addition, there was a strong anterior labeling in a vacuole, close to the flagellar pocket, that was distinct from the acidocalcisomes and that was identified by immunogold electron microscopy as the contractile vacuole complex. Taking together, the presence of an aquaporin in acidocalcisomes and the contractile vacuole complex of T. cruzi, provides support for the role of these organelles in osmotic adaptations of these parasites.

Fluid-phase endocytosis does not contribute to rapid fluid secretion in the malpighian tubules of the house cricket, Acheta domesticus

When the Malpighian tubules (Mt) of the house cricket (Acheta domesticus) are treated with dibutyryl adenosine 3', 5'-cyclic monophosphate (db-cAMP; 1 mM), which causes a doubling in secretion rate, more than 50% of the cell volume is occupied by vesicles within 420 sec of exposure. In view of the fact that the increase in vesiculation occurs concomitantly with stimulated fluid transport, we set out to determine whether the vesicles are formed as a result of fluid-phase endocytosis (pinocytosis) and subsequently used to transport fluid to the lumen as one means of increasing transport rate. We used fluorescent fluid-phase markers (Lucifer Yellow Carbohydrazide [LYCH] and Alexa 488 hydrazide) and an electron dense marker (cationized ferritin) to elucidate the degree of endocytosis that occurred with db-cAMP stimulation. We found that, although some fluid is taken into the cells of the mid-tubule via endocytosis, it does not coincide with the level of vacuolation present in stimulated tubules. The amount of LYCH transported into the primary urine by the db-cAMP-stimulated Mt decreased by 40% as compared to the unstimulated transport, and the rate of transport of LYCH was only 30% of the unstimulated tubules. In summary, our findings do not support the theory that the majority of the vesicles or vacuoles comprise intracellular, endocytotic compartments formed via a basolateral endocytotic pathway. We also found no evidence to support the functioning of vesicles or vacuoles as transcellular "shuttling" mechanisms to move fluid from the basal region to the apical membrane and into the lumen.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

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.

Evidence for the role of actin filaments in regulating cell swelling

Actin filaments could play a role in regulation of cell swelling by two distinct mechanisms. One is by a tensile mechanism involving the coordinated interaction of actin and actin-associated proteins with all plasma membrane domains. The actin-membrane linkage would restrain cell swelling in the event of the influx of water. In shark rectal gland cells, conditions that cause massive cell swelling (i.e., high K medium, exposure to mercurials) are associated with disruption of membrane-associated actin filaments. Under conditions that result in only moderate swelling (Na-pump inhibition, Li substitution) the actin filaments remain associated with the cell membrane. Thus, in this cell type, disruption of the actin-membrane organization is correlated with increased swelling. Another mechanism by which actin could limit cell swelling is via regulation of ion transport proteins that are activated by cell swelling. This could be accomplished by a vesicle transport and insertion mechanism that delivers ion transport units to the cell membrane or by interaction with transport proteins already present in the membrane. Cell-attached patch clamp studies of RCCT-28A cells exposed to hypotonic medium demonstrated the activation of Cl-channel activity coincident with an alteration in actin. Activation of the channel was mimicked by stretching the membrane. Exposure of inside-out patches to cytochalasins also increased Cl-channel activity. Treatment of isolated patches with phalloidin inhibited stretch-induced activation. Thus, regulation of a volume-sensitive Cl-channel appears to be directly related to the state of organization of actin filaments.

Structural reaction pattern of hepatocytes following exposure to hypotonicity

Isolated rat hepatocytes were exposed to hypotonic media (225 mosmol/l) for 5 and 15 min and processed for a quantitative electron microscopic stereologic analysis. Within 5 min of hypotonicity, the hepatocyte volume increased by 25% and thereafter displayed a volume regulatory decrease leading to mean cellular volume, which was 16% above that of controls. Stereologic analysis of the major subcellular compartment, the cytosol, showed an identical change as the whole cell. In contrast to that, the mitochondrial compartment increased in volume by 30% within the first 5 min of exposure and returned by regulatory volume decrease back to values of the isotonic controls after 15 min of hypotonicity. In contrast, hypotonicity (220 mosmol/l)-induced stimulation of flux through mitochondrial glutaminase and the glycine cleavage enzyme complex, as assessed by 14CO2 production from [1-14C]glutamine or [1-14C]glycine in isolated perfused rat liver persisted throughout a 15-min period of hypotonic exposure. Thus hypotonicity-induced alterations of mitochondrial metabolism apparently do not parallel the time course of mitochondrial volume changes. This suggests that persistent mitochondrial swelling is not required for functional alterations, but that the latter may be triggered by the initial swelling of mitochondria. Hypotonic exposure did not alter the nuclear volume of isolated hepatocytes. Cell membrane surface nearly doubled after 5 min of hypotonic exposure, but returned within 15 min of exposure to values observed in normotonic media. This may reflect the participation of exocytosis in hepatocyte volume regulation.

Morphology of astroglia in colony cultures following transient exposure to potassium ion, hypoösmolarity and vasopressin

Brain swelling is the major cause of delayed neuronal damage following injury to the central nervous system. Swelling of mouse astroglial cells was studied in colony cultures by light and electron microscopy. Swelling of suspended astroglial cells was studied by flow cytometry. Swelling caused by hypoösmolarity solution was more pronounced than that caused by 15 or 60 mM K+. Under both conditions swelling in both immature and mature astroglia was followed by a regulatory volume decrease. Arginine vasopressin caused mild astroglial swelling and atrial natriuretic peptide did not significantly affect cell volume. All changes in extracellular environment were associated with changes in the morphology of microvilli and varying amounts of membrane ruffling. Immature cells exhibited a delayed response to the application of atrial natriuretic peptide and less membrane ruffling following exposure to 60 mM K+ than mature astroglia. These nonspecific morphological changes are likely associated with changes in membrane ion pump activity.

The response of contractile and non-contractile vacuoles of Paramecium calkinsi to widely varying salinities

Paramecium calkinsi from tidal marshes survive a wide salinity range. Fluid output of contractile vacuoles of these cells decreased as salinity of the medium to which they were acclimated increased, and both pulse rate and vacuole volume were used to regulate output. When cells were first exposed to more dilute medium, contractile vacuoles greatly increased volume so that fluid output increased even though pulse rate decreased. In cells shifted to a more concentrated medium, contractile vacuole output decreased by decreasing pulse rate. The contractile vacuole is surrounded by a set of collecting structures which change form as the salinity changes. Distensible ampullae are found in media of low salinity and collecting canals are found in media of high salinity. When cells are shifted from high salinity to low, the number of ampullae increases and the number of canals decreases. When cells are shifted from low salinity to high, the number of ampullae decreases and the number of canals decreases. Other non-contracting vacuoles also appear in response to a hypoosmotic shock. These include vacuoles within the cell as well as "blisters" on the surface. The number and frequency of blisters increases with the size of the hypoosmotic shock. They detach from cells without resulting in any visible loss of cytoplasm. Non-contractile vacuoles may play a role in sequestering and removing excess water that the contractile vacuoles cannot handle.

Comparative studies of volume restoration following cold-stress induced swelling in renal tissues--I. Effects of ouabain, K+ free medium, colchicine and cytochalasin B on rat and rabbit kidney cortex slices

1. Cold-stress-induced swelling in rabbit and rat kidney cortex slices cannot be due to the sole inhibition of a Na+/K+ exchange system. In these tissues indeed, ouabain induces no swelling and an exchange of Na+ for K+ or a l/l basis. Inhibition of K+ extrusion at low temperature has also to be taken into consideration. 2. Volume restoration at 27 degrees C after cold-stress-induced swelling is inhibited by ouabain in rabbit slices, not in rat ones. The inhibition in rabbit slices is concomitant with an increase in Na+ at levels higher than equilibrium with the external medium. 3. Volume restoration does not seem to implicate colchicine or cytochalasin B sensitive processes.