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

Structure and dynamics of the contractile vacuole complex in Tetrahymena thermophila

The contractile vacuole complex (CVC) is a dynamic and morphologically complex membrane organelle, comprising a large vesicle (bladder) linked with a tubular reticulum (spongiome). CVCs provide key osmoregulatory roles across diverse eukaryotic lineages, but probing the mechanisms underlying their structure and function is hampered by the limited tools available for in vivo analysis. In the experimentally tractable ciliate Tetrahymena thermophila, we describe four proteins that, as endogenously tagged constructs, localize specifically to distinct CVC zones. The DOPEY homolog Dop1p and the CORVET subunit Vps8Dp localize both to the bladder and spongiome but with different local distributions that are sensitive to osmotic perturbation, whereas the lipid scramblase Scr7p colocalizes with Vps8Dp. The H+-ATPase subunit Vma4 is spongiome specific. The live imaging permitted by these probes revealed dynamics at multiple scales including rapid exchange of CVC-localized and soluble protein pools versus lateral diffusion in the spongiome, spongiome extension and branching, and CVC formation during mitosis. Although the association with DOP1 and VPS8D implicate the CVC in endosomal trafficking, both the bladder and spongiome might be isolated from bulk endocytic input.

A set of SNARE proteins in the contractile vacuole complex of Paramecium regulates cellular calcium tolerance and also contributes to organelle biogenesis

The contractile vacuole complex (CVC) of freshwater protists serves the extrusion of water and ions, including Ca(2+). No vesicle trafficking based on SNAREs has been detected so far in any CVC. SNAREs (soluble NSF [N-ethylmaleimide sensitive factor] attachment protein receptors) are required for membrane-to-membrane interaction, i.e. docking and fusion also in Paramecium. We have identified three v-/R- and three t/Q-SNAREs selectively in the CVC. Posttranscriptional silencing of Syb2, Syb6 or Syx2 slows down the pumping cycle; silencing of the latter two also causes vacuole swelling. Increase in extracellular Ca(2+) after Syb2, Syb6 or Syx2 silencing causes further swelling of the contractile vacuole and deceleration of its pulsation. Silencing of Syx14 or Syx15 entails lethality in the Ca(2+) stress test. Thus, the effects of silencing strictly depend on the type of the silenced SNARE and on the concentration of Ca(2+) in the medium. This shows the importance of organelle-resident SNARE functions (which may encompass the vesicular delivery of other organelle-resident proteins) for Ca(2+) tolerance. A similar principle may be applicable also to the CVC in widely different unicellular organisms. In addition, in Paramecium, silencing particularly of Syx6 causes aberrant positioning of the CVC during de novo biogenesis before cytokinesis.

Chapter 6: cubic membranes the missing dimension of cell membrane organization

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between “inside” and “outside”, survival, and death. Intracellular compartmentalization—governed by biomembranes as well—is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into “unusual”, highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Electrical properties and fusion dynamics ofin vitromembrane vesicles derived from separate parts of the contractile vacuole complex ofParamecium multimicronucleatum

The contractile vacuole complex of Paramecium multimicronucleatumtransforms into membrane-bound vesicles on excision from the cell. The I–V relationship was linear in a voltage range of–80 to +80 mV in all vesicles, despite being derived from different parts of the contractile vacuole complex. No voltage-gated unit currents were observed in membrane patches from the vesicles. Vesicles derived from the radial arm showed a membrane potential of >10 mV, positive with reference to the cytosol, while those derived from the contractile vacuole showed a residual (<5 mV) membrane potential. The electrogenic V-ATPases in the decorated spongiome are responsible for the positive potential, and Cl– leakage channels are responsible for the residual potential. The specific resistance of the vesicle membrane (∼6 kΩcm2) increased, while the membrane potential shifted in a negative direction when the vesicle rounded. An increase in the membrane tension (to∼5×10–3 N m–1) is assumed to reduce the Cl– leakage conductance. It is concluded that neither voltage- nor mechano-sensitive ion channels are involved in the control of the fluid segregation and membrane dynamics that govern fluid discharge cycles in the contractile vacuole complex.

The membrane vesicles shrank when the external osmolarity was increased,and swelled when the osmolarity was decreased, implying that the contractile vacuole complex membrane is water permeable. The water permeability of the membrane was 4–20×10–7 μm s–1Pa–1. The vesicles containing radial arm membrane swelled after initially shrinking when exposed to higher external osmolarity, implying that the V-ATPases energize osmolyte transport mechanisms that remain functional in the vesicle membrane. The vesicles showed an abrupt (<30 ms),slight, slackening after rounding to the maximum extent. Similar slackening was also observed in the contractile vacuoles in situ before the opening of the contractile vacuole pore. A slight membrane slackening seems to be an indispensable requirement for the contractile vacuole membrane to fuse with the plasma membrane at the pore. The contractile vacuole complex-derived membrane vesicle is a useful tool for understanding not only the biological significance of the contractile vacuole complex but also the molecular mechanisms of V-ATPase activity.

Cell volume control in Paramecium: factors that activate the control mechanisms

A fresh water protozoan Paramecium multimicronucleatum adapted to a given solution was found to swell until the osmotic pressure difference between the cytosol and the solution balanced the cytosolic pressure. The cytosolic pressure was generated as the cell swelled osmotically. When either one or both of these pressures was somehow modified, cell volume would change until a new balance between these pressures was established. A hypothetical osmolyte transport mechanism(s) was presumably activated when the cytosolic pressure exceeded the threshold value of approximately 1.5 x 10(5) Pa as the cell swelled after its subjection to a decreased osmolarity. The cytosolic osmolarity thereby decreased and the volume of the swollen cell resumed its initial value. This corresponds to regulatory volume decrease (RVD). By contrast, another hypothetical osmolyte transport mechanism(s) was activated when the cell shrank after its subjection to an increased osmolarity. The cytosolic osmolarity thereby increased and volume of the shrunken cell resumed its initial value. This corresponds to regulatory volume increase (RVI). The osmolyte transport mechanism responsible for RVD might be activated again when the external osmolarity decreases further, and the cytosolic osmolarity thereby decreases to the next lower level. Similarly, another osmolyte transport mechanism responsible for RVI might be activated again when the external osmolarity increases further, and the cytosolic osmolarity thereby increases to the next higher level. Stepwise changes in the cytosolic osmolarity caused by a gradual change in the adaptation osmolarity found in P. multimicronucleatum is attributable to these osmolyte transport mechanisms. An abrupt change in the amount of fluid discharged from the contractile vacuole seen immediately after changing the external osmolarity reduces an abrupt change in cell volume and thereby protects the cell from the disruption of the plasma membrane by excessive stretch or dehydration during shrinkage.

Hypo-osmotic or Ca2+-rich external conditions trigger extra contractile vacuole complex generation in Paramecium multimicronucleatum

The freshwater ciliated protozoan, Paramecium multimicronucleatum, usually possesses two contractile vacuole complexes (CVCs). The number of CVCs in a single cell, however, may vary from 1 to 7. We found that the number of cells that have more than two CVCs increased after the cells were exposed to a hypo-osmotic or a high Ca2+ condition. It is assumed that the biological significance of this increase in the number of CVCs is to enhance the cell's ability to eliminate excess water or Ca2+ from the cytosol. An extra CVC was either generated de novo in the posterior region of the cell or, when in the anterior region, by binary fission of the anterior CVC. Generation of these extra CVCs was not inhibited by aphidicolin, a potent inhibitor of DNA synthesis in the micronuclei of Paramecium, even though normal duplication of the CVC that accompanies normal cell division was completely inhibited by this inhibitor. These results suggest that generation of extra CVCs is controlled by a hypothetical regulatory mechanism that is activated either by a hypo-osmotic or by a Ca2+-rich condition and that differs from the regulatory mechanism that governs normal CVC duplication during cell division.

Relationship between the membrane potential of the contractile vacuole complex and its osmoregulatory activity inParamecium multimicronucleatum

The electric potential of the contractile vacuole (CV) of Paramecium multimicronucleatum was measured in situ using microelectrodes,one placed in the CV and the other (reference electrode) in the cytosol of a living cell. The CV potential in a mechanically compressed cell increased in a stepwise manner to a maximal value (approximately 80 mV) early in the fluid-filling phase. This stepwise change was caused by the consecutive reattachment to the CV of the radial arms, where the electrogenic sites are located. The current generated by a single arm was approximately 1.3×10-10 A. When cells adapted to a hypotonic solution were exposed to a hypertonic solution, the rate of fluid segregation, RCVC, in the contractile vacuole complex (CVC) diminished at the same time as immunological labelling for V-ATPase disappeared from the radial arms. When the cells were re-exposed to the previous hypotonic solution, the CV potential, which had presumably dropped to near zero after the cell's exposure to the hypertonic solution, gradually returned to its maximum level. This increase in the CV potential occurred in parallel with the recovery of immunological labelling for V-ATPase in the radial arm and the resumption of RCVC or fluid segregation. Concanamycin B, a potent V-ATPase inhibitor, brought about significant decreases in both the CV potential and RCVC. We confirm that (i) the electrogenic site of the radial arm is situated in the decorated spongiome, and (ii) the V-ATPase in the decorated spongiome is electrogenic and is necessary for fluid segregation in the CVC. The CV potential remained at a constant high level(approximately 80 mV), whereas RCVC varied between cells depending on the osmolarity of the adaptation solution. Moreover, the CV potential did not change even though RCVC increased when cells adapted to one osmolarity were exposed to a lower osmolarity, implying that RCVC is not directly correlated with the number of functional V-ATPase complexes present in the CVC.

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.

The ionic composition of the contractile vacuole fluid of Paramecium mirrors ion transport across the plasma membrane

In vivo K+, Na+, Ca2+, Cl- and H+ activities in the cytosol and the contractile vacuole fluid, the overall cytosolic osmolarity, the fluid segregation rate per contractile vacuole and the membrane potential of the contractile vacuole complex of Paramecium multimicronucleatum were determined in cells adapted to 24 or 124 mosm l(-1) solutions containing as the monovalent cation(s): 1) 2 mmol l(-1) K+; 2) 2 mmol l(-1) Na+; 3) 1 mmol l(-1) K+ plus 1 mmol l(-1) Na+; or 4) 2 mmol l(-1) choline. In cells adapted to a given external osmolarity i) the fluid segregation rate was the same if adapted to either K+ or Na+, twice as high when adapted to solutions containing both K+ and Na+, and reduced by 50% or more in solutions containing only choline, ii) the fluid of the contractile vacuole was always hypertonic to the cytosol while the sum of the ionic activities measured in the fluid of the contractile vacuole was the same in cells adapted to either K+ or Na+, at least 25% higher in cells adapted to solutions containing both K+ and Na+, and was reduced by 55% or more in solutions containing only choline, iii) the cytosolic osmolarity was the same in cells adapted to K+ alone, to Na+ alone or to both K+ and Na+, whereas it was significantly lower in cells adapted to choline. At a given external osmolarity, a positive relationship between the osmotic gradient across the membrane of the contractile vacuole complex and the fluid segregation rate was observed. We conclude that both the plasma membrane and the membrane of the contractile vacuole complex play roles in fluid segregation. The presence of external Na+ moderated K+ uptake and caused the Ca2+ activity in the contractile vacuole fluid to rise dramatically. Thus, Ca2+ can be eliminated through the contractile vacuole complex when Na+ is present externally. The membrane potential of the contractile vacuole complex remained essentially the same regardless of the external ionic conditions and the ionic composition of the fluid of the contractile vacuole. Notwithstanding the large number of V-ATPases in the membrane of the decorated spongiome, the fluid of the contractile vacuole was found to be only mildly acidic, pH 6.4.

The vacuolar-atpase of Paramecium multimicronucleatum: gene structure of the B subunit and the dynamics of the V-ATPase-rich osmoregulatory membranes

Previous studies have shown that the vacuolar-ATPase (V-ATPase) of the contractile vacuole complexes (CVCs) in Paramecium multimicronucleatum is necessary for fluid segregation and osmoregulation. In the current study, immunofluorescence showed that the development of a new CVC begins with the formation of a new pore around which the collecting canals form. The decorated membranes are then deposited around the newly formed collecting canals. Quick-freeze deep-etch techniques reveal that six 10-nm-wide V-ATPase V, sectors, tightly packed into a 20 x 30-nm rectangle, form two rows of these compacted sectors that helically wrap around the cytosolic side of decorated membrane tubules. During new CVC formation, packing of decorated tubules around mature CVCs was temporarily disrupted so that some of these decorated tubules became transformed into decorated vesicles. Freeze-fracturing of these decorated vesicles revealed a highly pitted E-face and a particulate P-face. The V-ATPase was purified for the first time in any ciliated protozoan and shown to contain, as in other cells, the V1 subunits A to E, and four 14-20 kDa polypeptides. The B subunit was cloned and found to be encoded by one gene containing four short introns. This subunit has 510 amino acid residues with a predicted molecular weight of 56.8 kDa, a value similar to B subunits of other organisms. Except for the N- and C-termini, it has a 75% sequence identity with other B subunits, suggesting that the B subunits in Paramecium, like other species, have been conserved and that the entire surface of this subunit may be important in interacting with other subunits.

Reversible vacuolation of T-tubules in skeletal muscle: mechanisms and implications for cell biology

The majority of investigations of the transverse tubules (T-system) of skeletal muscle have been devoted to their role in excitation-contraction coupling, with particular reference to contact with the sarcoplasmic reticulum and the mechanism of Ca2- release. By contrast, this review is concerned with structural and functional aspects of the vacuolation of T-tubules. It covers experimental procedures used in reversible vacuolation induced by the efflux-influx of glycerol and other small nonelectrolytes, sugars, and ions. The characteristics of the phenomenon, associated alterations in muscle function, and the swelling of analogous structures in nonmuscle cells are considered. Possible functions of reversible vacuolation in water balance, transport, membrane repair, muscle pathology, and fatigue are considered, and the potential application of reversible vacuolation in the transfection of skeletal muscle is discussed. In relation to the possible mechanisms involved in reversible vacuolation, particular attention is given to the dynamic and structural aspects of the opening and closing of T-tubules, the origin of vacuolar membranes, and the localized character of tubular swelling.

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.