Swelling detection for volume regulation in the primitive eukaryote Giardia intestinalis: a common feature of volume detection in present-day eukaryotes

Microfilament dynamics during HaCaT cell volume regulation

Cell volume is an important parameter in many physiological processes, and is closely regulated in many cell types. In those cells, swelling induced by hypotonic media is followed by an ion-driven regulatory volume decrease. In many cell types, this regulatory volume decrease requires an intact actin cytoskeleton. Therefore, we investigated the changes in the structure and polymerization state of the actin cytoskeleton in HaCaT keratinocytes during cell swelling and regulatory volume decrease. Disruption of the actin cytoskeleton by 2microM cytochalasin D inhibits regulatory volume decrease in HaCaT cells. Cells swollen in the presence of low concentrations of cytochalasin D (0.8microM, 305-250mosM) keep the elevated volume even after cytochalasin D removal. A further decrease of tonicity (250-200mosM) is again counteracted by regulatory volume decrease reaching the volume, which has been established at 250mosM. In contrast, no visible changes occurred in actin cytoskeleton morphology of EGFP-actin-transfected HaCaT cells during swelling or regulatory volume decrease. However, biochemical analysis showed an increase in total F-actin levels 90s after the onset of hypotonicity. The ratio of Triton-soluble to -insoluble actin also increased after hypotonic shock, suggesting that the measured increase in F-actin is primarily due to de novo polymerization and formation of short actin filaments, i.e., actin oligomers. These results show that a rapid reorganization of the actin cytoskeleton takes place after hypotonic treatment. This reorganization can influence signaling in response to hypotonicity either indirectly by means of sequestering or releasing actin-associated proteins, or directly by the interaction of short actin filaments with plasma membrane ion channels, and may be involved in determining a new volume set point.

Tonic shock induces detachment of Giardia lamblia

BACKGROUND

The parasite Giardia lamblia must remain attached to the host small intestine in order to proliferate and subsequently cause disease. However, little is known about the factors that may cause detachment in vivo, such as changes in the aqueous environment. Osmolality within the proximal small intestine can vary by nearly an order of magnitude between host fed and fasted states, while pH can vary by several orders of magnitude. Giardia cells are known to regulate their volume when exposed to changes in osmolality, but the short-timescale effects of osmolality and pH on parasite attachment are not known.

METHODOLOGY AND PRINCIPAL FINDINGS

We used a closed flow chamber assay to test the effects of rapid changes in media osmolality, tonicity, and pH on Giardia attachment to both glass and C2(Bbe)-1 intestinal cell monolayer surfaces. We found that Giardia detach from both surfaces in a tonicity-dependent manner, where tonicity is the effective osmolality experienced by the cell. Detachment occurs with a characteristic time constant of 25 seconds (SD = 10 sec, n = 17) in both hypo- and hypertonic media but is otherwise insensitive to physiologically relevant changes in media composition and pH. Interestingly, cells that remain attached are able to adapt to moderate changes in tonicity. By exposing cells to a timed pattern of tonicity variations and adjustment periods, we found that it is possible to maximize the tonicity change experienced by the cells, overcoming the adaptive response and resulting in extensive detachment.

CONCLUSIONS AND SIGNIFICANCE

These results, conducted with human-infecting Giardia on human intestinal epithelial monolayers, highlight the ability of Giardia to adapt to the changing intestinal environment and suggest new possibilities for treatment of giardiasis by manipulation of tonicity in the intestinal lumen.

Lack of Threshold for Anisotonic Cell Volume Regulation

Most cells possess mechanisms that are able to detect cellular volume shifts and to signal the initiation of appropriate volume regulatory responses. However, the identity and characteristics of the detecting mechanism remain obscure. In this study, we explored the influence of hypertonic and hypotonic challenges of varying magnitude on the characteristics of the ensuing regulatory volume increase (RVI) and regulatory volume decrease (RVD) of cultured bovine corneal endothelial cells (CBCECs). The main question we asked was whether a threshold of stimulation existed that would unleash a regulatory response. CBCECs (passage 1-3) were seeded on rectangular glass coverslips and grown for 1-2 days. We used a procedure based on detection of light scattering to monitor the transient volume changes of such plated cells when subjected to osmotic challenge. The osmometric responses were asymmetric: cells shrank faster than they swelled (by a factor of 3). Complete volume regulatory responses took 10-12 min. Bumetanide (50 microM) resulted in incomplete (50%) RVI. We found no threshold as the cells examined responded to hypertonic and hypotonic stimuli as low as 1%. There was some gradation as stimuli of <4% resulted in incomplete volume regulation. The degree of activation of the volume responses grew as an exponential buildup with the strength of the anisotonic challenge. We discuss how our observations are consistent with volume sensing mechanisms based on both ionic strength and the cytoskeleton.

Role of K+ and amino acids in osmoregulation by the free-living microaerophilic protozoon Hexamita inflata

The primitive free-living protozoon Hexamita inflata was found to maintain a cell volume of approximately 260 fI under standard culture conditions. On increasing the extracellular osmolality the volume decreased and the cells remained shrunken for >30 min. By contrast, a decrease in the external osmolality resulted in a transient increase in cell volume which was followed by an efficient 'regulatory volume decrease' (RVD). H. inflata contains high concentrations of amino acids, with alanine constituting over 70% of the total amino acid pool. Exposure to hypo-osmotic medium resulted in the loss from the cell of both amino acids and K+, via one or more swelling-activated pathways. The efflux of amino acids and K+, together with a charge-balancing counter-anion, accounted almost fully for the observed RVD. The pharmacological properties of the swelling-activated pathways differ from those of volume-sensitive transporters and channels described previously in other cell types.

High intracellular chloride delays the activation of the volume-sensitive chloride conductance in mouse L-fibroblasts

1. The relationship between cell volume and volume-sensitive Cl- conductance during hyposmotic cell swelling of patched cells and the effects of intracellular chloride on the conductance have been studied in mouse L-fibroblasts. To this end, swelling-activated current and cell volume were measured simultaneously in cells dialysed with low-Cl- (16 mM) or high-Cl- (130 mM) solutions using the whole-cell patch-clamp technique and videomicroscopy. 2. The increase in cell volume of patched cells and the volume-sensitive conductance saturated during a 4-5 min exposure to mildly hyposmotic solutions (15-20 % less than isosmotic). The swelling of patched cells varied considerably and was greater than the swelling of intact cells. No correlation between the maximal values of the volume-sensitive conductance and the maximal volumes of swollen cells was evident for cells dialysed with the low-Cl- solutions. 3. The amplitude of the volume-sensitive conductance decreased with a reduction in either extracellular or intracellular Cl- concentration; the size of the maximal conductance was not modulated by intracellular Cl- ions. 4. The activation of the volume-sensitive conductance was slower in high-Cl- cells than in low-Cl- cells whether it was induced by hypotonic cell swelling or by cell inflation; in low-Cl- cells the conductance saturated before the cell volume had reached its maximal value. 5. It is concluded that in patched cells an increase in cell volume triggers activation of the volume-sensitive Cl- conductance but does not determine its amplitude and that the rate of activation of the conductance is affected by the intracellular Cl- concentration.