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

Full-length transcriptomic analysis reveals osmoregulatory mechanisms in Coilia nasus eyes reared under hypotonic and hyperosmotic stress

In recent years, sea-level rise, caused by global warming, will trigger salinity changes. This will threaten the survival of aquatic animals. Till now, the osmoregulatory mechanism of Coilia nasus eyes has not been yet explored. Oxford Nanopore Technologies (ONT) sequencing was performed in C. nasus eyes during hypotonic and hyperosmotic stress for the first time. 22.5G clean reads and 26,884 full-length non-redundant sequences were generated via ONT sequencing. AS events, APA, TF, and LncRNA were identified. During hypotonic stress, 46 up-regulated DEGs and 28 down-regulated DEGs were identified. During hypertonic stress, 190 up-regulated DEGs and 182 down-regulated DEGs were identified. These DEGs were associated with immune, metabolism, and transport responses. The expression of these DEGs indicated that apoptosis and inflammation were triggered during hypotonic and hyperosmotic stress. To resist hypotonic stress, polyamines metabolism and transport of Na⁺ and Cl^- from inter-cellular to extra-cellular were activated. During hyperosmotic stress, amino acids metabolism and transport of myo-inositol and Na⁺ from extra-cellular to inter-cellular were activated, while Cl^- transport was inhibited. Moreover, different transcript isoforms generated from the same gene performed different expression patterns during hypotonic and hypertonic stress. These findings will be beneficial to understand osmoregulatory mechanism of C. nasus eyes, and can also improve our insights on the adaptation of aquatic animals to environmental changes.

Leishmania donovani amastigotes mobilize organic and inorganic osmolytes during regulatory volume decrease

The protozoan parasite Leishmania donovani encounters large fluctuations in osmolality as it cycles between its insect vector and human host. The flagellated promastigote exhibits regulatory volume responses involving organic and inorganic osmolytes, but little is known about volume regulation in the clinically relevant amastigote that multiplies within the parasitophorous vacuoles of mammalian host cells. Using a combination of morphological, X-ray microanalytical, and biochemical approaches we determined that non-motile amastigotes respond to hypotonic stress with (1) an amino acid and l-alanine-mediated regulatory volume decrease, and (2) a parallel release of Na+, K+, P (presumably as negatively charged phosphates), and subsequently Cl- from cytoplasm and the cell as a whole. In addition P, Zn2+, and subsequently Ca2+ increase in acidocalcisomes as Cl- content declines in this compartment. This evidence is the first to document subcellular translocation of, and thus a potential role for, zinc in volume regulatory responses. These coordinated changes in organic and inorganic osmolytes demonstrate that amastigote subcellular compartments, particularly acidocalcisomes, function in maintaining ionic homeostasis in the response of Leishmania amastigotes to hypo-osmotic stress.

Osmoregulation in the parasitic protozoan Tritrichomonas foetus

Tritrichomonas foetus was shown to undergo a regulatory volume increase (RVI) when it was subjected to hyperosmotic challenge, but there was no regulatory volume decrease after hypoosmotic challenge, as determined by using both light-scattering methods and measurement of intracellular water space to monitor cell volume. An investigation of T. foetus intracellular amino acids revealed a pool size (65 mM) that was similar to that of Trichomonas vaginalis but was considerably smaller than those of Giardia intestinalis and Crithidia luciliae. Changes in amino acid concentrations in response to hyperosmotic challenge were found to account for only 18% of the T. foetus RVI. The T. foetus intracellular sodium and potassium concentrations were determined to be 35 and 119 mM, respectively. The intracellular K(+) concentration was found to increase considerably during exposure to hyperosmotic stress, and, assuming that there was a monovalent accompanying anion, this increase was estimated to account for 87% of the RVI. By using light scattering it was determined that the T. foetus RVI was enhanced by elevated external K(+) concentrations and was inhibited when K(+) and/or Cl(-) was absent from the medium. The results suggested that the well-documented Na(+)-K(+)-2Cl(-) cotransport system was responsible for the K(+) influx activated during the RVI. However, inhibitors of Na(+)-K(+)-2Cl(-) cotransport in other systems, such as quinine, ouabain, furosemide, and bumetanide, had no effect on the RVI or K(+) influx in T. foetus.

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.

Osmoregulation inParamecium: in situ ion gradients permit water to cascade through the cytosol to the contractile vacuole

In vivo K+, Na+, Ca2+ and Cl-activities in the cytosol and the contractile vacuole fluid of Paramecium multimicronucleatum were determined in cells adapted to a number of external osmolarities and ionic conditions by using ion-selective microelectrodes. It was found that: (1) under standardized saline conditions K+ and Cl- were the major osmolytes in both the cytosol and the contractile vacuole fluid; and (2) the osmolarity of the contractile vacuole fluid, determined from K+ and Cl- activities only, was always more than 1.5 times higher than that of the cytosol. These findings indicate that excess cytosolic water crosses the contractile vacuole complex membrane osmotically. Substitution of choline or Ca2+ for K+ in the external solution or the external application of furosemide caused concomitant decreases in the cytosolic K+ and Cl- activities that were accompanied by a decrease in the water segregation activity of the contractile vacuole complex. This implies that the cytosolic K+ and Cl- are actively coimported across the plasma membrane. Thus, the osmotic gradients across both the plasma membrane and the membrane of the contractile vacuole complex ensure a controlled cascade of water flow through the cell that can provide for osmoregulation as well as the possible extrusion of metabolic waste by the contractile vacuole complex.

The antioxidant potential of pyruvate in the amitochondriate diplomonads Giardia intestinalis and Hexamita inflata

Giardia intestinalis and Hexamita inflata are microaerophilic protozoa which rely on fermentative metabolism for energy generation. These organisms have developed a number of antioxidant defence strategies to cope with elevated O(2) tensions which are inimical to survival. In this study, the ability of pyruvate, a central component of their energy metabolism, to act as a physiological antioxidant was investigated. The intracellular pools of 2-oxo acids in G. intestinalis were determined by HPLC. With the aid of a dichlorodihydrofluorescein diacetate-based assay, intracellular reactive oxygen species generation by G. intestinalis and H. inflata suspensions was monitored on-line. Addition of physiologically relevant concentrations of pyruvate to G. intestinalis and H. inflata cell suspensions was shown to attenuate the rate of H(2)O(2)- and menadione-induced generation of reactive oxygen species. In addition, pyruvate was also shown to decrease the generation of low-level chemiluminescence arising from the oxygenation of anaerobic suspensions of H. inflata. In contrast, addition of pyruvate to suspensions of respiring Saccharomyces cerevisiae was shown to increase the generation of reactive oxygen species. These data suggest that (i) in G. intestinalis and H. inflata, pyruvate exerts antioxidant activity at physiological levels, and (ii) it is the absence of a respiratory chain in the diplomonads which facilitates the observed antioxidant activity.