Drying and salting send different messages

Does the intracellular ionic concentration or the cell water content (cell volume) determine the activity of TonEBP in NIH3T3 cells?

The transcription factor, tonicity-responsive enhancer binding protein (TonEBP), is involved in the adaptive response against hypertonicity. TonEBP regulates the expression of genes that catalyze the accumulation of osmolytes, and its transcriptional activity is increased by hypertonicity. The goal of the present investigation was to investigate whether cell shrinkage or high intracellular ionic concentration induced the activation of TonEBP. We designed a model system for isotonically shrinking cells over a prolonged period of time. Cells swelled in hypotonic medium and performed a regulatory volume decrease. Upon return to the original isotonic medium, cells shrank initially, followed by a regulatory volume increase. To maintain cell shrinkage, the RVI process was inhibited as follows: ethyl-isopropyl-amiloride inhibited the Na+/H+ antiport, bumetanide inhibited the Na+-K+-2Cl− cotransporter, and gadolinium inhibited shrinkage-activated Na+ channels. Cells remained shrunken for at least 4 h (isotonically shrunken cells). The activity of TonEBP was investigated with a Luciferase assay after isotonic shrinkage and after shrinkage in a high-NaCl hypertonic medium. We found that TonEBP was strongly activated after 4 and 16 h in cells in high-NaCl hypertonic medium, but not after 4 or 16 h in isotonically shrunken cells. Cells treated with high-NaCl hypertonic medium for 4 h had significantly higher intracellular concentrations of both K+ and Na+ than isotonically shrunken cells. This strongly suggested that an increase in intracellular ionic concentration and not cell shrinkage is involved in TonEBP activation.

Naturally occurring and stress induced tubular structures from mammalian cells, a survival mechanism

Background

Tubular shaped mammalian cells in response to dehydration have not been previously reported. This may be due to the invisibility of these cells in aqueous solution, and because sugars and salts added to the cell culture for manipulation of the osmotic conditions inhibit transformation of normal cells into tubular shaped structures.

Results

We report the transformation of normal spherical mammalian cells into tubular shaped structures in response to stress. We have termed these transformed structures 'straw cells' which we have associated with a variety of human tissue types, including fresh, post mortem and frozen lung, liver, skin, and heart. We have also documented the presence of straw cells in bovine brain and prostate tissues of mice. The number of straw cells in heart, lung tissues, and collapsed straw cells in urine increases with the age of the mammal. Straw cells were also reproduced in vitro from human cancer cells (THP1, CACO2, and MCF7) and mouse stem cells (D1 and adipose D1) by dehydrating cultured cells. The tubular center of the straw cells is much smaller than the original cell; houses condensed organelles and have filamentous extensions that are covered with microscopic hair-like structures and circular openings. When rehydrated, the filaments uptake water rapidly. The straw cell walls, have a range of 120 nm to 200 nm and are composed of sulfated-glucose polymers and glycosylated acidic proteins. The transformation from normal cell to straw cells takes 5 to 8 hr in open-air. This process is characterized by an increase in metabolic activity. When rehydrated, the straw cells regain their normal spherical shape and begin to divide in 10 to 15 days. Like various types of microbial spores, straw cells are resistant to harsh environmental conditions such as UV-C radiation.

Conclusion

Straw cells are specialized cellular structures and not artifacts from spontaneous polymerization, which are generated in response to stress conditions, like dehydration. The disintegrative, mobile, disruptive and ubiquitous nature of straw cells makes this a possible physiological process that may be involved in human health, longevity, and various types of diseases such as cancer.

Desiccation Response of Mammalian Cells: Anhydrosignaling

Dehydration through evaporation, or air drying, is expected to have both similarities and differences to osmostress. Both stresses involve water loss, but the degree of dehydration will ultimately be more severe during desiccation. Despite the severity of desiccation stress, there are examples of organisms that can survive almost complete water loss, including resurrection plants and plant seeds, certain invertebrates among the nematodes, brine shrimps, tardigrades and bdelloid rotifers, and many microorganisms, including bakers' yeast. During desiccation, these organisms enter a state of suspended animation, a process known as anhydrobiosis ("life without water"). For other organisms, desiccation is lethal, but there is considerable interest in using what is known about anhydrobiosis to confer desiccation tolerance on sensitive cell types, such as mammalian cells. Success with this approach, which we have termed anhydrobiotic engineering, will require a more complete knowledge of the mechanisms of desiccation tolerance and the sensing and response of nontolerant organisms to extreme dehydration. With this goal in mind, we have attempted to characterize the response of human tissue culture cells to desiccation and to compare this response with osmotic upshift. This chapter describes some of the methods used to begin to uncover the response to evaporative water loss in human cell cultures.

Gene induction by desiccation stress in human cell cultures

One strategy for investigating desiccation tolerance is to use mammalian cells, which are sensitive to desiccation, as a model for testing putative adaptive mechanisms. However, how mammalian cells themselves respond to desiccation is poorly characterised. Although MAPK signal transduction pathways are activated by desiccation of human cells, hypertonicity-responsive genes AR, BGT1 and SMIT are not significantly induced, although they are proposed to be regulated by physiological changes which should occur during drying. To determine whether a response to desiccation occurs at the transcriptional level in human cells, we performed genome-wide microarray analysis. Twenty upregulated genes, including early stress response and transcription factor genes, were identified, most of which, e.g., EGR1, EGR3, SNAI1, RASD1 and GADD45B, were also induced by hypertonicity, indicating common regulatory mechanisms. Our data suggest that human cells can initiate a complex desiccation stress response distinct from, but overlapping with, that to hypertonic stress.