Phase separation in necrotic cells

Stability of Intracellular Protein Concentration under Extreme Osmotic Challenge

Cell volume (CV) regulation is typically studied in short-term experiments to avoid complications resulting from cell growth and division. By combining quantitative phase imaging (by transport-of-intensity equation) with CV measurements (by the exclusion of an external absorbing dye), we were able to monitor the intracellular protein concentration (PC) in HeLa and 3T3 cells for up to 48 h. Long-term PC remained stable in solutions with osmolarities ranging from one-third to almost twice the normal. When cells were subjected to extreme hypoosmolarity (one-quarter of normal), their PC did not decrease as one might expect, but increased; a similar dehydration response was observed at high concentrations of ionophore gramicidin. Highly dilute media, or even moderately dilute in the presence of cytochalasin, caused segregation of water into large protein-free vacuoles, while the surrounding cytoplasm remained at normal density. These results suggest that: (1) dehydration is a standard cellular response to severe stress; (2) the cytoplasm resists prolonged dilution. In an attempt to investigate the mechanism behind the homeostasis of PC, we tested the inhibitors of the protein kinase complex mTOR and the volume-regulated anion channels (VRAC). The initial results did not fully elucidate whether these elements are directly involved in PC maintenance.

Studying cell volume beyond cell volume

The first part of the paper describes two simple microscopic techniques that we use in our laboratory. One measures cell volumes in adherent cultures and the other measures cell dry mass; both measurements are done on the same instrument (a standard bright-field transmission microscope with only one or two narrow-band color filters added) and on the same cells. The reason for combining cell volume with dry mass is that the ratio of the two-dry mass concentration (MC)-is an important and insufficiently utilized biological parameter. We then describe a few applications of MC. The available experimental data strongly suggest its critical role in biological processes, including cell volume regulation. For example, most eukaryotic cells have surprisingly similar values of MC. Moreover, MC (and not cell volume) is tightly controlled in growing cell cultures at highly variable external osmolarities. We review the results showing that elevation of MC is a direct cause of shrinkage-induced apoptosis. Also, by focusing on MC, one can study heterogenous processes, such as necrotic swelling, or discriminate between apoptotic dehydration and the loss of cell fragments.

Comparison of cell volume measurements by fluorescence and absorption exclusion microscopy

There are two light microscopic methods for cell volume measurement based on volume exclusion. One method, sometimes referred to as FLEX, utilises negative staining by an external fluorescent dye, and cell volume is found from attenuation of fluorescence under a wide-field microscope. The other method (TTD) is based on exclusion of an external absorbing dye, resulting in an increased intensity of transmission image. In this work, we compared these two methods. TTD measurements were consistent, reproducible and identical to those obtained by confocal scanning. In our hands, FLEX based on either sodium fluorescein of fluorescent dextran, usually resulted in underestimation of cell volume, which were insignificant in shallow chambers but became more severe with increased chamber depth. We have not been able to exactly pinpoint the source of the problem; it may have been undetected accumulation of dye in the cells or, more likely, some unappreciated aspects of image formation under epi-illumination. We also discuss applicability of both methods to in-flow volume measurements. LAY DESCRIPTION: Cell volume is a parameter important for many cell biological and physiological applications, and many different methods have been proposed for its measurement. Two light microscopic methods based on volume exclusion deserve special attention due to their speed and simplicity. In one of them (transmission-through-dye, or TTD), cells are placed in a shallow chamber, and a strongly absorbing external dye is added to the cell-containing medium. The sample is imaged in transmission at a wavelength of maximum dye absorption. Because cells with intact membranes exclude the dye, they appear brighter on a transmission image, and their contrast quantitatively reflects cell thickness. By summation of thickness values over the cell area, cell volume can be obtained. The other method sometimes referred to as FLEX utilises exclusion of a fluorescent dye. Cells appear darker than the background under wide-field fluorescence observation in accordance with their thicknesses, and cell volume is computed by thickness summation over the area, like in TTD. In this work, we compared the accuracy of TTD and FLEX for volume measurements. TTD and confocal scanning produced virtually identical results, which suggests that TTD data are accurate. On the other hand, cell volumes measured by FLEX were consistently smaller than by TTD. The discrepancy always increased with the depth of the chamber, although the exact relationship varied between experiments. By contrast, TTD results were insensitive to chamber depth. Thus, it appears that FLEX underestimates cell volume. The reason for that is not entirely clear. Accumulation of the fluorescent dye inside the cell could be a possibility, although we found no evidence for that. Most likely, the reason lies with some unappreciated aspects of wide-field fluorescence image formation, which has not been well-characterised for the type of negative staining used in FLEX. In our opinion, TTD is the method of choice, at least for stationary cells. On the other hand, due to linear dependence of intensity on volume, FLEX might offer advantages for high-throughput flow volume imaging, although realisation of such an experiment has yet to be worked out.

Cell Volume Measurements by Optical Transmission Microscopy

Cell volume is an important parameter in studying cell adaptation to anisosmotic stress, activation of monovalent ion channels, and cell death. This article describes a method for measurement of the volumes of adherent cells using a standard light microscope. A coverslip with attached cells is placed in a shallow chamber in a medium containing a strongly absorbing and cell-impermeant dye, Acid Blue 9. When such a sample is imaged in transmitted light at a wavelength of maximum dye absorption (630 nm), the resulting contrast quantitatively reflects cell thickness; once the thickness is known at every point, the volume can be computed as well. Technical details, interpretation of data, and possible artifacts are discussed. © 2019 by John Wiley & Sons, Inc.