Interrogating the Electromechanical Regulation of Cellular Volume at the Single-Cell Level

Main Text

Cells have developed a complex regulatory mechanism to achieve homeostasis of their volume as well as, in some cases, to use its change to trigger processes such as mitosis, migration, and apoptosis (1, 2). In particular, it is believed that a delicate ionic balance across the cell membrane must be maintained for the cell to keep its volume, and disturbance in such equilibrium could be closely related to the development of diseases such as cancer (3) and neurological disorders (4). Interestingly, although the exact mechanism remains elusive, accumulating evidence has demonstrated that changes in the cellular volume can be triggered by a number of electromechanical stimuli. For example, cell shrinkage (or swelling) was observed when the ionic environment was perturbed by an applied electrical field (5). In addition, it has been found that active cross-membrane ion transport in cells could be activated by elevated membrane tension (6), eventually leading to changes in the cellular volume (7). Surprisingly, a recent study also revealed that leukemia cells can respond to variations in the surrounding hydrostatic pressure by adjusting their volume (8). Despite this progress, a major challenge faced here is to isolate the role of a specific cue on the volume regulation of cells from other contributing factors. For instance, it is conceivable that the membrane potential (i.e., the voltage difference across the cell membrane) could be altered as the cell starts to swell/shrink under the influence of osmotic/hydrostatic pressure shock, which in return will further induce cross-membrane ion transport and size change of cells. In addition, most existing studies focused on the average response of a large number of cells immersed in the culture medium, in which precise control of experimental conditions is difficult to achieve.

In this issue of Biophysical Journal, Yellin et al. (9) present a promising method that could address these outstanding problems. Specifically, by using the whole-cell patch-clamp setup, Yellin and co-workers were able to precisely control the membrane potential of a single cell as well as monitor its volume change in an accurate and continuous manner. It is conceivable that, in combining such a setup with optical trapping (Fig. 1) or micropipette aspiration, the role of membrane tension in regulating the cell volume can be systematically examined. Indeed, it has been reported in (9) that disruption of cortical contraction in cells resulted in their swelling. Interestingly, Yellin et al. (9) also found that, even when the surrounding osmolarity was kept at the same level, altering the concentration percentage of chloride or potassium ions in the culture fluid led to distinct volumetric response of cells, highlighting the importance of cross-membrane transport of individual ion species in the volume regulation of cells. Fortunately, on the basis of the platform developed in (9), it becomes possible to quantitatively measure, after proper fluorescent labeling (8, 10), how the concentrations of different intracellular ions evolve under precisely controlled experimental conditions (e.g., at fixed level of membrane potential and/or extracellular concentration of individual ions), which will greatly help us understand the mechanism(s) behind such transmembrane ion exchange. Finally, by taking into account both the passive and active (by different ion channels/pumps) cross-membrane transport of major ions as well as water molecules, Yellin et al. (9) propose a mathematical model to quantitatively explain the observed volumetric response of cells under different electromechanical cues in their experiment. Interestingly, the model also suggests that as cells grow and age, their volume will increase linearly with the total amount of cell proteins, whereas the concentrations of intracellular proteins and ions will remain more or less constant. These predictions certainly provide insights on how cells maintain homeostasis during their life cycle.

Figure 1.

Changes in the cell volume can be triggered by a number of electromechanical signals such as variations in the surrounding osmotic or hydrostatic pressure, the membrane potential that can be controlled by a patch clamp, and the membrane tension level, which can be altered by, for example, optical pulling or disruption of cortical contraction in the cell. To see this figure in color, go online.

It must be pointed out that the study by Yellin et al. (9) focuses on HN31 cancer cells. However, it has been well-documented that the expression level and functioning of ion and water channel proteins in tumor cells and their normal counterparts can be very different (3, 5), which ultimately may result in their distinct reactions against the same volume-changing cue. In this regard, with the help of the powerful method developed in (9), it will be interesting to see whether a quantitative link between the volumetric response of cells and their pathological status can be established.

Editor: Vivek Shenoy.