Correlation of single-cell ICP-MS intensity distributions for the study of heterogeneous cellular responses to environmental stresses
Biological cells in a population can behave heterogeneously upon uniform external stimulation, i.e., responses of subpopulations of the cells are different. Therefore, distributions of cell properties, rather than the average, are more meaningful for the characterization of biological cells under stresses. Change in the symmetry of the distribution is also an indicator of heterogeneous cellular responses. Single-cell ICP-MS measures the contents of essential and absorbed elements of individual cells in a population and naturally gives the distributions of ICP-MS intensity of the measured elements. We propose the correlation of the distributions of ICP-MS intensity and other cellular properties for the characterization of cellular responses to external stresses. A novel correlation method is adopted: the corresponding percentiles of distributions of two cell properties are correlated in a plot. The linearity of the correlation plots is a measure of the similarity of the two distributions under external stresses. Linear plots signify similar skewness and cellular responses, and vice versa. In addition, changes in the slope of the plots versus the strength of external stress are a measure of the relative sensitivity of the two properties to the external stress. The well-characterized toxicology of Cr(VI) on C. vulgaris was used as a test model. Mg was used as an intrinsic biomarker. Distributions of the ICP-MS intensity of Mg and cell volume become asymmetric under Cr(VI) stress, indicating heterogeneous cellular responses to Cr(VI) stresses. However, each pair of distributions is of similar shape and skewness and the correlation plots are linear, suggesting that the two properties are related. Moreover, the slope of the correlation plots increases with the Cr(VI) concentration, i.e., the cellular Mg content per unit cell volume increases upon Cr(VI) stress. The single-cell ICP-MS intensity of the absorbed Cr also correlates linearly with Mg intensity. The strong correlation of the ICP-MS intensity of Mg and Cr and the simultaneous increase in both cellular Mg and Cr contents indicate that the increase in cellular Mg contents is related to the uptake of toxic Cr(VI) ions by the algal cells. The proposed method of single-cell ICP-MS measurement and distribution correlation appears to be a feasible technique for direct label-free characterization of cellular responses to external stresses.