Dynamics of single-cell mass, volume and stiffness during mitosis

Abstract

Cells undergo dynamic changes in cell shape and mechanics during mitosis, a short cell-cycle stage dedicated to separating replicated chromosomes into two daughter cells. Recently, several groups have reported that mitotic biophysical changes are essential for proper mitotic spindle function, consequently affecting mitotic fidelity and development of cancer. However, studying biophysical dynamics in mitosis is challenging because both the magnitude and duration of mitotic changes are heterogeneous across the cell population. In this thesis, I demonstrate new methods to monitor single-cell mass, volume and mechanical properties throughout mitosis with temporal resolution of <1 min. We utilize the suspended microchannel resonator (SMR) which is a fluid-filled cantilever capable of measuring cell buoyant mass by the change of SMR resonant frequency. First, we monitor the volume and density of single cells in suspension by consecutively weighing them in two fluids of different densities. We find that mitotic cells reversibly increased their volume by more than 10% over a 20-min period after mitotic entry through osmotic regulation. Next, using the SMR and a protein synthesis assay, we quantify the mass accumulation and translation rates of single cells between mitotic stages. Various animal cell types displayed persistent mass accumulation during mitosis and cytokinesis with mitotic-stage specific growth rate dynamics. Finally, we quantify mechanical properties via acoustic scattering of waves from a cell inside the SMR. Through simulations, experiment with hydrogels and chemical perturbation of cells, we show that our readout from acoustic scattering measures stiffness. Cells maintained constant stiffness throughout interphase but show dynamic changes during mitosis. Altogether, continuous monitoring of single-cell physical parameters -- mass, volume and stiffness -- has revealed biophysical dynamics during mitosis that have not been previously observed.