Separating Water Content from Network Dynamics in Cell Nuclei with Brillouin Microscopy.

Probing forces, deformations, and mechanical properties of cells are the hallmark of mechanobiology. Many techniques have been developed to this end that are largely based on deforming the cells and measuring the reaction force. In cells, an alternative approach was implemented in the mid-2010s based on Brillouin light scattering (BLS), which allows extracting acoustic parameters at picosecond timescales. In all of these measurements, the response of the cell is quantified with a colloquial "stiffness" that encompasses both the contribution of load-bearing structures and volume changes, to much confusion. To clarify the interpretation of the hypersonic data obtained from BLS spectra, we vary the relative volume fraction of an intracellular water and solid network by applying osmotic compressions to single cells. In the nucleus, we observe a nonlinear increase in the sound velocity and attenuation with increasing osmotic pressure that we fit to a poroelastic model. By comparing BLS data to volume measurements, our approach demonstrates clearly that BLS shift alone is mostly sensitive to water content while the additional analysis of the linewidth allows identifying the contribution of the biopolymer-based network dynamics in living cells.