Force Spectrum Microscopy Reveals Active Diffusive-Like Fluctuations in Living Cells

Abstract

A living cell is a far-from-equilibrium dynamic material, replete with active force generation due to molecular motors and other enzymatic activity. While motors often produce highly directed motion, the aggregate yet incoherent effect of all active processes can create randomly fluctuating forces that can lead to diffusive-like, but inherently non-thermal motion. These fluctuating forces drive many basic cellular processes, making it essential to characterize their behavior; however, there is no existing technique to measure these forces. Here we introduce a new assay to quantify the random forces directly, by combining measurements of the random motion of probe particles with independent micromechanical measurements of the cytoplasm, to quantitatively determine the spectrum of these active force fluctuations. These active forces substantially enhance intracellular movement over a broad range of length scales. Moreover, we show that this force spectrum is a sensitive assay to probe motor activity in cells: The fluctuations are three times larger in malignant cancer cells than in their benign counterparts.