Linking particle dynamics to intracellular micromechanics in living cells

Mechanical properties of cells are important to cellular functions. To gain a better understanding of the driving forces and viscoelastic properties responsible for the overall mechanical behavior of living cells, intracellular particle tracking technique is frequently applied to particles naturally embedded, or artificially introduced, into the cytoplasm. However, claims on the suitability of certain probes to characterize the dynamics (especially the viscoelasticity) of the cytoskeleton have not always met consensus, and comparisons between different probes in the same cells have rarely been made. We explored a dual-probes system of endogenous lipid granules (EG) and injected latex spheres (BIP) to relate single particle dynamics to local microenvironments inside human endothelial cells. Analyzing mean-squared-displacement (MSD) versus time functions, we found that the two probes display remarkably different dynamics. Interventions with a range of drugs targeted at the cytoskeleton corroborate that both the driving force and the local micro-environment are entirely different for two different intracellular probes. We conclude that EG dynamics is strongly ATP sensitive and intimately connected to the dynamics of the microtubules. In contrast, BIP are mostly embedded in the actin network and display a more passive dynamics that however depends on acto-myosin contractility. These assertions indicate the availability of distinct microrheological probes to dissect the different contributions of actin and microtubule networks to the mechanical behavior of living cells. We tested dual-probes system in two pairs of tumor cell lines. The malignant cells show higher probe dynamics comparing to the benign tumor cells, indicating a softer cellular mechanical environments.