To examine how mechanical changes at the cellular level affect macroscopic tissue properties, the team used intracellular particle tracking microrheology (PTM) to study the mechanical properties of cells exposed to the chemotherapy drug paclitaxol. Understanding these cellular changes should enable optimization of elastographic techniques for treatment monitoring (Phys. Med. Biol. 58 923).

"We chose PTM because it is a passive microrheology technique that does not apply any external strain or stress to the cell," explained Ahmed El Kaffas, (now a PhD candidate at the University of Toronto). "Instead, PTM allows us to locally measure mechanical properties based on the Brownian motion of embedded particles and without damage to the specimen. This is particularly important when conducting longitudinal studies of cell death, as cells in advanced stages of decay are sensitive to the application of external forces."

El Kaffas and colleagues examined MCF-7 breast cancer cells, treating six culture dishes of cells with paclitaxol and leaving six dishes untreated. One treated and one control dish were stained with a cell death detection kit, and subsequent fluorescence imaging showed that over 60% of treated cells were dead after 24 hr. Untreated cells remained mostly viable. Light microscopy revealed cells to be mostly viable prior to paclitaxol exposure, with morphological changes evident 24 hr after exposure.

Particle tracking

For PTM studies, cells were injected with 0.19 µm diameter carboxyl-modified fluorescent polystyrene microspheres (such that each cell contained two to five particles). After 24 hr, two dishes were treated with paclitaxol and two left untreated. The researchers then recorded videos (at 25 fps) of the motion of the embedded particles at 0, 6, 12 and 24 hr after treatment.

They then used MATLAB routines to individually track particles in the videos, examining 20–40 tracks per time point. Representative tracks showed that particles travelled greater distances at the start of treatment than at 24 hr after drug exposure, indicating stiffening of the cytoplasm.

To further study the cells' mechanical properties, the researchers computed the mean square displacement (MSD) of the particles for a range of lag-times (the time-scale over which the displacement is measured), in treated and untreated cells. At 0.1 s, the average MSD amplitude of treated cells decreased between 0 and 6 hr after paclitaxol exposure, and between 0 and 24 hr. At a 1 s lag-time, MSD decreased between 0 and 12 hr, and 0 and 24 hr. No statistically significant differences in MSD were found for the untreated cells.

Finally, the researchers calculated the average elastic and viscous moduli of the cytoplasm of treated cells. For low-frequency shear modulus measurements (1 rad/s), the elastic modulus increased from 0.022 to 191.3 Pa at 24 hr after drug exposure, while the viscous modulus increased from 1.1 to 10.1 Pa. At higher frequencies (10 rad/s), the increases were from 3.3 to 191.8 Pa and from 13.4 to 15.1 Pa, for the elastic and viscous moduli, respectively.

The ratio of the elastic to viscous moduli showed that at 0 and 6 hr, the intracellular microenvironment was strongly viscous at lower frequencies and more elastic at higher frequencies. At 12 hr, the ratio was approximately unity over most frequencies, and 24 hr after treatment the ratio indicated a predominantly elastic microenvironment. These results suggest that the elastic and viscous contributions change as a function of both probing frequency and treatment time.

El Kaffas and colleagues concluded that PTM can detect changes in mechanical properties of treated cells, and that the cytoplasm of treated cells stiffens as the cells respond to chemotherapy. Such mechanical changes could potentially be exploited to monitor treatment response.

"Our next step will be to assess how mechanical properties change at different spatial locations within the cell, and which cell components are most closely linked to bulk mechanical property changes during treatment. This will provide us with insight that we hope will allow us to develop better imaging tools to more precisely monitor treatment response," El Kaffas told medicalphysicsweb. "We also hope to use PTM to associate cell mechanics with physiological conditions of tumours, in order to develop tools that can assess changes in tumour physiology."

Related articles in PMB
Investigating longitudinal changes in the mechanical properties of MCF-7 cells exposed to paclitaxol using particle tracking microrheology
Ahmed El Kaffas et al Phys. Med. Biol. 58 923
Imaging the elastic properties of tissue: the 20 year perspective
K J Parker et al Phys. Med. Biol. 56R1