Probing the Micrheology of Mesenchymal Stem Cell Migration to Tumors

Abstract

Mesenchymal stem cells (MSCs) are excellent candidates for the development of cell-based gene delivery systems; however, extended cell culture, required for therapeutic development, alters MSC morphology, reducing MSC migration upon reinfusion. Spontaneous migration of MSCs to tumors is mediated by tumor secretion of proangiogenic chemokines. Multiple particle tracking microrheology was used to investigate the effect of tumor-secreted molecules on MSC viscoelasticity, which was correlated with MSC migration and morphology. Within 24 hours after MSC treatment with tumor-conditioned media (TCM), MSCs were elongated, with more than 5-fold difference in the length of lamellipodia. Within 24 hours, the migration of MSCs, measured using a Boyden chamber assay, toward TCM was increased 10-fold over control media. The mean squared displacements (MSDs) of 100-nm carboxylated polystyrene particles, injected into the cytoplasm of human MSCs using the Biolistic Particle Injection System, were determined with 33 ms temporal and 5 nm spatial resolution using multiple particle tracking. The frequency dependent elastic and viscous moduli were calculated from the complex shear moduli, which were determined from the Fourier transform of the time-dependent MSDs, using the frequency-dependent Stokes-Einstein equation. Pretreatment of MSCs with TCM resulted in rapid changes in cytoplasmic viscoelasticity with a 9.8-fold increase in the average elastic moduli, which increased from 35 to 344 dyn/cm2, and a 3.5-fold decrease in the average viscous moduli, which was reduced from 99 to 28 dyn/cm2, within 1 hour (n = 6-8 cells per group). We hypothesize that tumor-secreted molecules increase MSC mobility by altering cytoskeletal organization. Changes in MSC viscosity may be in part to reduced actin cross-linking during cytoskeletal reorganization. Increased MSC rigidity may be due to MSC elongation, which leads to the formation of polymer entanglements as the ratio of cell length to width is greatly increased.