Physical regulation of cells and its applications

Abstract

This thesis focuses on the important question of whether and how the behavior and fate of biological cells can be regulated by the applied physical signals. First of all, we showed that, similar to osmotic shocks, a sudden change in the medium hydrostatic pressure led to cross-membrane ion fluxes and resulted in sluggish reversible volumetric deformation of cells (usually taking ~1 hour to complete). On the other hand, it was found that active ion transport across the cell membrane will not be triggered by a gradually varied extracellular osmolarity, suggesting that the response of cells to changes in their micro-environment depends more on the manner of how those changes are introduced, rather than their magnitudes. Interestingly, under the same osmotic stimuli, cancer cells underwent larger volumetric changes and moved much faster in the microchannel compared to their normal counterparts. We showed that such distinct response is likely due to the overexpression aquaporin-4 (AQP4) in tumor cells, with knockout of such water channel proteins resulting in a markedly reduced cellular volume change and migration capability.

Next, we examined how mechanical stimuli like pressure shock and sonic excitation influence the fate of live cells. It was found that both cytoskeleton and DNA damages will be induced by the elevated surrounding pressure, eventually leading to cell cycle arrest and apoptosis were observed. In addition, cell death can also be triggered by sonic vibrations. Specifically, we showed that soft cancerous cell lines are prone to cell death in the low-frequency range. In comparison, the mortality rate of stiff non-cancerous cell lines will undergo a sharp increase around a relatively high excitation frequency, indicating that the phenomenon of resonance may play a role here. This is corroborated by the observation that the frequency for inducing maximum cell death will shift to larger values when cells are treated with drugs known to cause cytoskeletal stiffening.

Last but not the least, we have developed two methods to characterize the mechanical response of cells. Specifically, by combing optical trapping with fluorescence imaging, a technique allowing us to probe the coupled adhesion and deformation characteristics of suspension cells was established. In addition, a method for the quantitative analysis of the membrane resealing response of cells was also developed. Interestingly, it was found that micron-sized membrane pores (introduced by mechanical puncturing) in nasopharyngeal, lung and colorectal tumor cells can all reseal ~3 times faster than those in their normal counterparts. Furthermore, a big difference in the resealing time between drug resistance/sensitive and well-differentiated/undifferentiated pairs of cancer cells has also been observed, demonstrating the potential of using membrane resealing time as a novel maker for the diagnosis and classification of cancer in the future.