Active cellular forces in variable mechanical microenvironments

Doctoral thesis, 2016

Our bodies are immensely complex structures, with various systems, organs, and processes that are not only biochemically driven, but strongly dependent on physical mechanics.  At the heart of all these processes are cells, which must generate forces to adhere, spread, differentiate, grow, and move. The cells are also exposed to a diverse number of mechanical stimuli from their environment including fluid shear stress, and exerted stress from neighbouring cells. Cells sense and respond to all these forces through a diverse set of mechanisms generally referred to as mechanotransduction. This force communication between cells and their environment regulates cell function, and has increasingly come into focus for much research in the last two decades. However, there are still fundamental questions to be answered in regards to the physical nature of cells and their responses to microenvironment mechanical stimuli. This thesis contributes to understanding the relationship between forces and mechanotransduction by studying cell behaviour on variable mechanical microenvironments fabricated from different polymer gels. 3D composite hydrogels were formed using combinations of alginate, collagen and alginate modified with RGD (Arginiylglycylaspartic acid) resulting in gels having well-defined mechanics representing physiological and pathological stiffnesses of the extracellular matrix found in vivo. Cells were cultured within these 3D hydrogels to form multicellular aggregates (MCAs), and studied the proliferation of cells and growth of the MCAs, finding that these MCAs grew faster and bigger in stiffer gels. As MCAs increased in size, their morphology changed from spherical to elliptical, and this transition occurred faster and more frequently in the stiffer 3D hydrogels. It is also found that the nuclei of the cells in MCAs appeared compressed, which led to the proposed idea of using nuclei as novel endogenous pressure sensors in MCAs. Using this approach, we found that the pressure on nuclei appear uniform throughout spherical MCAs, but have a radial dependence in elliptical MCAs, with the largest pressures apparent at the outer edge of the MCA. These findings suggest that cells in the MCAs have different compressive stresses on nuclei, which may be key in mechanosensory processes. To understand the relationship between cell forces, movement, and substrate stiffness, polydimethylsiloxane (PDMS) substrates with stiffness gradients was developed. It was found that all investigated cells preferentially moved to the stiffer part of the substrate, with a velocity dependent on the local modulus and the gradient. The forces and work exerted by the cell during movement were examined, and found that both the traction force and strain energy for fibroblasts scaled linearly with the local modulus. To examine how these relations, change in pathology, benign and metastatic cancers was studied, finding that highly metastatic cancer cells move slower and exert larger forces. Interestingly, the strain energy which represents the work expended by the cell on the substrate increased sharply above 30 kPa for highly metastatic cancer cells, and this was not observed for the benign cells. This increase in strain energy suggests that metastatic cells may have an adaptive motility that is better able to increase force generation in response to microenvironment mechanics, and is consistent with previous reports that these cells display a more adaptive metabolism.