Khalilgharibi, N; (2017) The rheology of living cell monolayers. Doctoral thesis , UCL (University College London).

Abstract

Cell monolayers are a group of tissues in the body that, compared to other tissue types, have a simple structure, yet they play critical roles by acting as mechanical and physical barriers and partitioning the body into distinct organs. Cell monolayers are continuously exposed to external stresses applied to different extents and at various rates. Previous studies have shown that the mechanical response of monolayers to a sudden deformation is biphasic, however, what mechanisms are involved in these two phases is poorly understood. The aim of this work is to identify these mechanisms, their respective importance and their respective timescales. For this purpose, a new mechanical testing setup, based on a previous device developed in the Charras lab[1] was designed, calibrated and used to perform stress relaxation experiments on monolayers. ATP depletion experiments revealed that the first phase of the relaxation (t < 5 s) is dominated by physical ATP-independent processes, while the second phase (5 < t < 100 s) is dominated by biological ATP-dependent processes. Monitoring the changes in cell shape after stretch showed that cellular-level processes do not contribute to relaxation, suggesting that only molecular scale processes are at play. Chemical and genetic perturbations were used to identify the molecular origins of the ATP-dependent phase. It was shown that presence of the actin cytoskeleton is essential for the second phase of the relaxation. Furthermore, myosin contractility is a key player in the second phase, such that its perturbations lead to a decrease in the relaxation rate. The role of different crosslinkers in the active relaxation was also investigated. Finally, a computational model developed by the group of Dr José J Muñoz was adapted to simulate monolayer stress relaxation and to determine contributions of the cytoplasm and intercellular junctions. [1] Harris et al. PNAS. (2012).

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