Computational modelling to predict mechanosensing of fibroblast cells in response to different material properties

We present a computational model of the mechanosensing of a fibroblast cell seeded on the materials with different stiffnesses and thicknesses. The model can predict the critical thickness of a given biomaterial that a cell can sense and the dynamic change of stress fibres and focal adhesions through its incorporation of the dynamic characteristics of stress fibre contraction and focal adhesion. We show that the cell-cell communication via elastic substrate induces the orientation of stress fibres. The cell-cell interaction through compliant substrate has a small but significant effect on enhancing the cell depth sensing capability in terms of interfacial displacement and stress fibre concentration. The framework developed here is important for a thorough understanding of processes where substrates are deformed such as in wound healing process and the design of bioactive coatings for tissue engineering.

Vitronectin acts as a key regulator of adhesion and migration in umbilical cord-derived MSCs under different stress conditions

Mesenchymal stem cell (MSC)-based cellular therapy gets compromised as adverse microenvironmental conditions like nutrient deprivation, ischemia, hypoxia affect migration and engraftment, in addition to viability, of MSCs at the target site post transplantation. To improve the treatment efficacy, it is critical to identify factors involved in regulating migration and adhesion of MSCs under such microenvironmental stress conditions. In our study, we observed that Wharton's jelly-MSCs (WJ-MSCs) exhibited increase in cell spread area and adhesion with reduction in cellular migration under serum starvation. The changes in adhesion and migration characteristics were accompanied by extensive stress fibre formation and altered ECM gene expression with notable induction in vitronectin (VTN) expression and reduction in MMP-1 expression. Molecular and phenotypic correlative studies advocated the possible role of VTN and not MMP-1, in regulating adhesion and migration of WJ-MSCs. NF-kb was found to be the positive regulator of VTN expression while ERK pathway regulated it negatively. Further investigation with inhibition of these signalling pathways or VTN knockdown studies under serum starvation established the correlation between increase in VTN expression and increased cellular adhesion with corresponding reduction in migration. VTN knockdown under serum starvation also led to reduction in actin stress fibre along with reversal in expression of several ECM genes. Additionally, VTN induction being absent in hypoxia-treated WJ-MSCs, the hypoxic cells showed no significant change in the adhesion and migration properties. However, when VTN expression was induced under hypoxia by ERK pathway inhibition, similar increase in cell spread area and adhesion was observed. Our study thus highlights VTN as a factor which is induced under serum starvation stress and possibly affects the adhesion and migration properties of WJ-MSCs.

In silico stress fibre content affects peak strain in cytoplasm and nucleus but not in membrane for uniaxial substrate stretch

Existing in silico models for single cell mechanics feature limited representations of cytoskeletal structures that contribute substantially to the mechanics of a cell. We propose a micromechanical hierarchical approach to capture the mechanical contribution of actin stress fibres. For a cell-specific fibroblast geometry with membrane, cytoplasm and nucleus, the Mori-Tanaka homogenization method was employed to describe cytoplasmic inhomogeneities and constitutive contribution of actin stress fibres. The homogenization was implemented in a finite element model of the fibroblast attached to a substrate through focal adhesions. Strain in cell membrane, cytoplasm and nucleus due to uniaxial substrate stretch was assessed for different stress fibre volume fractions and different elastic modulus of the substrate. A considerable decrease of the peak strain with increasing stress fibre content was observed in cytoplasm and nucleus but not the membrane, whereas the peak strain in cytoplasm, nucleus and membrane increased for increasing elastic modulus of the substrate.

Entropic forces drive cellular contact guidance

Contact guidance—the widely-known phenomenon of cell alignment induced by anisotropic environmental features—is an essential step in the organization of adherent cells, but the mechanisms by which cells achieve this orientational ordering remain unclear. Here we seeded myofibroblasts on substrates micropatterned with stripes of fibronectin and observed that contact guidance emerges at stripe widths much greater than the cell size. To understand the origins of this surprising observation, we combined morphometric analysis of cells and their subcellular components with a novel statistical framework for modelling non-thermal fluctuations of living cells. This modelling framework is shown to predict not only the trends but also the statistical variability of a wide range of biological observables including cell (and nucleus) shapes, sizes and orientations, as well as stress-fibre arrangements within the cells with remarkable fidelity. By comparing observations and theory, we identified two regimes of contact guidance: (i) guidance on stripe widths smaller than the cell size (w ≤ 160 μm), which is accompanied by biochemical changes within the cells, including increasing stress-fibre polarisation and cell elongation, and (ii) entropic guidance on larger stripe widths, which is governed by fluctuations in the cell morphology. Overall, our findings suggest an entropy-mediated mechanism for contact guidance associated with the tendency of cells to maximise their morphological entropy through shape fluctuations.