A Novel Cell Traction Force Microscopy to Study Multi-Cellular System

Abstract

Traction forces exerted by adherent cells on their microenvironment can mediate many critical cellular functions. Accurate quantification of these forces is essential for mechanistic understanding of mechanotransduction. However, most existing methods of quantifying cellular forces are limited to single cells in isolation, whereas most physiological processes are inherently multi-cellular in nature where cell-cell and cell-microenvironment interactions determine the emergent properties of cell clusters. In the present study, a robust finite-element-method-based cell traction force microscopy technique is developed to estimate the traction forces produced by multiple isolated cells as well as cell clusters on soft substrates. The method accounts for the finite thickness of the substrate. Hence, cell cluster size can be larger than substrate thickness. The method allows computing the traction field from the substrate displacements within the cells' and clusters' boundaries. The displacement data outside these boundaries are not necessary. The utility of the method is demonstrated by computing the traction generated by multiple monkey kidney fibroblasts (MKF) and human colon cancerous (HCT-8) cells in close proximity, as well as by large clusters. It is found that cells act as individual contractile groups within clusters for generating traction. There may be multiple of such groups in the cluster, or the entire cluster may behave a single group. Individual cells do not form dipoles, but serve as a conduit of force (transmission lines) over long distances in the cluster. The cell-cell force can be either tensile or compressive depending on the cell-microenvironment interactions.

Geometric guidance of integrin mediated traction stress during stem cell differentiation

“…As cells exert traction stresses on deformable matrices, focal adhesions are reinforced and there have been several reports that size, density and turnover of focal adhesions influence differentiation [40,41]. As cells were cultured on our patterned matrices that differentially affect lineage outcome, we employed traction force microscopy (TFM) to determine the tractions exerted by MSCs by obtaining measurements of the micro-bead displacement within PAAm hydrogels [37]. MSCs cultured on fibronectin matrices were able to exert higher traction stress than cells adherent to laminin or collagen.…”

“…Matrix proteins were patterned as described above. An Olympus IX81 fluorescent microscope and 20x objective was used to obtain the live cell images [37]. Throughout the experiment, temperature and carbon dioxide levels were maintained at 37°C and 5% respectively.…”

“…In order to assess the displacement of beads under the null-force condition, cells were removed from the surface using sodium dodecyl sulfate (SDS, Fisher Inc.), resulting in the gel returning to its relaxed initial state without cells. To characterize the gel displacements, the images before and after cell removal were analyzed using Matlab digital image correlation programs published in [37] to obtain the 2D displacement field (ux, uy). The resolution of the algorithm is 1/10 of pixel size, i.e.…”

“…~ 33 nm, and signal-to-noise ratio reaches 40. The detailed procedures of cell traction computation using finite element method can be found in a previous report [37]. In brief, our computation employed a mixed boundary condition model, by prescribing zero traction at all nodes outside the cell (Fx=Fy=Fz=0) and the obtained 2D displacement field (ux, uy) as well as Fz=0 at the nodes within the cell boundaries.…”

“…We did not measure uz during the experiments. Our theoretical derivation suggests that for elastic biomaterial substrates with Poisson’s ratio close to 0.5, such as PA gels, prescribing Fz=0 for all surface nodes results in an error of less than 2% in the calculation of in-plane forces Fx and Fy [37]. …”