Viscoplasticity Enables Mechanical Remodeling of Matrix by Cells Sungmin Nam, Joanna Lee, Doug G. Brownfield, Ovijit Chaudhuri  Biophysical Journal  Volume 111, Issue 10, Pages 2296-2308 (November 2016) DOI: 10.1016/j.bpj.2016.10.002 Copyright © 2016 Biophysical Society Terms and Conditions

Figure 1 Creep and recovery tests characterize plasticity of cell-culture materials. (A) The schematic explains general features of the material response in a creep and recovery test. Initially, materials exhibit instantaneous elastic response to applied stress, and strain of materials gradually increases over time. At the end of the creep test, materials exhibit a maximum or total strain. After release of the creep test, materials undergo elastic and viscoelastic recovery, but may leave a residual, or irreversible, strain, which is indicative of plastic deformation. Also shown are typical mechanical responses of (B) collagen gels, (C) rBM matrix, (D) agarose gels, (E) alginate gels, (F) fibrin gels, and (G) polyacrylamide gels (PAM) under creep and recovery tests at different stresses. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 2 Degree of plasticity in cell-culture materials is enhanced at increasing timescales of imposed stress. The degree of plasticity of (A) collagen gels, (B) rBM matrix, (C) agarose gels, (D) alginate gels, (E) fibrin gels, and (F) polyacrylamide gels are obtained as a function of different times of imposed creep, or creep time, during creep and recovery tests. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 3 Degree of plasticity is enhanced with increasing stress in collagen gels, but not in the other materials. The degree of plasticity of (A) collagen gels, (B) rBM matrix, (C) agarose gels, (D) alginate gels, and (E) fibrin gels, was obtained by creep and recovery tests at different stresses. Data are shown as the mean ± SD; n = 4. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 4 Different cell-culture materials exhibit distinct behaviors of viscoplasticity. Shown is a plot of the degree of plasticity in tested materials as a function of stress for collagen gels (blue), rBM matrix (green), agarose gels (red), alginate gels (orange), and fibrin gels (purple). The type of line indicates the timescale for the creep test in the creep and recovery tests. Solid lines represent a creep time of 3600 s for all the tested materials except rBM matrix. The dash-dotted line for rBM matrix indicates a creep time of 1200 s, and the dashed lines represent a creep time of 300 s for all the tested materials. At increasing timescales, all the materials exhibit higher degrees of plasticity. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 5 Degree of plasticity is regulated by covalent cross-linking in collagen gels. The degree of plasticity for untreated, GTA-cross-linked, and tTG-cross-linked collagen gels was obtained by creep and recovery tests with a creep time of 3600 s. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 6 Standard viscoelastic models combined with viscoplastic elements capture the viscoplastic behaviors of materials. The schematics describe mechanical models incorporating a viscoplastic element into (A) the SLS and (B) the Burgers model. The normalized degree of plasticity from experimental results is compared with the corresponding computational results from the mechanical model embedding nonlinear plastic flow (the Norton-Hoff plastic element) for collagen gels (C) and linear plastic flow (the Bingham plastic element) for rBM matrix, agarose gels, alginate gels, and fibrin gels (D). A comparison of the degree of plasticity from experimental and computational results at different timescales for (E) collagen gels (blue) and (F) rBM matrix (green) is shown. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 7 Cells locally realign collagen fibers, and the reorientation of fibers is retained after cells are removed. Images are shown of collagen gels (A) containing cells transfected to express RFP-actin, and (B) after lysing of cells. The inset in (A) shows a fluorescent image of actin. (C) Collagen fibers at regions of interest (ROIs) near cells (left and center), where strong actin fluorescent emissions are detected, and far from cells (right). (D) Polar distribution of fiber orientation at the corresponding ROI in (C). (E) Collagen fibers after lysing cells at the same ROIs shown in (C). (F) Polar distribution of fiber orientation at the ROIs in (E). Red and blue polar distributions represent the fiber orientation at the regions near and far from cells, respectively. Scale bar, 10 μm. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions

Figure 8 Quantitative assessment reveals that plastic remodeling of collagen gels by cells is mediated by β1 integrin, the cross-linking state of the collagen gel, and the magnitude of cellular force, but not by proteolytic activities. (A) A schematic to describe the analysis of fiber orientation. Each fiber is represented with a vector, p→, that connects the end points of the fiber. Given a selected point along the cell periphery, another vector, d→, connects the point along the cell periphery to the center of vector p→. The length of this vector is used as a measure of the distance from the cell periphery to the fiber. (B) FOI as a function of distance from cells (left) before and (right) after lysing cells. Red dotted lines indicate the average value of the FOI at each distance, and yellow lines indicate the average value of the FOI at random networks, which is ∼0.637 (see Materials and Methods). (C) Image of cells in collagen gels prelysis (left) and postlysis (right) with a graphical representation of the FOI evaluated at local points on the cell boundary superimposed on the image. Yellow arrows represent the regions where collagen fibers are highly aligned and high FOI is measured; the scale bar for the FOI is located on the right. (Inset) a fluorescent image of actin. Yellow arrows inset indicate the regions where strong actin fluorescent emissions are detected. (D) Image of a cell in collagen gel under inhibition of β1-integrin binding with a blocking antibody, with a graphical representation of the FOI around the cell superimposed on the image. (E–G) Plasticity around cells assessed under inhibition of actin polymerization with CytoD (E), in covalently cross-linked collagen gels (F), and under inhibition of MMP activity with GM6001 (G). p-values are calculated by Spearman’s correlation in (E) and by Student’s t-test in (F) and (G). ∗p < 0.05; ∗∗p < 0.01; ns, no significant difference. Scale bar, 10 μm. To see this figure in color, go online. Biophysical Journal 2016 111, 2296-2308DOI: (10.1016/j.bpj.2016.10.002) Copyright © 2016 Biophysical Society Terms and Conditions