Cell mechanics, structure, and function are regulated by the stiffness of the three-dimensional microenvironment

Abstract

This study adopts a combined computational and experimental approach to determine the mechanical, structural, and metabolic properties of isolated chondrocytes cultured within three-dimensional hydrogels. A series of linear elastic and hyperelastic finite-element models demonstrated that chondrocytes cultured for 24 h in gels for which the relaxation modulus is <5 kPa exhibit a cellular Young's modulus of ∼5 kPa. This is notably greater than that reported for isolated chondrocytes in suspension. The increase in cell modulus occurs over a 24-h period and is associated with an increase in the organization of the cortical actin cytoskeleton, which is known to regulate cell mechanics. However, there was a reduction in chromatin condensation, suggesting that changes in the nucleus mechanics may not be involved. Comparison of cells in 1% and 3% agarose showed that cells in the stiffer gels rapidly develop a higher Young's modulus of ∼20 kPa, sixfold greater than that observed in the softer gels. This was associated with higher levels of actin organization and chromatin condensation, but only after 24 h in culture. Further studies revealed that cells in stiffer gels synthesize less extracellular matrix over a 28-day culture period. Hence, this study demonstrates that the properties of the three-dimensional microenvironment regulate the mechanical, structural, and metabolic properties of living cells.

Figure 1

Experimental data used for modeling chondrocyte mechanical properties. The data show gross viscoelastic stress relaxation (a and b) and cell deformation (c and d) in compressed alginate (a and c) and agarose (b and d) constructs. The freshly isolated cells were cultured in the 3D constructs for 24 h and then held at 20% static compression. Applied stress and cell deformation were recorded over the subsequent 60-min period.

Figure 2

Details of the composite finite-element mesh for compression of cells in 3D hydrogels. The cell is modeled as a spherical inclusion within the gel such that the entire 3D model has rotational symmetry. Roller boundary conditions were applied to the bottom of the compression platen, with frictionless contact between the gel and the platen. The bottom of the construct was fixed in the vertical direction and the displacement was applied on the top of the construct. The symmetric boundary condition was applied at the center lines of the cell and 3D scaffold.

Figure 3

Linear elastic model with a cell modulus of 5.3 kPa provides the best fit to the data for cell deformation in 1.2% and 2% alginate gel. (a) The results of three different linear elastic models with effective moduli (E_cell) of 3, 5.3, and 8 kPa are shown against the experimental data from Fig. 1 (26). (b) The corresponding R values for these and other models are plotted against modulus, indicating that the highest R value corresponds to a cell modulus of 5.3 kPa.

Figure 4

Hyperelastic models predict cell deformation in compressed alginate constructs. The results of three different hyperelastic models fitted to the experimental data of instantaneous gross stress and cell strain in 20% compressed alginate constructs as shown in Fig. 1 (26). The Ogden Model (a), the Polynomial model (b), and the NH model (c) produced effective cell moduli of 4.3, 4.4, and 3.7 kPa, respectively.

Figure 5

Relationship between the effective cell modulus, E_cell, and the predicted cell strain in compressed 1% and 3% agarose enables analysis of temporal changes in cell mechanics. (a) The influence of E_cell on the cell strain estimated using the linear elastic model. The hyperelastic Ogden model produced accurate estimates of cell strain in 1% agarose (14%) when optimized to experimental data for cells cultured for 24 h in alginate. However, the Ogden model overestimated the cell strain in 3% agarose (31%), suggesting that the modulus in 3% agarose may be substantially larger than that predicted in the softer alginate. The linear elastic models best match the experimental cell strain values in 1% and 3% agarose at moduli of 5.3 kPa and 20 kPa, respectively. (b) The relationship between modulus and cell strain may be used to derive the temporal changes in cell mechanics for chondrocytes in 1% and 3% agarose after 1 h and 24 h in culture. Values represent mean cell moduli ±SD based on the mean experimental cell strain values at 20% gross compression.

Figure 6

Relative influence of the cytoplasm and the nucleus on gross cell mechanics. (a) The results of the linear elastic models with nucleus moduli (E_nuc) of 3, 11, and 36 kPa and cytoplasmic modulus (E_cyto) of 3 kPa. The models have been fitted to the experimental data of instantaneous gross stress and cell strain in 20% compressed alginate constructs, as shown in Fig. 1. (b) The corresponding R values for linear elastic models with a nucleus modulus of 3–40 kPa and a cytoplasmic modulus of 2, 3, 4, or 5 kPa. The data indicate that the best fit occurs with a nucleus modulus of ∼10 kPa. (c) Contour plot showing the relative effect of cytoplasm and nucleus moduli (E_cyto and E_nuc) on the resulting cell modulus (E_cell) based on the linear elastic models. The contours are indicated separately for a cell modulus of 5.3 and 20 kPa, representing the predicted values for cells in 1% and 3% agarose.

Figure 7

Cells in 3% agarose develop greater cortical actin organization compared to those in 1% agarose. (a) Confocal section images showing the chondrocytes after 24 h in 1% and 3% agarose gel. F-actin was labeled with Alexa 564-Phalloidin (red, outer ring) and the nuclei with Hoescht (blue, central area). Scale bars, 5μm. The level of actin organization was quantified by measuring the mean Alexa 564-Phalloidin intensity within the cortical region and dividing by the cytoplasmic intensity to yield the cortical actin ratio, which was normalized to values at 1 h to account for variation in stain penetration. (b) The normalized cortical actin ratio increased with time in culture and at 18 and 24 h was significantly greater for cells in 3% agarose compared to cells in 1% agarose (^∗∗∗p < 0.001). Values are represented as the mean ± SE (n = 40–80).

Figure 8

Cells in 3% agarose develop greater chromatin condensation compared to those in 1% agarose. (a) Confocal section images of chondrocytes in 1% and 3% agarose gel showing the nuclei labeled with Hoescht and the higher level of chromatin condensation in the 3% gel after 24 h in culture. Scale bars, 5 μm. (b) Corresponding images that have been digitally filtered to reveal the density of edges, from which the chromatin condensation parameter was calculated (see text for details). (c) Chromatin condensation decreased with time in culture, but by 24 h the levels were significantly greater for cells in 3% agarose (^∗∗∗p < 0.001). Values are represented as the mean ± SE (n = 60).

Figure 9

Cells in 2% agarose synthesize more proteoglycan than cells in 3% agarose. (a) sGAG content retained within the 2% and 4% agarose constructs. (b) Cumulative amount of sGAG released to the associated culture media over a 28-day culture period. Values represent mean ± SD for n = 6 constructs. Differences are statistically significant at p < 0.05. (c and d) Staining with Safranin O revealed that the proteoglycan formed a dense pericellular matrix around individual cells and groups of recently divided cells, with a more elongated columnar morphology in the stiffer scaffolds. In some cases the cells and matrix expanded out of the agarose constructs (arrow).