Fig. 1.

Spatially resolved intracellular microrheology. (A) Bright-field image of an RPE1 cell plated on a crossbow micropattern. The 2-µm-diameter beads were internalized in the cell (black arrow). The dashed and dotted lines show the micropattern and cell contours, respectively. (Scale bar, 10 µm.) (B, Top) Schematic of the step relaxation experiment. The bead is initially trapped at the center of the optical tweezers. A time t = 0 s, the cell is moved in a stepwise fashion to the left on a distance X_s = 0.5 μm. The bead is displaced on a distance X_b < X_s due to the viscoelastic nature of its microenvironment, then its position relaxes toward the trap center. (Bottom) Single and averaged bead displacement plots. Error bars represent SEM (n = 152 beads from 147 cells). (C) Distribution of the beads locations relative to the micropattern. Position (x = 0, y = 0) denotes the center of the crossbow micropattern. $\theta$ denotes the angle between the bead position and the polarity axis ($\theta =0$) of the crossbow micropattern. The dashed blue contour represents the average nucleus shape (Fig. S1B). (D) The relaxation strongly depends upon the bead location within the cell: bead 1 located in the central region of the micropattern relaxes more slowly than bead 2 located at the periphery. The blue lines are fits to the experimental data using the power-law (PL) model. (E) Maps showing the spatial distribution of the storage modulus (G′), the loss modulus (G″), the shear modulus G₀, and the G′/G″ ratio. The maps are color-coded from blue (low values) to red (high values). Data are from n = 152 beads from 147 cells. The dashed white contour represents the average nucleus shape (Fig. S1B). (F) Polar plot of the shear modulus (G₀) as a function of the angle $\theta$. Data were pooled and averaged in 30° intervals.