Plasmonic imaging of subcellular electromechanical deformation in mammalian cells

Abstract

A membrane potential change in cells is accompanied with mechanical deformation. This electromechanical response can play a significant role in regulating action potential in neurons and in controlling voltage-gated ion channels. However, measuring this subtle deformation in mammalian cells has been a difficult task. We show a plasmonic imaging method to image mechanical deformation in single cells upon a change in the membrane potential. Using this method, we have studied the electromechanical response in mammalian cells and have observed the local deformation within the cells that are associated with cell-substrate interactions. By analyzing frequency dependence of the response, we have further examined the electromechanical deformation in terms of mechanical properties of cytoplasm and cytoskeleton. We demonstrate a plasmonic imaging approach to quantify the electromechanical responses of single mammalian cells and determine local variability related to cell-substrate interactions.

Keywords: cell electromechanics; electromechanical coupling; electromechanical deformation; plasmonic imaging.

Fig. 1

Plasmonic imaging of cellular mechanical deformation. (a) Schematic diagram of experimental setup. Cells are plated on the gold-coated coverslip and are imaged with plasmonic microscopy via a fast camera. Membrane potential modulation waveform was applied to the cell with a micropipette in a whole-cell patch clamp configuration. (b) Membrane potential change induces mechanical deformation on the membrane. (c) Plasmonic images were analyzed by applying FFT along time, and the amplitude and phase-shift images at the modulation frequency were obtained, to quantify the deformation amplitude and direction.

Fig. 2

Membrane potential-induced mechanical deformation in cells. (a) Transmitted and (b) plasmonic images of a cell whose membrane potential is modulated at 37 Hz using a glass micropipette. (c) Membrane potential and current associated with the potential modulation. (d) Amplitude images of the cell at the frequency of the applied potential modulation (37 Hz). (e) Phase image of the cell. (f) Spectral response of the plasmonic image intensity of the whole cell and a background region (without the cell), where the peak at the modulation frequency from the whole cell region is the mechanical deformation of the cell. Scale bar: $10\mu \mathrm{m}$.

Fig. 3

Cell deformation and its dependence on the membrane potential. (a) Raw plasmonic and (b) differential plasmonic images of a depolarized cell from $-600$ to 100 mV, where the white line marks the edge of the cell, the green and blue dashed lines indicate a cell edge and center regions selected for quantification, respectively. (c) Stepwise change of the membrane potential polarization. (d) Time profiles of the mechanical deformation in the cell edge (top) and center (bottom) regions in response to the stepwise membrane potential polarization. (e) Relationship of the mechanical deformation in the cell edge and center regions at the steady state of each potential steps, where the error bars indicate the temporal standard deviation of the deformation for each potential step (averaged over 94 cycles). Scale bar: $10\mu \mathrm{m}$.

Fig. 4

Cell deformation and its dependence on the membrane potential modulation frequency. (a)–(e) FFT image of cellular mechanical deformation in response to membrane potential modulation frequencies, where the brightness and color indicate the amplitude and phase of the FFT images. (f) Mechanical model of cell deformation. (g), (h) Mechanical deformation in the cell edge and center regions versus modulation frequency, where the data are plotted in log-log scale and fitted with a linear equation. Scale bar: $20\mu \mathrm{m}$.

Fig. 5

Modification of the mechanical properties of the cell with cytochalasin D. (a), (b) Potential-induced cell deformation of (a) cytochalasin D-treated cells and (b) normal cells, where the white lines indicate the phase boundary. (c) Averaged phase shift of the whole cell; $p<0.01$, $t$-test. (d), (e) Model illustration of the potential-induced cell mechanical deformation of the cytochalasin D-treated cells and normal cells. Scale bar: $10\mu \mathrm{m}$.