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CELLULAR IMPEDANCE EXPLAINED

Positioned between reductionistic biochemical assays and whole organism in vivo experimentation, cell-based assays serve as an indispensable tool for basic and applied biological research. However, the utility of many cell-based assays is diminished by: (1) the need to use labels, (2) incompatibility with continuous monitoring (i.e. only end point data is produced), (3) incompatibility with orthogonal assays, and (4) the inability to provide an objective/quantitative readout. Each of these shortcomings is, however, overcome by the non-invasive, label-free, and real-time cellular impedance assay.

The functional unit of a cellular impedance assay is a set of gold microelectrodes fused to the bottom surface of a microtiter plate well (Figure 1). When submersed in an electrically conductive solution (such as buffer or standard tissue culture medium), the application of an electric potential across these electrodes causes electrons to exit the negative terminal, pass through bulk solution, and then deposit onto the positive terminal to complete the circuit. Because this phenomenon is dependent upon the electrodes interacting with bulk solution, the presence of adherent cells at the electrode-solution interface impedes electron flow. The magnitude of this impedance is dependent on the number of cells, the size and shape of the cells, and the cell-substrate attachment quality. Importantly, neither the gold microelectrode surfaces nor the applied electric potential (22 mV) have an effect on cell health or behavior.

Figure 1.  Overview of cellular impedance apparatus.  A side view of a single well is shown before and after cells have been added.  Neither the electrodes nor the cells are drawn to scale (they have been enlarged for clarity).  In the absence of cells electric current flows freely through culture medium, completing the circuit between the electrodes.  As cells adhere to and proliferate on the electrodes current flow is impeded, providing an extremely sensitive readout of cell number, cell size/morphology, and cell-substrate attachment quality.

IMPEDANCE ELECTRODES

The gold microelectrode biosensors in each well of ACEA’s electronic microtiter plates (E-Plates®) cover 70-80% of the surface area (depending if a view area is present). Rather than the simplified electrode pair depicted in Figure 1, the electrodes in each well of an E-Plate are linked into “strands” that form an interdigitating array (Figure 2). This arrangement enables populations of cells to be monitored simultaneously and thereby provides exquisite sensitivity to: the number of cells attached to the plate, the size/morphology of the cells, and the cell-substrate attachment quality.

Figure 2.  Impedance electrodes on ACEA’s E-Plates.  (A) Simplified schematic of the interdigitated electrodes used in each well of an E-Plate.  Electrodes are not drawn to scale (only a few are shown, and they have been enlarged for clarity).  Though cells can also be visualized on the gold electrode surfaces, the electrode-free region in the middle of the well facilitates microscopic imaging (brightfield, fluorescence, etc.).  (B) Photograph of a single well in a 96-well E-Plate.  (C) Zoomed in brightfield image of shadowed electrodes and unstained human cells.  (D) Gold electrodes and crystal violet stained human cells, as viewed in a compound microscope.

REAL-TIME IMPEDANCE TRACES EXPLAINED

The impedance of electron flow caused by adherent cells is reported using a unitless parameter called Cell Index (CI), where CI = (impedance at time point n – impedance in the absence of cells)/nominal impedance value. Figure 3 provides a generic example of a real-time impedance trace throughout the course of setting up and running an apoptosis experiment. For the first few hours after cells have been added to a well there is a rapid increase in impedance. This is caused by cells falling out of suspension, depositing onto the electrodes, and forming focal adhesions. If the initial number of added cells is low and there is empty space on the well bottom cells will proliferate, causing a gradual yet steady increase in CI. When cells reach confluence the CI value plateaus, reflecting the fact that the electrode surface area that is accessible to bulk media is no longer changing. The addition of an apoptosis inducer at this point causes a decrease in CI back down to zero. This is the result of cells rounding and then detaching from the well bottom. While this generic example involves drug addition when cells are confluent, impedance-based assays are extremely flexible and can also evaluate the rate and extent of initial cell adhesion to the electrodes, or the rate and extent of cell proliferation.

 Figure 3.  Generic real-time impedance trace for setting up and running an apoptosis assay.  Each phase of the impedance trace, and the cellular behavior it arises from, is explained in the text.

Moving beyond the generic example shown above, Figure 4 shows actual real-time impedance data acquired using E-Plates in ACEA’s xCELLigence® real-time cell analysis (RTCA) instruments. Figure 4A shows impedance traces for the first two hours after A549 cells have been added to an E-Plate, the wells of which were previously coated with differing concentrations of collagen IV. While Figure 4B demonstrates the change in cell index that occurs within the first few minutes of exposing HeLa cells to the GPCR agonist dopamine, Figure 4C evaluates NK cell-mediated cytolysis of cancer cells over the course of 20 hours. Figure 4D highlights the variety of changes that can occur in cell index depending on a drug’s mechanism of action.

Figure 4.  Examples of real-time impedance traces obtained using E-Plates and xCELLigence RTCA instruments.  (A) Real-time monitoring of A549 cell adhesion to E-Plate wells that had been pre-coated with different concentrations of collagen IV.  Note the correlation between impedance values (Cell Index) and the number of adherent cells visible in the microscope.  (B) Real-time impedance traces for HeLa cells exposed to different concentrations of the GPCR agonist dopamine.  The black arrow indicates the time of dopamine addition.  (C) Real-time impedance traces for NK 92 cell-mediated cytolysis of MCF7 breast cancer cells.  (D) Real-time impedance traces for A549 cells exposed to drugs displaying a variety of mechanisms of action.

CORRELATING IMPEDANCE WITH CELLULAR PHENOMENA

RTCA provides a quantitative readout of cell number, proliferation rate, cell size/shape, and cell-substrate attachment quality. Because these physical properties are the product of thousands of different genes/proteins, RTCA can provide an extremely wide field of view on cell health and behavior. Everything from endothelial barrier function and chemotaxis to filopodia dynamics and immune cell-mediated cytolysis have successfully been analyzed on xCELLigence instruments. Despite the breadth of their reach, xCELLigence assays are still capable of interrogating very specific biochemical and cellular phenomena. Appropriate use of controls and/or orthogonal techniques make it possible to correlate the features of an impedance trace with specific cellular/molecular phenomena.