Applied_Biophysics_ECIS_Real_Time_Cell_Growth_Monitoring__Signal

Many compounds of interest in biological and medical research are those molecules that specifically bind to cell surface receptors. When the receptor receives these ligands, subsequent changes in a signal transduction pathway result in the enhancement or attenuation of a cellular response. Assays capable of detecting and quantifying the receptor-ligand interaction are valuable tools both in fundamental studies of cell behaviour and in the development of new drugs. Many modern assays are based upon detection of receptor-ligand binding event directly. Although binding of a compound is an essential component of signal transduction, this event alone does not assure that the biological response affected by the receptor is altered. Assays where one monitors cellular response directly, can avoid these potential false positives and provide more reliable information regarding the efficacy of compounds. We have explored the use of ECIS to monitor the signal transduction pathways activated by G protein coupled receptors (GPCR). This assay is based upon the widely accepted conjecture that GPCR activation, regardless of the second messenger, results in alterations of the cell’s cytoskeletal elements. This culminates in morphological changes, and this is precisely the type of event detected in real time and with great sensitivity by the ECIS biosensor.
	In preliminary studies using this approach, several receptors have been successfully studied, including the muscarinic receptor monitored in a dose response experiment reported in the figure above. To prepare for this experiment, CHO cells, engineered to over express the muscarinic receptor were first grown to confluence over a period of 24 hours in the ECIS wells. At time zero in the figure, the cell-covered electrodes exhibited resistance values from 4.1 to 5.4 k ohms – normal variation in the measurement. At the arrow, the control received a small volume of balanced salt solution, whereas the other wells received the same vehicle but containing different concentrations of the muscarinic agonist, carbachol (final concentrations are listed in the figure). Following agonist addition, there is a steep rise in the impedance lasting for 15 to 20 minutes. The impedance then plateaus and begins a gradual return to the baseline value. The control shows only a slight response to the vehicle addition. The impedance changes are very significance – doubling in some cases.
To be certain the response monitored by ECIS is in fact the specific activation of the muscarinic receptor, we first estimated the EC50 value from the dose response data. Using the maximum value obtained following carbachol addition as indicative of the response, the data gave an EC50 of 1 micromolar, consistent with values reported in the literature for this receptor-ligand pair. Next, we carried out experiments with an antagonist for the receptor, perenzipine (PZP), to see if we could block the carbachol response. In the figure below these results are presented. Here, at the first arrow, we first administer different concentrations of PZP, followed in 15 minutes (the second arrow) with a fixed dose of 20 mM carbachol. Controls are included with only buffer addition at both times and with no PZP addition to see the normal carbachol response. The ability of PZP to block the carbachol activation is clearly demonstrated and indicates that we in fact are seeing morphological changes associated with activation of the muscarinic receptor and the ensuing signal transduction cascade.
The use of whole cell sensors for these signal transduction measurements is especially significant in drug discovery efforts. Drugs must function at the cellular level to ultimately have an effect upon tissue and the whole organism. The complex molecular events that result in a cellular response to a particular compound involve a series of signalling and feedback circuits. Simply detecting binding of a compound to a receptor does not assure the efficacy of the compound in other aspects of the transduction mechanism such as receptor activation, second messenger production, etc. Using whole cell sensors such as ECIS in drug assays eliminates these potential stumbling points. In addition, with the whole cell system one can also evaluate the toxicity of compounds and the effect of drugs upon different cell types.