A Novel In Vitro Wound Healing Assay to Evaluate Cell Migration

The goal of this procedure is to show how to evaluate the ability of some immunomodulatory molecules, such antimicropeptides, to stimulate cell migration, which is a rate-limiting event during the wound-healing process. Generally speaking, the basic procedure to study cell migration in vitro involves the creation of cell monolayer, the production of pseudo-wound, the capture of images at different time intervals until wound closure is reached, and analysis of the image sequence to quantify cell migration. Now, the advantage of this method over the classic assay, which is based on a manually made scratch, is due to the usage of special silicone culture inserts to create the pseudo-wound field, which is completely free of cells and with a very well-defined width, 500 micrometer.

In addition, thanks to a web-based, automated image analysis program, it is possible to rapidly receive within minutes quantitative data on the speed of wound closure and cell migration. So overall, this is a reliable experimental technique that can be applied to any kind of adherent cells with very high reproducibility of data. Here, it is provided, as an example, the effect of a frog skin antimicropeptide on the migration of bronchial epithelial cells and how pretreatment of these cells with specific inhibitors can give information on the molecular mechanism underlying such events.

More precisely, it will be shown how the peptide-induced cell migration involves activation of epidermal growth factor receptor. To begin experiment, remove the flask containing cells from the cell culture incubator, and check the circumference on an inverted microscope equipped with an automatic image acquisition system. If 90%of complete confluence is reached, move to a biosafety hood, and carefully aspirate the culture medium using a 10-milliliter plastic pipette under sterile conditions and gently discard the medium into waste bottle.

Wash the cells with six milliliter PBS without calcium and magnesium, that is, CMF. Gently rock the flask manually and discard CMF. Then, add other 10 milliliter CMF to further remove cell debris and non-attached cells.

At this point, close the flask, and put it back into the cell culture incubator for 10 minutes. Take out the flask, discard CMF, and gently add two milliliter trypsin-EDTA to detach cells from the plastic surface. Gently rock the flask manually, allowing the solution to completely cover the cells, and incubate the flask for additional 10 minutes.

Afterwards, remove the flask from the incubator, and check whether the trypsin addition process is complete. If this is the case, the cells will be in suspension and will appear rounded under the microscope. To inactivate trypsin, add 10 milliliter culture media supplemented with 10%heat-inactivated bovine serum, and carefully wash the flask.

Collect the cell suspension, transfer this volume into a conical 50-milliliter tube, and centrifuge the sample. After centrifugation, check the pellet formation, aspirate the supernatant, and resuspend the cell pellet in six milliliter culture medium supplemented with serum. Pipette the cell suspension up and down for several times to break up any clumps formation.

This process may take some minutes. Afterwards, take out 10 microliter with a micropipette tip, and inject the volume under the cover glass of a Burker or Neubauer chamber and count the cell number. In each well of a 12-well plate, put the culture insert with the help of sterile tweezers.

Each insert is provided of two compartments separated by a 500-micrometer wall, and there's a sticky underside to allow adhesion. However, by means of tweezers, apply some pressure along the insert edges to fix them to the surface of the plate. Fill each compartment of the insert with 70 microliter of cell suspension containing about 3.5 by 10 to fourth cells per chamber.

Importantly, the concentration of cells applied into each compartment depends on the type of cells. It is recommended to use a cell density that leads to complete confluence within 24 hours. Under the microscope at four-fold magnification, check that cells are not leaking from the insert compartments, and then incubate the plate for 24 hours.

After incubation, gently remove the inserts with sterile tweezers without breaking the cell monolayers, and transfer the insert onto absorbent paper. Wash the cells by adding one milliliter medium per well. Be careful not to add the medium directly on top of cell monolayers, to avoid their disruption.

Close the plate, and gently rock it manually. Aspirate the medium, and replace it with one milliliter of fresh medium per well. Visualize the cell-free gaps under the microscope, and acquire images at time zero.

Withdraw the media from the wells, and add PBS supplemented with calcium and magnesium. Shake the plate, remove PBS, and finally add the test compounds to the wells. For untreated control samples, one millimeter medium is used.

Incubate the plate. For each sample, observe the cell migration under the microscope at four-fold magnification, and acquire images after 15, 20, and 24 hours incubation. During this step, try to capture images in the same areas as for time zero.

The choice of time intervals at which images are captured depends on the migration speed of the selected cells. When the experiment is finished, select images for each sample at the different time intervals, and create a zip file. It is important to have at least three replicates for each sample at each time interval.

Open the automated Image Analysis Wimasis program, and upload the zip file using the lowest magnification. The program will automatically provide by email a summary file containing the experimental data of cell-covered area and scratch areas as percentages of each single sample, for example, samples 1a and 2a, at the selected time points. Calculate a mean value of all samples at time zero.

Normalize all data with respect to the mean value at time zero. Calculate a mean value of all samples at each time point and the relative standard error. By using two-way ANOVA test, perform statistical analysis.

The obtained data can be plot into a graph as a histogram showing the percentage of cell-covered area at different time points for different sample groups. These histograms show the wound closure obtained after 15, 20, and 24 hours treatment of bronchial cells with different concentrations of the antimicropeptide esculentin 1-21-1c or esculentin 1-21 in comparison to the control. As pointed out by the histograms and some representative micrographs, esculentin 1-21-1c is more efficient in promoting re-epithelialization of pseudo-wound in bronchial cells induced in the almost-complete closure of the wound field within approximately 20 hours at one micromolar, while a higher concentration is needed for esculentin 1-21, that is, 10 micromolar.

To study the role of some receptors, such as epidermal growth factor receptor, on cell migration, pretreat the cell monolayers with a specific inhibitor, for example, the compound AG1478. To this purpose, before removing the insert, aspirate the medium in each compartment, wash it once, and add 70 microliter of media supplemented with five micromolar AG1478. Incubate the plate for 30 minutes.

Then, remove the insert with tweezers, and proceed as described earlier. These are the final data showing that the peptide-induced cell migration involves activation of epidermal growth factor receptor. Indeed, after pretreatment with inhibitor AG1478, the percentage of cell-covered area is significantly reduced at all time intervals compared to the results found when the cells are not pre-incubated with AG1478 before adding the peptide.

In conclusion, the described method has a high potential to significantly improve the understanding on the migratory feature of different types of cells, as well as on the selection of the most promising wound-healing promoters and/or inhibitors, within a short time and with a high accuracy.