Using Terminal Transferase-mediated dUTP Nick End-labelling (TUNEL) and Caspase 3/7 Assays to Measure Epidermal Cell Death in Frogs with Chytridiomycosis

The overall goal of this experiment is to observe the affect of amphibian fungal pathogen batrachochytrium dendrobatidis on apoptosis and cell death in the epidermal tissue of frogs. This method can help us understand wildlife disease such as if infection causes cell death or, conversely, if there are immune mechanisms against infection. The main advantage of this technique is that it can be performed on small tissue biopsy samples so that small individuals can be repeatedly sampled throughout the course of infection.

To begin this procedure, prepare hydrophilic glass slides with sample tissue as outlined in the text protocol. To deparaffinize the tissue sections, wash the slides three times with xylene for five minutes each in a coplin jar. Next, wash the slides with 100%ethanol two times for five minutes each.

After this, wash the slides with 95%ethanol and 70%ethanol for three minutes each. Then, wash the slides once with PBS for five minutes. To prepare experimental and negative control slides, pretreat the tissue by adding freshly diluted proteinase K directly to the slides.

Incubate the slides for 10 minutes at room temperature. Then, wash the slides in PBS two times for two minutes each. To prepare positive control slides, pretreat the tissue with DN buffer then incubate the slides at room temperature for five minutes.

After this, dissolve DNase one in DN buffer to a final concentration of 0.1 micrograms per milliliter. Add the prepared DNase solution to the slides. Incubate the slides for 15 minutes at room temperature.

Then, wash the slides with distilled water in a coplin jar five times for three minutes each. Quench the reaction in 3%hydrogen peroxide in PBS for five minutes at room temperature. Next, wash the slides in PBS two times for two minutes each.

Then, add equilibrium buffer to the tissue on the slides. Incubate for 10 seconds at room temperature. Then, tap off the excess liquid around the tissue section.

Dilute terminal deoxynucleotidyl transferase enzyme in reaction buffer. Then, add it to the tissue on the experimental and positive control slides. Incubate in a humidified chamber for one hour at 37 degrees Celsius.

After this, place the slides in a coplin jar containing the working concentration of stop buffer. Agitate for 15 seconds and then incubate the slides at room temperature for 10 minutes. Then, wash the slides with PBS three times for one minute each and tap off the excess liquid.

Next, add the anti-digoxigenin conjugate, prewarmed to room temperature, directly onto the tissue. Incubate the slides for 30 minutes at room temperature in a humidified dark chamber. After the incubation is complete, wash the slides with PBS four times for two minutes each.

Tap off any excess liquid. Then, add mounting medium to the slides. Add 15 microliters of DAPI solution to the mounting medium on the slides.

Cover each slide with a cover slip and seal with nail polish. Allow the slides to dry in the dark. Using a fluorescent microscope, observe the slides immediately upon completion of the standing process.

Take photos at 400 times at random intervals along each skin section. Count DAPI-stained blue cells to ensure at least 100 cells per skin section per animal are visible. Next, count the number of TUNEL-positive cells which appear as red single cells with clear cell edges.

To begin, use an inverted light microscope at 40 times to take a photo of each of the frog’s toes. Draw lines around the edge of the toe clip and use imaging software to estimate the area within the shape. To evaluate the surface area of the three-dimensional skin sample, multiply the estimated area by pi.

To extract proteins from a frozen frog toe sample, place the sample in a 1.5 milliliter screw cap microcentrifuge tube containing 100 microliters of sample buffer and two stainless steel beads. Use a bead beater at maximum speed to lyse the cells. After this, place the tube on ice for three minutes.

Then, centrifuge the tube to collect the supernatant. To perform the caspase 3/7 assay in a 384-well plate, add 10 microliters of sample buffer as blank and 10 microliters of protein extract in triplicate. Then, pipette 10 microliters of commercially available caspase 3/7 reagent to the protein extract and blank wells.

Then, shake the plate gently for 15 seconds to mix the two reagents. Incubate at room temperature in dark for 30 minutes. Finally, use a luminescent plate reader to measure the luminescence of the sample.

In this study, the infected animals depict significantly more TUNEL-positive cells in comparison to control animals as illustrated by red or pink-stained cells. In control animals, there are few TUNEL-positive cells, and they are not concentrated in the epidermis. In infected animals, the TUNEL-positive cells are more frequent in the epidermis.

TUNEL-positive cells are more concentrated at and directly adjacent to the sights of infected cells. Also, there are more widespread TUNEL-positive cells over the epidermis and in epidermal layers that are deeper than where the fungal pathogen resided. In infected animals, there are more TUNEL-positive cells in all three skin types with a particularly high level in the thigh and venter skin.

In the caspase assay, the infection intensity is positively correlated with caspase 3/7 activity. Additionally, caspase 3/7 activity in infected and uninfected animals differed over time. Caspase 3/7 activity decreased within the first few weeks after inoculation and then increased toward the end of week seven in infected animals.

Caspase 3/7 activity remained constant in uninfected individuals. While attempting the procedure for the TUNEL assay, it’s important to read the slides on the same day as the assay to avoid loss of sensitivity and to keep the slides in a cool dark place to protect the dyes. After watching this video, you should have a good understanding of how to detect apoptosis and cell death using two different detection methods in small tissue samples like amphibian skin.