Immunofluorescence Assay to Detect PM_2.5-Mediated DNA Damage in Fish Embryos

All procedures involving animal models have been reviewed by the local institutional animal care committee and the JoVE veterinary review board.

1. PM_2.5 sampling and organic compound extraction

NOTE: PM_2.5 was collected in an urban area in Suzhou, China, August 1-7, 2015, as described previously.

1. Bake 47 mm quartz membrane filters in a 500 °C muffle furnace for 2 h to remove the organic components.
2. Place a filter in a PM_2.5 sampler for 24 h of uninterrupted sampling.
3. Remove the filter and dry for 24 h at room temperature.
4. Quantify the filter with an analytical balance.
5. Extract organic components from the filter by Soxhlet extraction using dichloromethane as a solvent.
6. Dry the EOM by a rotary evaporation in a 60 °C water bath and nitrogen flow. Dissolve EOM in DMSO and store at -20 °C.

2. Zebrafish embryo collection and treatment

1. Maintain the zebrafish at 28.5 ± 0.5 °C in a re-circulating aquaculture system with a 14 h light and 10 h dark photoperiod cycle.
2. Place healthy adult zebrafish into a tank at a 2:1 male to female ratio.
3. The next day, collect the embryos and wash them with system water (i.e., zebrafish breeding water).
4. Select and randomly divide zebrafish embryos demonstrating a normal development (uniform size, full grains, and no egg coagulation) into 4 groups in individual glass Petri dishes with a diameter of 7 cm (about 50 embryos per dish).
5. Treat the embryos with PM_2.5 (5 mg/L) in the presence or absence of NAC at 0.25 μM from 2 hpf until 72 hpf. Use DMSO as a vehicle control to a final concentration at 0.1% (v/v).

3. Morphological observation of zebrafish embryos and cardiac dissection

1. At 72 hpf, transfer embryos to glass slides and observe under a stereo microscope. Record heart malformations, such as pericardial edema, altered looping, and decreased size.
2. Calculate malformation rates (the percentage of embryos with heart defects out of the total living embryos) and analyze differences between groups using one-way ANOVA followed by Turkey's Multiple Comparison Test (p < 0.05 = statistically significant).
3. Anesthetize the embryos with 0.6 mg/mL MS-222 to immobilize them on glass slides.
4. Record heart beats for 30 s and quantify heart rates using ImageJ software.
5. Dissect hearts from zebrafish embryos with a disposable syringe needle under a stereo microscope. Caution: Avoid destroying the heart shape.

4. Immunofluorescence Assay

1. To use an immunofluorescence assay to detect PM_2.5 induced DNA damage in the heart of zebrafish embryos, use a hydrophobic barrier pen to draw a circle on a clean glass side.
2. Add 50 μL of 4% paraformaldehyde to 1.25 mL of Phosphate Buffered Saline (PBS) to make a fixative solution.
3. Place 3 dissected hearts into one hydrophobic barrier pen circle and incubate for 20 min at room temperature.
4. Decant the solution under the microscope and dry the samples at room temperature for at least 5 min, so that the hearts completely attach to the glass slides.
5. Wash the slides three times in PBS with 0.1% Tween 20 (PBST) for 5 min per wash.
6. Add 50 μL of bovine serum albumin (BSA) to 1000 μL of PBST to obtain a 5% BSA solution and incubate the slides in a humid chamber for 1 h to block non-specific antibody binding.
7. Decant the solution and wash the samples three times with PBST for 5 min per wash.
8. Dilute 2 μL of mouse monoclonal antibody against 8-OHdG and 2 μL of rabbit polyclonal antibody against γH2AX in 296 μL of PBST to obtain a working primary antibody cocktail solution.
9. Incubate the heart samples with 50 μL of the primary antibody cocktail solution against 8-OHdG and γH2AX in a humidified chamber for at least one hour at room temperature or overnight at 4 °C (Overnight incubation can increase signal intensity).
10. Decant the solution and wash the samples three times with PBST for 5 min per wash.
11. Dilute 1 μL of FITC-labeled goat anti-mouse secondary antibody and 1 μL of cy3 goat anti-rabbit secondary antibody in 498 μL of PBST to obtain a working secondary antibody cocktail solution and incubate the samples with the secondary antibodies (1:500 in PBST) for 1 h at room temperature in the dark.
12. Decant the solution and wash the samples three times with PBS for 5 min per wash protected from light.
13. Add 20 μL of DAPI (4',6-diamidino-2-phenylindole) to the samples for nuclear staining for 30 min at room temperature.
14. Apply a coverslip to slide and seal with nail polish to prevent drying and movement. Then image the samples under a fluorescence microscope and quantify the fluorescence signal of heart area using ImageJ software. Calculate the relative changes with the average of DMSO control samples. Determine the statistical significance of the data as in step 3.2.