TUNEL staining for apoptotic DNA fragmentation
Materials Required
Principle
TUNEL staining detects DNA strand breaks by using terminal deoxynucleotidyl transferase to add labeled nucleotides to exposed 3′-OH DNA termini, generating either microscopic staining in fixed cells or tissue sections, or fluorescence/cytometric signal in cell suspensions[1][2][3]. TUNEL positivity reflects DNA fragmentation but should not be interpreted alone as definitive apoptosis, because TUNEL can also label necrotic, autolytic, mechanically damaged, or DNA-repair-associated DNA breaks[4][5][6][7].
MCE has not independently verified the accuracy of these methods. They are for reference only.
Experimental Materials
• Use phosphate-buffered saline for washing, paraformaldehyde or formalin-fixed material for morphology preservation, permeabilization/proteolysis reagents such as proteinase K where required for DNA-end accessibility, DNase-treated material as a positive control, and TdT-omitted reaction mixture as a negative control[2][5].
• Use TdT enzyme with labeled dUTP, biotin-dUTP, BrdUTP, or fluorochrome-tagged deoxynucleotides to label DNA breaks; BrdUTP labeling followed by anti-BrdU detection is a reported cytometric variant with high sensitivity, and DAPI or propidium iodide can be used to counterstain total DNA for microscopy or cytometry[2][3].
• Use a humidified incubation chamber for slide reactions, a light or fluorescence microscope for tissue/cell imaging, and a flow or laser-scanning cytometer when the assay is performed in cell suspensions with DNA-content counterstaining[2][3].
• Use TdT enzyme with labeled dUTP, biotin-dUTP, BrdUTP, or fluorochrome-tagged deoxynucleotides to label DNA breaks; BrdUTP labeling followed by anti-BrdU detection is a reported cytometric variant with high sensitivity, and DAPI or propidium iodide can be used to counterstain total DNA for microscopy or cytometry[2][3].
• Use a humidified incubation chamber for slide reactions, a light or fluorescence microscope for tissue/cell imaging, and a flow or laser-scanning cytometer when the assay is performed in cell suspensions with DNA-content counterstaining[2][3].
Experimental Procedure
• Prepare fixed cultured cells, frozen sections, paraffin sections, or single-cell suspensions according to the sample format; for tissue sections, preserve morphology and expose DNA breaks sufficiently for TdT access, because fixation and pretreatment strongly affect TUNEL sensitivity and background[2][5].
• Include one DNase-treated positive-control specimen and one negative-control specimen processed without TdT enzyme in every staining run[2].
• Wash fixed samples in buffer, permeabilize or protease-treat when required, equilibrate the sample in TdT-compatible reaction conditions, incubate with TdT plus labeled nucleotide, stop the enzymatic reaction, wash away unincorporated label, detect the incorporated label directly by fluorescence or indirectly through antibody/streptavidin-based detection, counterstain nuclei when needed, and mount for imaging or cytometric acquisition[2][3].
• For colorimetric tissue TUNEL, perform TdT-mediated labeling followed by enzyme-linked detection and chromogenic visualization; for fluorescent microscopy, detect fluorophore-labeled nucleotide incorporation; for flow cytometry, label DNA strand breaks with BrdUTP or fluorochrome-tagged nucleotide and combine with DNA-content staining such as propidium iodide or DAPI[2][3].
• For archival tissue sections, optimize pretreatment to the fixation method, because Labat-Moleur et al. reported that microwave pretreatment at pH 3 for formalin-fixed tissues or pH 10.6 for Bouin-fixed tissues combined with proteolysis improved detection while reducing background, but also concluded that no single protocol fits all tissue-processing conditions[5].
• Score TUNEL-positive cells as cells with nuclear signal above the negative-control background, report results as the percentage or density of TUNEL-positive cells rather than as “percent apoptosis” unless apoptosis is confirmed by independent morphology or molecular markers[4][5][7].
• Use DNase-treated samples to confirm that DNA ends can be labeled, TdT-omitted samples to estimate nonspecific detection/background, and independent apoptosis readouts such as cleaved caspase-3, Annexin V, PARP cleavage, morphology, or DNA laddering when the biological conclusion requires apoptosis-specific interpretation[2][4][7].
• Include one DNase-treated positive-control specimen and one negative-control specimen processed without TdT enzyme in every staining run[2].
• Wash fixed samples in buffer, permeabilize or protease-treat when required, equilibrate the sample in TdT-compatible reaction conditions, incubate with TdT plus labeled nucleotide, stop the enzymatic reaction, wash away unincorporated label, detect the incorporated label directly by fluorescence or indirectly through antibody/streptavidin-based detection, counterstain nuclei when needed, and mount for imaging or cytometric acquisition[2][3].
• For colorimetric tissue TUNEL, perform TdT-mediated labeling followed by enzyme-linked detection and chromogenic visualization; for fluorescent microscopy, detect fluorophore-labeled nucleotide incorporation; for flow cytometry, label DNA strand breaks with BrdUTP or fluorochrome-tagged nucleotide and combine with DNA-content staining such as propidium iodide or DAPI[2][3].
• For archival tissue sections, optimize pretreatment to the fixation method, because Labat-Moleur et al. reported that microwave pretreatment at pH 3 for formalin-fixed tissues or pH 10.6 for Bouin-fixed tissues combined with proteolysis improved detection while reducing background, but also concluded that no single protocol fits all tissue-processing conditions[5].
• Score TUNEL-positive cells as cells with nuclear signal above the negative-control background, report results as the percentage or density of TUNEL-positive cells rather than as “percent apoptosis” unless apoptosis is confirmed by independent morphology or molecular markers[4][5][7].
• Use DNase-treated samples to confirm that DNA ends can be labeled, TdT-omitted samples to estimate nonspecific detection/background, and independent apoptosis readouts such as cleaved caspase-3, Annexin V, PARP cleavage, morphology, or DNA laddering when the biological conclusion requires apoptosis-specific interpretation[2][4][7].
Troubleshooting
Problem: Weak or absent TUNEL signal in a sample expected to contain DNA fragmentation.
• Possible cause: DNA ends may be inaccessible because fixation and tissue processing affect staining sensitivity.• Literature-supported solution: Adjust pretreatment to the fixation/sample type and confirm assay competence with a DNase-treated positive control[2][5].
Problem: High background or widespread labeling of cells without apoptotic morphology.
• Possible cause: TUNEL can label non-apoptotic DNA breaks and can produce nonspecific labeling depending on tissue processing.• Literature-supported solution: Interpret staining with morphology, TdT-omitted controls, and independent apoptosis markers rather than using TUNEL alone[4][5][6][7].
Problem: Apparent apoptosis in necrotic or damaged tissue.
• Possible cause: TUNEL does not reliably discriminate apoptosis from necrosis, autolysis, or other DNA-damage states.• Literature-supported solution: Remove apoptosis-specific claims unless supported by additional evidence such as apoptotic morphology or caspase-based assays[4][6][7].
References:
- [1]. Gavrieli Y, et al. Identification of programmed cell death in situ via specific labeling of nuclear DNA fragmentation. J Cell Biol. 1992;119(3):493-501. [Content Brief]
- [2]. Kyrylkova K, et al. Detection of apoptosis by TUNEL assay. Methods Mol Biol. 2012;887:41-47. [Content Brief]
- [3]. Darzynkiewicz Z, et al. Analysis of apoptosis by cytometry using TUNEL assay. Methods. 2008;44(3):250-254. [Content Brief]
- [4]. Grasl-Kraupp B, et al. In situ detection of fragmented DNA (TUNEL assay) fails to discriminate among apoptosis, necrosis, and autolytic cell death: a cautionary note. Hepatology. 1995;21(5):1465-1468. [Content Brief]
- [5]. Labat-Moleur F, et al. TUNEL apoptotic cell detection in tissue sections: critical evaluation and improvement. J Histochem Cytochem. 1998;46(3):327-334. [Content Brief]
- [6]. Charriaut-Marlangue C, et al. A cautionary note on the use of the TUNEL stain to determine apoptosis. Neuroreport. 1995;7(1):61-64. [Content Brief]
- [7]. Mirzayans R, et al. Do TUNEL and other apoptosis assays detect cell death in preclinical studies? Int J Mol Sci. 2020;21(23):9090. [Content Brief]
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