Protocol for Human brain tissue 3D immunohistochemistry
Starting with fresh brain tissue
1.Immerse whole/hemi-sected human brain tissue in 10% neutral buffered formalin solution for 3-4 weeks. To minimize fixation artefact, the brain is suspended in the solution using an inverted porous disposable hairnet. Alternatively, cut brain tissue into 1cm3 and immerse into 10% neutral buffered formalin solution for 7 days.
Starting with formalin-fixed paraffin-embedded (FFPE) tissue
1.Melt away excess paraffin wax at 60ºC in an incubation oven.
2.Retrieve tissue from molten paraffin wax and put into a screw-capped glass bottle with xylene, incubate at 55 – 60ºC for at least 6 hours or overnight.
Caution: General safety issues with xylene must be observed (e.g. use of fume hood, gloves and lab coat). Waste must be appropriately disposed of.
3.Immerse the tissue into 100% tetrahydrofuran (THF) in a screw-capped glass bottle to elute all the xylene out and ensure complete deparaffinisation, incubate for at least 1 hour at room temperature.
4.Immerse the tissue into 80% THF in water in a screw-capped glass bottle for at least 1 hour at room temperature.
5.Immerse the tissue into 50% THF in water in a screw-capped glass bottle for at least 1 hour at room temperature.
6.Immerse the tissue into 20% THF in water in a screw-capped glass bottle for at least 1 hour at room temperature.
7.Immerse the tissue into SBC solution and incubate at 55oC. Proceed to the usual 3D histology protocol starting from the SDS treatment step.
8.If the tissue is deemed not well-fixed, immerse in neutral buffered formalin for an additional 3 days at room temperature or overnight at 37ºC.
Starting with formalin-fixed tissue
1.Cut the tissue to the desired size using a razor blade, microtome blade or disposable scalpel. Alternatively, use a vibrotome or circular saw for more accurate sampling. Try to ensure a flat surface is cut as this will enable better quality images at the later stage. We recommend cutting the tissue to no larger than 3mm thick due to limitation in antibody penetration and imaging objectives working distance.
2.Immerse tissue into SBC and incubate at 37-55ºC. (Optional)
•The timing depends on the duration of fixation, the antigens to be stained, as well as the nature or brain region of the tissue to be imaged. In general:
oThe longer the tissue fixed in formalin, the longer the SDS treatment recommended.
oWhite matter regions require longer SDS treatment \(at least 10 days is recommended) than grey matter, where 0-5 days may be sufficient for a 1 mm-thick slice.
oSDS or incubation at high temperatures \(50-55ºC) for a long duration has an antigen retrieval effect, therefore the harsher the antigen retrieval method required in tissue sections, the longer the SDS treatment recommended for intact tissue blocks.
oThe longer the SDS-treatment, the less brownish discoloration, autofluorescence, and nonspecific binding of antibodies to lipofuscin. Therefore, whenever time allows, longer SDS treatment is recommended.
•Do not over-incubate; in general, around three months of incubation is the maximum.
•If the tissue is fixed for less than a week, immerse in formalin for a further 3 days at room temperature or overnight at 37ºC.
3.Wash the tissue in PBST at 37ºC for 3 x 2 hours [Tissue can be left in solution for up to 24 hours].
•To minimize tissue damage, tissue should be handed with care and should remain in the same container unless otherwise stated.
•To wash the tissue, use a plastic pipette to carefully extract all the solution around the tissue and apply new solution using a clean pipette.
- Incubate the tissue with primary antibody diluted in PBST at 37ºC. Start with a low concentration (e.g. 1:500 to 1:100 dilution) and supplement antibody over the next 1 – 2 days to the desired final dilution (e.g. 1:20 – 1:50 dilution)
•The denser the antigens within the tissue, the larger the amount of antibody required. Using a lower concentration to begin with can facilitate antibody diffusion deeper within the tissue.
•Penetration of antibodies is mostly governed by the density of the antigens within the tissue, and antibodies can diffuse through roughly 1 – 2 mm upon overnight incubation. Therefore, prolonged incubation is not necessary and futile in most cases in enhancing antibody penetration.
•A 3-day incubation period after the final targeted dilution has been achieved is more than sufficient for practical imaging purposes.
5.Wash the tissue in PBST at 37ºC for 3x 2 hours and leave overnight.
- Incubate the tissue with secondary antibody diluted in PBST at 37ºC. Protect from light using aluminium foil.
•The strategy for antibody addition and incubation period should be the same as that for the primary antibody used.
•Due to the brownish discoloration of human brain tissue, fluorophores with absorbance-emission spectra as bathochromatically-shifted as possible are recommended.
•A nuclear stain (such as DAPI, TO-PRO3, Propidium iodide) and/or fluorophore-conjugated lectin can be added during the secondary antibody incubation.
- Wash the tissue in PBST at 37ºC for 3 x 2 hours and leave overnight.
- Remove the tissue from the washing solution. Blot off excess washing solution from the tissue gently using Kimwipe paper, and place the tissue into the OPTIClear solution, incubate at 37ºC for about 6 hours to clear the tissue.
•A volume of OPTIClear 3-times that of the tissue volume is usually sufficient.
•Complete tissue transparency is normally reached within 3 hours of incubation, and will not improve further after 15 hours of incubation. If necessary, swirl the contents gently and occasionally to facilitate penetration of the reagents into the tissue.
•The tissue should be stored in OPTIClear at room temperature protected from light before imaging (up to 1 week).
•For prolonged storage, tissue should be placed in PBS at 4ºC. The addition of 0.01% sodium azide is recommended to avoid microbial growth
Carefully place the tissue into a glass-bottomed imaging dish for confocal imaging.
•Close apposition of sample with the glass is required for an inverted confocal microscope. To ensure this, tip the cleared tissue from its container into the imaging dish. Then use a clean pipette to carefully extract excess OPTIClear solution around the tissue. The tissue will adhere to the bottom of the imaging dish once all surrounding solution has been extracted. Any excess solution can also be carefully blotted off around the tissue using Kimwipe paper.
Apply a few drops of OPTIClear on the tissue just before imaging to prevent tissue being dried up during prolonged imaging and cover the imaging dish with a lid.
Proceed to confocal imaging.
Protocol for 3D Nissl staining
Both SDS-treated tissue and long-fixed tissue retrieved from formalin fixation solutions can be used directly. 3D Nissl staining should be performed prior to all other chemical stains and 3D immunohistochemistry.
1.Incubate the tissue in Nissl staining solution for 1 – 3 hours at room temperature.
•After staining the tissue should look like Figure 2a (adapted from Lai et al [15]), where grey matter is stained intensely purple while white matter is minimally stained.
- Regress the staining by placing the tissue into regression solution at room temperature. Swirl and refresh the regression solution until no observable violet tint is coming out of the tissue.
•The regression process should not be prolonged for more than 30 minutes, as it may cause precipitation of tissue proteins and subsequent difficulties in immunohistochemistry. Over-regression leading to poor staining may also occur and requires additional Nissl staining.
•If the tissue is thick and large (> 5 mm-thick, or > 2 cm x 2 cm in height and width), then serial dehydration through dilutions of regression solution in water is recommended to avoid warping of tissue, which can be difficult to revert.
•After regression the tissue should look like Figure 2b (adapted from Lai et al [15]), where grey matter is virtually non-stained or a slight tint of violet remains.
- Proceed to 3D immunohistochemistry starting with washing in PBST for 6 hours at 37ºC, or to other chemical stains after appropriate washing in 1x PBS. If direct imaging is desired, place the tissue into OPTIClear and incubate for 3 hours to overnight at 60ºC.
•If the tissue has not been completely regressed, some violet dyes may come out of the tissue and turn the OPTIClear solution violet. This would not cause problem in imaging and no further washings or refresh of the OPTIClear solution is needed as the fluorescent spectrum of cresyl violet changes when it binds to Nissl substances and lipofuchsins.
•After clearing, the tissue should look paradoxically darker, as the transparency reveals the darkly-stained underlying tissue substances, which is shown in Figure 2c (adapted from Lai et al [15]).
•The tissue should be stored in OPTIClear at room temperature protected from light before imaging (up to 1 week).
•For prolonged storage, tissue should be placed in PBS at 4ºC. The addition of 0.01% sodium azide is recommended to avoid microbial growth
- Image the tissue with 488 nm Argon laser excitation. Adjust the detection spectra range accordingly to obtain good contrast and avoid any autofluorescence. 570 – 700 nm is a good empirical start range.
Protocol for lipophilic dye (e.g. DiI) retrograde labelling
Tissue without prior contact with detergents should be used for lipophilic dye tracing. In principle, other lipophilic tracer dyes can be used, but may have different diffusion speeds or abilities. DiI, DiA, DiD, DiR, and CM-DiI has been proven to diffuse well in formalin-fixed tissue.
Gently place a crystal or powder of a lipophilic dye on to the piece of brain tissue.
Use a needle to poke the crystal deep below the tissue surface to secure the position of the DiI crystal.
Carefully soak the tissue in 1x PBS to keep the tissue moist.
Cover the plate or lid and seal it carefully without disturbing the system inside. Incubate the setup at 37ºC free from any disturbances for a variable period of time.
•In our hands, a 3 – 4 mm diffusion distance was achieved with 10 days of tracing in a 5 year-fixed brain. Tissue fixed for a shorter duration may have faster diffusion rates.
After the tracing is deemed to be sufficient, blot of excess liquid on the tissue and place the tissue in OPTIClear for clearing for 6 – 15 hours at 37ºC.
•The tissue should be stored in OPTIClear at room temperature protected from light before imaging (up to 1 week).
•For prolonged storage, tissue should be placed in PBS at 4ºC. The addition of 0.01% sodium azide is recommended to avoid microbial growth
- Proceed to confocal imaging.