Organotypic Slice Cultures to Study Oligodendrocyte Dynamics and Myelination

NOTE: All animal procedures are following the guidelines and have been approved by the Institutional Animal Care and Use Committee (IACUC) at the University of Connecticut.

NOTE: For these experiments constitutive NG2cre¹⁸ (JAX#008533) and inducible NG2creER¹⁶ (JAX#008538) transgenic mice crossed to Z/EG¹⁹ (JAX#003920) or gtRosa26:YFP reporter²⁰ (JAX#006148) lines respectively were used to image NG2 cells and their progeny. To image mature oligodendrocytes, PLPDsRed transgenic mice²¹ were used. For consistent survival, slices can be isolated from mice up to postnatal day 10 from both the forebrain and cerebellum.

1. Slice Preparation

1. In a culture hood, place tissue culture inserts in six well plates and add 1 ml slice culture medium (recipe in reagents section). Put the plates in the cell culture incubator at 37 °C with 5% CO₂ at least 2 hr prior to dissection.
2. Sterilize all tools and tissue chopper with 70% ethanol.
3. Bubble dissection buffer (recipe in reagents section) with 95%  O₂, 5% CO₂ on ice for at least 15 min prior to the start of dissection.
4. Anesthetize mouse pups by placing them on ice for 5-10 min according to approved animal protocols. Ascertain the appropriate depth of anesthesia by confirming that the mice do not respond to tail and toe pinching.
5. Decapitate animals using sharp scissors following animal protocols. Quickly remove the skull over the forebrain and cerebellum by first making a sagittal cut followed by lateral incision, using sterilized tools. Cut cranial nerves from the ventral surface of the brain and cerebellum by carefully rolling the tissue to the side.
6. Place tissue in a sterile 35 mm dish containing ice-cold oxygenated dissection buffer.
7. Using a razor blade, sever the cerebellum and forebrain. Then make a mid-sagittal cut through the forebrain and cerebellum, separating the hemispheres.
8. Cut 300 μm-thick coronal and sagittal sections of the forebrain and cerebellum with a manual tissue chopper. NOTE: A manual tissue chopper allows rapid processing from dissection to culture incubation without the need for agarose embedding. A vibratome can also be used as described previously⁹.
9. Transfer separated slices into freshly bubbled cold dissection buffer on ice.
10. Use small dissecting or weighing spatulas to separate individual sections and transfer them to preincubated six well dishes with culture inserts.
11. Remove any excess dissection buffer from the surface of the culture insert using a disposable transfer pipette or a P 200 pipet tip. Two to three forebrain or three cerebellar slices can be placed on one culture insert.
    NOTE: For consistent results with forebrain cultures, it is ideal to take slices spanning the anterior one-third of the corpus callosum (Figure 1A) in order to study both gray and white matter regions in the same slice. Each animal usually yields 6 forebrain slices and 6 cerebellum slices.
12. Place the six well plates in the incubator and replace the slice medium with 1 ml of slice medium 1 day after slice preparation and then every other day until slice fixation.
13. Depending on the experiment, use slices after 5-7 days in culture. Use only slices that become transparent after the first few days and discard those that have uneven opaque regions. NOTE: Opaque regions within slices are visible by eye and appear as a clump of dark cells under phase contrast. After the technique has been mastered, dead opaque slices occur rarely, in less than 1-5% of slices.

2. Time-Lapse Imaging of NG2 cell Division and Oligodendrocyte Differentiation

NOTE: To perform time lapse imaging of NG2 cell proliferation and differentiation we use NG2cre:ZEG mice¹⁶ which express EGFP in NG2 cells and their progeny (Figure 1C-E). Reporter expression in this line is sufficiently robust to obtain live images of GFP+ cells in slices immediately after sectioning for a time period of at least four weeks. The clearest images are obtained after the initial thinning period of the slice, which occurs over the first 3-5 days in culture.

1. Throughout the experiment, keep the slices in a cell culture incubator at 37 °C and remove only for brief periods of time (less than 15 min per slice), keeping the lid on the six well plate while imaging.
2. Using an inverted fluorescence microscope equipped with a digital camera, capture images at the first time point using a 10X objective. NOTE: First time points will vary by experiment and can be the day when the slices were prepared or any time point after.
3. Capture images from multiple regions at time point one in both gray and white matter areas of the slice. Note the region imaged by specific landmarks within the slice and by mapping the image location on a schematic that can be used for subsequent imaging sessions. Useful landmarks can include edges of the slices and/or orientation of white matter tracts. Image four to eight locations on each slice at each time point.
4. Capture subsequent images at an interval of 4-6 hr if examining NG2 cell division and oligodendrocyte differentiation, ideally over 5-7 days.
5. Capture a final image of all regions and fix the slices immediately. Add 1 ml of fixative (4% PFA with 0.1M l-lysine 0.01 M sodium meta-periodate) to the bottom of the culture insert, then add another 1 ml on top of the slices. Take care not to squirt the fixative directly on the slice as it can detach from the membrane. Fix the tissue for 30 min  and proceed with immunohistochemistry using developmental stage-specific markers as described in section 4.
6. Wash slices with 0.2 M sodium phosphate buffer 3 x 10 min. If necessary, store slices at 4 °C in 0.2 M sodium phosphate buffer for 1-3 days before immunostaining but ideally perform staining within 24 hr. Proceed with immunohistochemistry as described below in section 4 to determine the proportion of cells that have differentiated into oligodendrocytes (Figure 2D-F).

3. Fate Mapping of NG2 cell Progeny Using Inducible Reporter Transgenic Mice in Slice Cultures

NOTE: To track the fate of NG2 cells from a particular time point in the culture, inducible NG2creER:YFP transgenic mice can be used.

1. Induce Cre recombination and reporter expression by adding 100 nM 4OHT dissolved in ethanol to the culture medium. NOTE: Reporter positive cells should appear within 1-2 days at an induction efficiency of ~20-25% YFP+NG2+/NG2+ cells and at this time point will all be cells at the NG2 cell stage (Figure 2A-C)
2. Fix and wash the slices at different time points after various manipulations (described in sections 2.5 and 2.6).

4. Slice Immunohistochemistry

1. Cut the membranes with the slices attached out of the inserts using a scalpel.
2. Transfer slices to individual wells of a 24 well plate containing phosphate buffered saline (PBS) using a set of tweezers, taking care not to disrupt the composition of the slice when picking up the membrane.
3. Transfer the slices to a 24 well plate with blocking solution containing 5% normal goat serum (NGS) and 0.1% Triton-X100 in PBS for 1 hr.
4. Incubate the slices in primary antibody O/N in 5% NGS in PBS at 4 °C. For identifying cells in the NG2 cell stage, use antibodies to NG2 and PDGFRα. For identifying differentiated oligodendrocytes, use an antibody to the adenomatous polyposis coli antigen (APC; clone CC1).
5. On the second day wash slices in PBS 3 x15 min, then incubate in secondary antibodies in 5% NGS in PBS for 1 hr at RT. Wash slices in PBS 3 x15 min and mount on slides. Mount with slices up and membrane facing the slide so that the membrane will not be between the objective and the tissue.