Isolating Mouse Brain Vessels: A Protocol to Isolate Intact Vessels from Murine Brain Sample

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

1. Dissection

NOTE: Sterile conditions are not required unless vessels are used for cell culture purposes. Prepare isolation vessel solutions: B1, add 1.5 mL of HEPES 1M to 150 mL of HBSS; B2, add 3.6 g of Dextran to 20 mL of B1; B3, add 1 g of BSA to 100 mL of B1.

1. Prepare a 150 mL beaker with 20 mL of B1. Keep on ice and cover with parafilm to avoid air contamination.
2. Deeply anesthetize the mouse under a hood with a small paper towel soaked in 1 mL of pure Isoflurane that is added into the cage. Anesthesia is verified by a lack of reaction to a toe pinch. Kill the mouse by cervical dislocation.
   NOTE: These steps are accomplished in compliance with national and institutional regulations.
3. Optional: Perform intracardiac perfusion with 20 mL of PBS 1x to eliminate the blood content.
4. Section the skin with a scalpel from the neck to the nose and pull it away. Remove all contaminating hairs with PBS 1x.
5. To open the skull, first, insert scissors anteriorly to the olfactory bulb, and open the scissors to rupture the skull in two parts.
6. Carefully remove the brain using a brain spatula. Dissect out the choroid plexus from the lateral ventricles as they would contaminate the blood vessel preparation.
   NOTE: Optional: The final preparation will contain parenchymal and meninges vessels. If not desired, meninges can be peeled off.
7. Transfer the brain into the beaker containing B1 solution on ice. Up to 8 brains can be treated together.

2. Brain Tissue Homogenization

1. Using two scalpels, manually and vigorously beat the brain in the B1 solution subsequently obtaining small pieces of about 2 mm.
2. Homogenize the preparation with an automatized Dounce homogenizer, performing 20 strokes at 400 rpm. Ensure that the glass tube is maintained in ice and that the upper part of the douncer is in solution when moving up and down, so as to prevent the formation of air pockets. If several samples are prepared, wash the douncer with ionized water between each homogenization.

3. Vessel Purification

1. Transfer the homogenate into a 50 mL plastic tube and proceed to the centrifugation at 2,000 x g for 10 min at 4 °C. A large white interface (mostly myelin) will form on the top of the vessel pellet (red if no perfusion was performed).
2. Discard the supernatant. The vessel pellet and the white interface will remain attached together. Add 20 mL of ice-cold B2 solution and shake the tube manually and vigorously for 1 min.
3. Proceed to the second centrifugation at 4,400 x g for 15 min at 4 °C. The myelin will now form a dense white layer at the surface of the supernatant.
4. Carefully detach the myelin layer from the tube walls by holding the tube and slowly rotating it to allow the supernatant to pass along the walls. Discard myelin with the supernatant. The pellet containing the vessels remains attached at the bottom of the tube.
5. Blot the inside wall of the tube with an absorbent paper wrapped around a 5 mL plastic pipette and remove all residual fluids, avoid touching the vessel pellet. Keep the tube upside-down on an absorbent paper to drain any remaining liquid.
6. Suspend the pellet in 1 mL of ice-cold B3 solution by pipetting up and down with low-binding tips, keeping the tube on ice, then add another 5 mL of B3 solution. Make sure that vessels are dispersed as much as possible and do not form aggregates.

4. Filtration

1. Prepare a beaker on ice with 30 mL of ice-cold B3 solution. Cover with parafilm to avoid air contamination.
2. Place a 20 μm-mesh filter on a modified filter holder on the top of a becker flask and equilibrate by applying 10 mL of ice-cold B3 solution.
3. Pour the vessel preparation on the filter and rinse the vessels with 40 mL of ice-cold B3 solution.
4. Recover the filter using clean forceps and immediately immerse it in the beaker containing the B3 solution. Detach the vessels from the filter by shaking it gently.
5. Pour the beaker content in a 50 mL plastic tube and centrifuge at 2,000 x g for 5 min at 4°C.
   NOTE: Alternatively, the brain vessels’ suspension from step 3.6 can be filtered onto a 100 μm-mesh filter. In this case, larger vessels are preferentially retained on the filter while the flow-through contains microvessels (mainly capillaries), which are then filtered on a 20 μm-mesh filter as above.
6. Resuspend the pellet of microvessels in 1 mL of ice-cold B3 solution and transfer it by pipetting it into a 1.5 mL Eppendorf tube. Centrifuge at 2,000 x g for 5 min at 4 °C.