Figure 1.

Schematic of the exosome isolation protocol from solid brain tissue. Fresh frozen (−80°C) human frontal cortex was sliced with a razor blade on ice while frozen to generate 1–2 cm long, 2–3 mm wide sections. The cut sections are dissociated while partially frozen in 75 U/ml of collagenase type 3 in Hibernate-E at 37°C for a total of 20 min. The tissue is returned to ice immediately after incubation and protease and phosphatase inhibitors are added. The tissue is spun at 300 × g for 5 min at 4°C (pellet is used as the brain homogenate + collagenase control), the supernatant transferred to a fresh tube, spun at 2000 × g for 10 min at 4°C, then at 10,000 × g for 30 min at 4°C. The EV-containing supernatant is overlaid on a triple sucrose cushion (0.6 M, 1.3 M, 2.5 M) and ultracentrifuged for 3 h at 180,000 × g to separate vesicles based on density. The top of the gradient is discarded and fractions designated 1, 2 and 3 are collected and the refractive index is measured. Each fraction is further ultracentrifuged at 100,000 × g to pellet the vesicles contained in each fraction. Each preparation is validated by a combination of techniques including electron microscopy and RNA and protein analysis. Note – some tissue samples will not be amenable to this method. Post-mortem delay, storage time and the number of freeze-thaw cycles will negatively impact on tissue quality and result in contamination of the fractions with cellular debris and non-exosome vesicles.