Matrix Metalloproteinase-9 Mediates the Deleterious Effects of α2-Antiplasmin on Blood-Brain Barrier Breakdown and Ischemic Brain Injury in Experimental Stroke

During acute brain ischemia, alpha 2-antiplasmin markedly enhances brain injury, blood-brain barrier breakdown and matrix metalloproteinase-9 (MMP-9) expression. Although alpha 2-antiplasmin inhibits fibrin thrombus-degradation, and MMP-9 is a collagen-degrading enzyme altering blood-brain barrier, both have similar deleterious effects on the ischemic brain. We examined the hypothesis that MMP-9 is an essential downstream mediator of alpha 2-antiplasmin's deleterious effects during brain ischemia. Middle cerebral artery thromboembolic stroke was induced in a randomized, blinded fashion in mice with increased blood levels of a2-antiplasmin. There was a robust increase in MMP-9 expression (immunofluorescence) in the ischemic vs. the non-ischemic hemisphere of MMP-9(+/+) but not MMP-9(-/-) mice, 24 h after stroke. Brain swelling and hemorrhage were significantly increased in the ischemic vs. the non-ischemic hemisphere of MMP-9(+/+) mice. By comparison to MMP-9(+/+) mice, the ischemic hemispheres of MMP-9(-/-) mice showed a similar to 6-fold reduction in brain swelling (p < 0.001) and a similar to 9fold reduction in brain hemorrhage. Brain infarction (p < 0.0001) and TUNEL-positive cell death (p < 0.001) were significantly diminished in the ischemic hemisphere of MMP-9(-/-) mice vs. MMP-9(+/+) mice. Ischemic breakdown of the blood-brain barrier and fibrin deposition were also significantly reduced in MMP-9(-/-) mice vs. MMP-9(+/+) mice (p < 0.05), as measured by quantitative immunofluorescence. We conclude that MMP-9 deficiency ablates many of the deleterious effects of high alpha 2-antiplasmin levels, significantly reducing blood-brain barrier breakdown, TUNEL-positive cell death, brain hemorrhage, swelling and infarction. This suggests that the two molecules may be in a shared pathway in which MMP-9 is essential downstream for the deleterious effects of a2-antiplasmin in ischemic stroke. (C) 2018 IBRO. Published by Elsevier Ltd. All rights reserved.