Role of blood-brain barrier leakage during stroke

Stroke is the leading disease accounting for death and disability in the world. It is often caused by the occlusion of blood vessels supplying the brain. Stroke causes the opening of blood-brain barrier (BBB), the specific vascular structure that regulates delivery of substances to the brain. The integrity of the barrier structure depends on a dynamic interaction between endothelial cells, glial cells, and neurons, collectively called the neurovascular unit. BBB breakdown during stroke would allow plasma constituents to enter the brain and possibly damage cells. In this study, I sought to investigate if BBB leakage of plasma toxic factors might contribute to the pathology of ischemic injury. To address the above question, the first step was to establish the correlation between BBB leakage and tissue injury. Focal ischemia was produced in a rat model of the middle cerebral artery occlusion (MCAo). High molecular weight dextran- fluorescein isothiocyanate (FITC) was employed to label vascular leakage. A consistent pattern of vascular labeling by dextran-FITC was observed within the ischemic core. Ultrastructural examination showed evidence of cytotoxic edema and severely disrupted vascular membrane associated with the presence of dextran-FITC. Histology revealed that the regional distribution of the severe vascular disruption correlated with the area of the ischemic infarction and neuronal injury. I concluded from these experiments that dextran-FITC can serve as a marker for severe vascular disruption and is useful in further studies of the patho-anatomic mechanisms of vascular- disruption mediated tissue injury. The next step was to identify the toxic factor that leaks into the parenchymal tissue. Among many candidates, thrombin is one of the earlier mediators responding to endothelial damage. In our stroke model, infusion of thrombin intra-arterially during ischemia greatly exacerbated blood-brain barrier breakdown and severe vascular disruption. Vascular disruption was blocked by intravenous infusion of the direct thrombin inhibitor argatroban. Greater numbers of dying cells were found in regions of severe vascular disruption, and interventions that reduced vascular leakage also reduced the numbers of dying cells, labeled by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL). These experiments suggested a key role for thrombin in mediating cell demise during ischemia. The next question was to ask how thrombin mediates cell death. Thrombin may contribute to ischemic injury by potentiating coagulation activity and/or acting on protease-activated receptor 1 (PAR1) on brain cells. To test the specific cellular pathways activated by thrombin, PAR1 antagonist was infused via the jugular vein during ischemia and protected the brain from further vascular damage. Arterial infusion of PAR1 agonist peptide exacerbated the vascular disruption and tissue injury. Immunohistochemistry revealed that PAR1 was activated in regions with vascular leakage. Inhibition of p38 mitogen-activated protein kinase (MAPK), a downstream effector of PAR1, alleviated the ischemic injury. Infusion of an antagonist against platelet aggregation, however, did not affect the ischemic vascular injury and tissue injury. Together, these data suggested a critical role for PAR1/MAPK pathway in thrombin-mediated ischemic injury. Further, the anti- thrombotic function of thrombin antagonists seemed less beneficial, suggesting that thrombin mediated thrombosis during ischemia plays a less significant role than PAR-1 mediated thrombin toxicit