Antithrombotic, procoagulant, and fibrinolytic mechanisms in cerebral circulation: implications for brain injury and protection.
Maintaining a delicate balance among anticoagulant, procoagulant, and fibrinolytic pathways in the cerebral microcirculation is of major importance for normal cerebral blood flow. Under physiological conditions and in the absence of provocative stimuli, the anticoagulant and fibrinolytic pathways prevail over procoagulant mechanisms. Blood clotting is essential to minimize bleeding and to achieve hemostasis; however, excessive clotting contributes to thrombosis and may predispose the brain to infarction and ischemic stroke. Conversely, excessive bleeding due to enhanced anticoagulatory and fibrinolytic mechanisms could predispose the brain to hemorrhagic stroke. Recent studies in the author's laboratory indicate that brain capillary endothelium in vivo produces thrombomodulin (TM), a key cofactor in the TM-protein C system that is of major biological significance to the antithrombotic properties of the blood-brain barrier (BBB). The BBB endothelium also expresses tissue plasminogen activator (tPA), a key protein in fibrinolysis, and its rapid inhibitor, plasminogen activator inhibitor (PAI-1). The procoagulant tissue factor is normally dormant at the BBB. There is a vast body of clinical evidence to document the importance of hemostasis in the pathophysiology of brain injury. In particular, functional changes caused by major stroke risk factors in the TM--protein C, tPA/PAI-1, and tissue factor systems at the BBB may result in large and debilitating infarctions following an ischemic insult. Thus, correcting this hemostatic imbalance could ameliorate drastic CBF reductions at the time of ischemic insult, ultimately resulting in brain protection. Delineation of the molecular mechanisms of BBB-mediated hemostasis will likely contribute to future stroke prevention efforts and brain protection strategies.