Matrix metalloproteinase activation and blood–brain barrier breakdown following thrombolysis

Thrombolysis with tissue plasminogen activator (tPA) is the only pharmacotherapy available for cerebral ischemia. However, the use of tPA can increase the risk of hemorrhage due to blood–brain barrier (BBB) breakdown. Recent evidence suggests that increased activation of matrix metalloproteinases (MMPs) may be involved in this breakdown. This study examines the temporal profile of MMP-2 and -9 following tPA administration to ischemic rats.Male Sprague–Dawley rats were randomly assigned to one of four groups (Sham-tPA; Sham-Saline; Ischemia-tPA; Ischemia-Saline; group n = 6, total N = 120). Focal embolic ischemia was induced by middle cerebral artery occlusion through injection of an autologous clot. One hour post-surgery, tPA (10 mg/kg) or saline was delivered intravenously and animals were euthanized at 3, 6, 12, or 24 h after onset of ischemia. Infarct volume was measured by TTC staining; BBB components examined immunohistochemically; and MMP activation measured by gelatin zymography.Our results show that tPA significantly reduced infarct volumes (overall infarct volume-Sham-tPA: 5.80 ± 4.55 [mean ± SE]; Sham-Saline: 5.00 ± 4.23; Ischemia-tPA: 186.1 ± 73.45; Ischemia-Saline: 284.8 ± 88.74; all P < 0.05). Treatment with tPA was also associated with the activation of MMP-9 at 6, 12, and 24 h following ischemia. No temporal changes were observed in MMP-2 activation, although tPA administration increased its activity compared to saline treatment. Analyses of immunohistochemistry showed that destruction of components of the BBB followed MMP-9 activation. Thus, increased MMP-9 activation may, in part, be responsible for the increases in hemorrhagic transformation reported with use of tPA. Our study is the first to demonstrate the temporal profile of MMP activation following thrombolysis with tPA in a model of thrombotic focal cerebral ischemia.