Parabiosis Discriminates the Circulating, Endothelial, and Parenchymal Contributions of Endogenous Tissue-Type Plasminogen Activator to Stroke

BACKGROUND: Intravenous injection of alteplase, a recombinant tPA (tissue-type plasminogen activator) as a thrombolytic agent has revolutionized ischemic stroke management. However, tPA is a more complex enzyme than expected, being for instance able to promote thrombolysis, but at the same time, also able to influence neuronal survival and to affect the integrity of the blood-brain barrier. Accordingly, the respective impact of endogenous tPA expressed/present in the brain parenchyma versus in the circulation during stroke remains debated. METHODS: To address this issue, we used mice with constitutive deletion of tPA (tPA Null tPA-deficient mice) or conditional deletion of endothelial tPA (VECad vascular endothelial-Cadherin-Cre-recombinase-Cre ∆tPA ). We also developed parabioses between tPA Null and wild-type mice (tPA WT ), anticipating that a tPA WT donor would restore levels of tPA to normal ones, in the circulation but not in the brain parenchyma of a tPA Null recipient. Stroke outcomes were investigated by magnetic resonance imaging in a thrombo-embolic or a thrombotic stroke model, induced by local thrombin injection or FeCl 3 application on the endothelium, respectively. RESULTS: First, our data show that endothelial tPA, released into the circulation after stroke onset, plays an overall beneficial role following thrombo-embolic stroke. Accordingly, after 24 hours, tPA Null /tPA Null parabionts displayed less spontaneous recanalization and reperfusion and larger infarcts compared with tPA WT /tPA WT littermates. However, when associated to tPA WT littermates, tPA Null mice had similar perfusion deficits, but less severe brain infarcts. In the thrombotic stroke model, homo- and hetero-typic parabionts did not differ in the extent of brain damages and did not differentially recanalize and reperfuse. CONCLUSIONS: Together, our data reveal that during thromboembolic stroke, endogenous circulating tPA from endothelial cells sustains a spontaneous recanalization and reperfusion of the tissue, thus, limiting the extension of ischemic lesions. In this context, the impact of endogenous parenchymal tPA is limited.