Abstract W P238: Selective Deletion of Pi3-kinase Gamma in the Brain Inhibits Tissue Plasminogen Activator-induced Brain Hemorrhage

Abstract

Background: PI3-kinase gamma (PI3Kγ), a key modulator in inflammation, is involved in many inflammatory diseases. We recently reported that PI3Kγ deficiency protects the brain from ischemic injury in PI3Kγ-/- mice by reducing blood-brain barrier damage and tissue infarction through inhibition of NF-kB-dependent inflammation, mainly involved in reduced MMP-9 activity in ischemic brain endothelium, microglia activation, and neutrophil infiltration into the ischemic brain. Meanwhile, we reported that ischemic brain endothelium, activated microglia, and infiltrating neutrophils represent the major cellular sources of PI3Kγ expressed in the brain within 24 h after cerebral ischemia. Brain hemorrhage is a serious complication of recombinant tissue plasminogen activator (tPA) therapy for ischemic stroke. In this study, we further investigated potential involvement of PI3Kγ, in particular the expression of PI3Kγ in the brain, in tPA-induced brain hemorrhage after ischemic stroke. Methods and Results: Focal ischemic stroke was induced by transient middle cerebral artery occlusion and reperfusion. PI3Kγ deficiency in PI3Kγ-/- mice not only reduced tissue infarction but also almost completely blocked brain hemorrhage induced by delayed tPA treatment at 6 hours after stroke. Bone marrow chimeras reveal that chimeric mice lacking PI3Kγ in the brain (WT BM →KO) blocked the delayed tPA-induced brain hemorrhage effectively as found in PI3Kγ-/- mice while chimeric mice lacking PI3Kγ expressed in leukocytes (KO BM →WT) had minimal effect on the tPA-induced brain hemorrhage. These effects could be attributed to a profound inhibition of MMP-9 activity in ischemic brain endothelium found in both PI3Kγ-/- mice and (WT BM →KO) chimeric mice but not in (KO BM →WT) chimeric mice, as determined by double IHC staining of MMP-9 in brain endothelial cells and by zymographic analysis of MMP-9 activity using microvessels isolated from the brain. Conclusion: Our findings suggest that PI3Kγ in the brain may play a predominant role in delayed tPA-induced brain hemorrhage after ischemic stroke. Our future study will define the role of PI3Kγ derived from brain endothelial cells versus microglia by generation of tissue/cell-specific knockout mice using the Cre-loxP system.