Abstract
Treatment of acute ischemic stroke with the thrombolytic tissue plasminogen activator (tPA) can significantly improve neurological outcomes; however, thrombolytic therapy is associated with an increased risk of intra-cerebral hemorrhage (ICH). Previously, we demonstrated that during stroke tPA acting on the parenchymal side of the neurovascular unit (NVU) can increase blood–brain barrier (BBB) permeability and ICH through activation of latent platelet-derived growth factor-CC (PDGF-CC) and signaling by the PDGF receptor-α (PDGFRα). However, in vitro, activation of PDGF-CC by tPA is very inefficient and the mechanism of PDGF-CC activation in the NVU is not known. Here, we show that the integrin Mac-1, expressed on brain microglia/macrophages (denoted microglia throughout), acts together with the endocytic receptor LRP1 in the NVU to promote tPA-mediated activation of PDGF-CC. Mac-1-deficient mice (Mac-1−/−) are protected from tPA-induced BBB permeability but not from permeability induced by intracerebroventricular injection of active PDGF-CC. Immunofluorescence analysis demonstrates that Mac-1, LRP1, and the PDGFRα all localize to the NVU of arterioles, and following middle cerebral artery occlusion (MCAO) Mac-1−/− mice show significantly less PDGFRα phosphorylation, BBB permeability, and infarct volume compared to wild-type mice. Bone-marrow transplantation studies indicate that resident CD11b+ cells, but not bone-marrow-derived leukocytes, mediate the early activation of PDGF-CC by tPA after MCAO. Finally, using a model of thrombotic stroke with late thrombolysis, we show that wild-type mice have an increased incidence of spontaneous ICH following thrombolysis with tPA 5 h after MCAO, whereas Mac-1−/− mice are resistant to the development of ICH even with late tPA treatment. Together, these results indicate that Mac-1 and LRP1 act as co-factors for the activation of PDGF-CC by tPA in the NVU, and suggest a novel mechanism for tightly regulating PDGFRα signaling in the NVU and controlling BBB permeability.
Highlights
Stroke is a leading cause of morbidity and the fifth leading cause of mortality in the United States [1]
We test the hypothesis that Mac-1 and LRP1 coordinate to enhance tissue plasminogen activator (tPA)-mediated platelet-derived growth factor-CC (PDGF-CC) activation by acting as co-factors for tPA in the neurovascular unit (NVU). Using both a cell-based PDGF receptor-α (PDGFRα) activation assay and a photothrombotic model of ischemic stroke, we show that both Mac-1 and LRP1 are required for efficient activation of PDGF-CC by tPA and the subsequent phosphorylation of the PDGFRα
Under pathologic conditions such as cerebral ischemia, this tight control is rapidly compromised; the early loss of barrier integrity is transient and there is a period of recovery, which is followed by a second phase of increased BBB permeability [35, 37]
Summary
Stroke is a leading cause of morbidity and the fifth leading cause of mortality in the United States [1]. There are two types of stroke, ischemic and hemorrhagic. The majority of strokes are ischemic, with hemorrhagic stroke accounting for approximately 10–13% of strokes [47]. The current standard of care for patients with ischemic stroke is thrombolytic therapy with tPA [52]. Thrombolysis carries a significant risk of intra-cerebral hemorrhage [24, 29, 39, 64, 65], and due in part to this increased risk of hemorrhagic conversion, it is estimated that only 5–7% of ischemic stroke patients receive intravenous tPA, with another 1–2% receiving intra-arterial therapy [34, 52]. The limited use of thrombolytic therapy demands further study of the mechanisms by which tPA leads to an increased risk of hemorrhagic conversion in stroke patients
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