Microglial-mediated PDGF-CC activation increases cerebrovascular permeability during ischemic stroke

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-017-1749-z) contains supplementary material, which is available to authorized users.

Emerging links between cerebrovascular and neurodegenerative diseases—a special role for pericytes

“…Recently, it has been demonstrated that PDGF signaling (PDGF-CC/ PRGFRa) may become induced downstream of tPA as a consequence of tPA-mediated cleavage and activation of latent PDGF-CC, and that modulation of PDGF may improve post-stroke BBB function [175]. Directly after acute ischemic stroke, tissue plasminogen activator (tPA) is used for thrombolysis, but beyond a few hours-long time window of therapeutic opportunity, tPA treatment is associated with intracerebral hemorrhage, a consequence of induced BBB breakdown.…”

Immune responses in stroke: how the immune system contributes to damage and healing after stroke and how this knowledge could be translated to better cures?

“…The primary inflammatory responses by microglia are to clear up debris and repair injured tissue. However, they may also promote secondary inflammation‐associated damage as multiple studies have indicated that microglia serve as pro‐inflammatory responders and increase damage following stroke . Specifically, evidence suggests that microglia contribute to increased neurovascular breakdown and permeability by prompting activation of platelet‐derived factors, and up‐regulating expression of matrix metalloproteinases (MMPs) such as MMP‐9, during the early stages of stroke.…”

“…CD11b‐positive immune cells, which encompass both microglia and infiltrating macrophages, express abundant levels of angiopoietin‐like protein 2 within the brain during tMCAO, which contributes to increased levels of the pro‐inflammatory cytokines IL‐1 β and TNF‐ α to promote endothelial dysfunction and subsequent neurological deficits . Additionally, CD11b + cells (expressed by both infiltrating macrophages and microglia) can signal through the platelet‐derived factor receptor on arterioles to promote BBB permeability and the incidence of haemorrhage during stroke . In contrast, depleting CCR2 + monocyte infiltration increases BBB permeability and risk of haemorrhage after stroke, suggesting that infiltrating macrophages may play a role in maintaining brain vasculature .…”

“…16,75 Additionally, CD11b + cells (expressed by both infiltrating macrophages and microglia) can signal through the platelet-derived factor receptor on arterioles to promote BBB permeability and the incidence of haemorrhage during stroke. 17 In contrast, depleting CCR2 + monocyte infiltration increases BBB permeability and risk of haemorrhage after stroke, suggesting that infiltrating macrophages may play a role in maintaining brain vasculature. 77,139 In support of this, it has recently been shown that the myeloid cell and astrocyte-derived matricellular glycoprotein osteopontin is critically involved in astrocyte-mediated protection of the neurovascular unit after ischaemia; macrophage-specific depletion of matricellular glycoprotein osteopontin resulted in the failure of astrocytes to properly maintain cerebral blood vessels and BBB integrity after ischaemic stroke.…”

Proteases: Pivot Points in Functional Proteomics

“…Tissue-type plasminogen activator (tPA) was approved in 1996 for clot dissolution in ischemic stroke, however its use is restricted to the first 3 hours post-stroke, and carries an established risk for bleeding. One study attributed the increased blood-brain barrier permeability to tPA-catalyzed activation of platelet-derived growth factor-CC, with Mac-1 integrin and LRP1 acting as co-factors in this reaction [211]. Another study suggested increased MMP-9 activity as a potential cause for increased bleeding risk [212], while yet another report pointed toward a connection of tPA-induced bleeding with hyperglycemia [213].…”