Brain-derived neurotrophic factor (BDNF), a cognate ligand for the tyrosine kinase receptor B (TrkB) receptor, mediates neuronal survival, differentiation, synaptic plasticity, and neurogenesis. However, BDNF has a poor pharmacokinetic profile that limits its therapeutic potential. Here we report the identification of 7,8-dihydroxyflavone as a bioactive high-affinity TrkB agonist that provokes receptor dimerization and autophosphorylation and activation of downstream signaling. 7,8-Dihydroxyflavone protected wild-type, but not TrkB-deficient, neurons from apoptosis. Administration of 7,8-dihydroxyflavone to mice activated TrkB in the brain, inhibited kainic acid-induced toxicity, decreased infarct volumes in stroke in a TrkB-dependent manner, and was neuroprotective in an animal model of Parkinson disease. Thus, 7,8-dihydroxyflavone imitates BDNF and acts as a robust TrkB agonist, providing a powerful therapeutic tool for the treatment of various neurological diseases.

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 perme-ability 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. Abstract Treatment of acute ischemic stroke with the thrombolytic tissue plasminogen activator (tPA) can significantly improve neurological outcomes; however, throm-bolytic 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 micro-glia throughout), acts together with the endocytic receptor Enming Joseph Su and Chunzhang Cao contributed equally to this work. Daniel A. Lawrence and Li Zhang contributed equally to this work. Electronic supplementary material The online version of this article (

Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: tissue-type plasminogen activator (tPA); plasminogen (Plg); middle cerebral artery occlusion (MCAO); blood-brain barrier (BBB); LDL receptor-related protein (LRP); urokinase-type plasminogen activator (uPA); receptor-associated protein (RAP); N-methyl-Daspartate (NMDA); protease-activated receptor (PAR).

Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: tissue-type plasminogen activator (tPA); plasminogen (Plg); middle cerebral artery occlusion (MCAO); blood-brain barrier (BBB); LDL receptor-related protein (LRP); urokinase-type plasminogen activator (uPA); receptor-associated protein (RAP); N-methyl-Daspartate (NMDA); protease-activated receptor (PAR).

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The low-density lipoprotein receptorrelated protein (LRP) is a member of the LDL receptor gene family that binds several ligands, including tissue-type plasminogen activator (tPA). tPA is found in blood, where its primary function is as a thrombolytic enzyme, and in the central nervous system where it mediates events associated with cell death. Cerebral ischemia induces changes in the neurovascular unit (NVU) that result in brain edema. We investigated whether the interaction between tPA and LRP plays a role in the regulation of the permeability of the NVU during cerebral ischemia. We found that the ischemic insult induces shedding of LRP's ectodomain from perivascular astrocytes into the basement membrane. This event associates with the detachment of astrocytic end-feet processes and the formation of areas of perivascular edema. The shedding of LRP's ectodomain is significantly decreased in tPA deficient (tPA ؊/؊ ) mice, is increased by incubation with tPA, and is inhibited by the receptor-associated protein (RAP). IntroductionThe low-density lipoprotein receptor-related protein (LRP) is a member of the LDL receptor gene family composed of a 515-kDa heavy chain noncovalently bound to an 85-kDa light chain containing a transmembrane and a cytoplasmic domain. 1 LRP mediates the internalization of apoE-enriched lipoprotein particles, 2 ␣-2-macroglobulin-protease complexes, 3 and several other ligands, including plasminogen activators, proteinase-inhibitor complexes, clotting factors, and the amyloid precursor protein (APP). 1 LRP has also been implicated in cellular signal transduction pathways 4 and neurotransmission. 5 Like other signaling receptors such as Notch 6 and APP, 7 LRP undergoes a ␥-secretase-likedependent cleavage of its cytoplasmic site with release of its intramembranous domain. 8 This process is preceded by shedding of the receptor's ectodomain which may increase substrate availability for the enzymes that are required for the cleavage of the intramembranous or cytosolic sites. 8,9 Tissue-type plasminogen activator (tPA) is a ligand for LRP, 10,11 on binding to LRP induces a transient tyrosine phosphorylation of its cytoplasmic domain, and induces increased synthesis of 13 Animal studies have demonstrated that following the onset of cerebral ischemia there is an increase in endogenous tPA activity within the ischemic tissue, [14][15][16] and that either genetic deficiency of tPA 14,17 or its inhibition with neuroserpin 15,18 are associated with neuronal survival and decrease in the volume of the ischemic lesion.The neurovascular unit (NVU) is a dynamic structure consisting of endothelial cells, the basal lamina, astrocytic end-feet processes, pericytes, and neurons. 19,20 One of the functions of the NVU is to form a barrier, known as the blood-brain barrier (BBB), that regulates the entry of selected molecules from the blood into the central nervous system (CNS). 21,22 During cerebral ischemia the permeability of the NVU increases, resulting in the development of cerebral edema, 23,24 which is a ...

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a member of the tumor necrosis factor superfamily. TWEAK acts on responsive cells via binding to a small cell surface receptor named Fn14. Recent studies have demonstrated that TWEAK can stimulate numerous cellular responses including cell proliferation, migration, and proinflammatory molecule production, but the role of this cytokine in cardiovascular disease and stroke has not been established. The present study investigated whether TWEAK or Fn14 expression was regulated in a murine model of cerebral ischemia and whether TWEAK played a role in ischemia-mediated cell death. We found that TWEAK and Fn14 were expressed by primary mouse cerebral cortex-derived astrocytes and neurons cultured in vitro. Also, both the TWEAK and Fn14 proteins were present at elevated levels in the ischemic penumbra region after middle cerebral artery occlusion. Finally, we report that intracerebroventricular injection of a soluble Fn14-Fc decoy receptor immediately after middle cerebral artery occlusion significantly reduced infarct volume and the extent of microglial cell activation and apoptotic cell death in the ischemic penumbra. We conclude that the cytokine TWEAK may play an important role in ischemia-induced brain injury and that inhibition of TWEAK expression or function in the brain may represent a novel neuroprotective strategy to treat ischemic stroke.