Tissue-type plasminogen activator (t-PA) can modulate permeability of the neurovascular unit and exacerbate injury in ischemic stroke. We examined the effects of t-PA using in vitro models of the bloodbrain barrier. t-PA caused a concentrationdependent increase in permeability. This effect was dependent on plasmin formation and potentiated in the presence of plasminogen. An inactive t-PA variant inhibited the t-PA-mediated increase in permeability, whereas blockade of lowdensity lipoprotein receptors or exposed lysine residues resulted in similar inhibition, implying a role for both a t-PA receptor, most likely a low-density lipoprotein receptor, and a plasminogen receptor. This effect was selective to t-PA and its close derivative tenecteplase. The truncated t-PA variant reteplase had a minor effect on permeability, whereas urokinase and desmoteplase were ineffective. t-PA also induced marked shape changes in both brain endothelial cells and astrocytes. Changes in astrocyte morphology coincided with increased F-actin staining intensity, larger focal adhesion size, and elevated levels of phosphorylated myosin. Inhibition of Rho kinase blocked these changes and reduced t-PA/plasminogenmediated increase in permeability. Hence plasmin, generated on the cell surface selectively by t-PA, modulates the astrocytic cytoskeleton, leading to an increase in blood-brain barrier permeability. IntroductionThe plasminogen-activating enzyme system is widely appreciated for its role in fibrinolysis and thrombolysis 1 and in other areas related to remodeling of the extracellular matrix. 2 However, this enzyme system also has a major impact in the CNS under both physiologic and pathologic circumstances. 3,4 The literature has amassed much data implicating tissue-type plasminogen activator (t-PA), plasmin, or both in cognitive function, memory, and anxiety 3 and in addictive behavior. 5,6 Under pathologic conditions, including ischemic stroke and traumatic brain injury, t-PA has been shown to facilitate neurotoxic events via potentiation of glutamate receptor signaling. 3,7 Although direct neurotoxicity of t-PA has been demonstrated by some studies 7,8 but not others, 9 t-PA also has been shown to modulate permeability of the neurovascular unit. [10][11][12] Hence, under pathologic conditions, the deleterious consequences of t-PA may be because of direct neurotoxicity, increased permeability of the blood-brain barrier (BBB), or a combination.Much effort has been devoted to understanding the mechanism by which t-PA modulates BBB permeability. t-PA delivered intravenously has been shown to cross brain endothelial cells via transcytosis without compromising BBB integrity, 10,13 whereas others have shown that t-PA can enter the parenchyma during pathologic conditions where it further enhances BBB breakdown. 12 In some paradigms of BBB breakdown, 12,14 the damaging effect of t-PA has been reported as plasmin-dependent 15,16 or -independent 11,12,17 contingent on the cellular context and time-frame of the experiments performed. Hence, t...

The neurovascular unit provides a dynamic interface between the circulation and central nervous system. Disruption of neurovascular integrity occurs in numerous brain pathologies including neurotrauma and ischaemic stroke. Tissue plasminogen activator is a serine protease that converts plasminogen to plasmin, a protease that dissolves blood clots. Besides its role in fibrinolysis, tissue plasminogen activator is abundantly expressed in the brain where it mediates extracellular proteolysis. However, proteolytically active tissue plasminogen activator also promotes neurovascular disruption after ischaemic stroke; the molecular mechanisms of this process are still unclear. Tissue plasminogen activator is naturally inhibited by serine protease inhibitors (serpins): plasminogen activator inhibitor-1, neuroserpin or protease nexin-1 that results in the formation of serpin:protease complexes. Proteases and serpin:protease complexes are cleared through high-affinity binding to low-density lipoprotein receptors, but their binding to these receptors can also transmit extracellular signals across the plasma membrane. The matrix metalloproteinases are the second major proteolytic system in the mammalian brain, and like tissue plasminogen activators are pivotal to neurological function but can also degrade structures of the neurovascular unit after injury. Herein, we show that tissue plasminogen activator potentiates neurovascular damage in a dose-dependent manner in a mouse model of neurotrauma. Surprisingly, inhibition of activity following administration of plasminogen activator inhibitor-1 significantly increased cerebrovascular permeability. This led to our finding that formation of complexes between tissue plasminogen activator and plasminogen activator inhibitor-1 in the brain parenchyma facilitates post-traumatic cerebrovascular damage. We demonstrate that following trauma, the complex binds to low-density lipoprotein receptors, triggering the induction of matrix metalloproteinase-3. Accordingly, pharmacological inhibition of matrix metalloproteinase-3 attenuates neurovascular permeability and improves neurological function in injured mice. Our results are clinically relevant, because concentrations of tissue plasminogen activator: plasminogen activator inhibitor-1 complex and matrix metalloproteinase-3 are significantly elevated in cerebrospinal fluid of trauma patients and correlate with neurological outcome. In a separate study, we found that matrix metalloproteinase-3 and albumin, a marker of cerebrovascular damage, were significantly increased in brain tissue of patients with neurotrauma. Perturbation of neurovascular homeostasis causing oedema, inflammation and cell death is an important cause of acute and long-term neurological dysfunction after trauma. A role for the tissue plasminogen activator-matrix metalloproteinase axis in promoting neurovascular disruption after neurotrauma has not been described thus far. Targeting tissue plasminogen activator: plasminogen activator inhibitor-1 complex signalling or dow...

Removal of dead cells in the absence of concomitant immune stimulation is essential for tissue homeostasis. We recently identified an injury-induced protein misfolding event that orchestrates the plasmin-dependent proteolytic degradation of necrotic cells. As impaired clearance of dead cells by the innate immune system predisposes to autoimmunity, we determined whether plasmin could influence endocytosis and immune cell stimulation by dendritic cells – a critical cell that links the innate and adaptive immune systems. We find that plasmin generated on the surface of necrotic cells enhances their phagocytic removal by human monocyte-derived dendritic cells. Plasmin also promoted phagocytosis of protease-resistant microparticles by diverse mouse dendritic cell sub-types both in vitro and in vivo. Together with an increased phagocytic capacity, plasmin-treated dendritic cells maintain an immature phenotype, exhibit reduced migration to lymph nodes, increase their expression/release of the immunosuppressive cytokine TGF-β, and lose their capacity to mount an allogeneic response. Collectively, our findings support a novel role for plasmin formed on dead cells and other phagocytic targets in maintaining tissue homeostasis by increasing the phagocytic function of dendritic cells while simultaneously decreasing their immunostimulatory capacity consistent with producing an immunosuppressive state.

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